AUXIN RESPONSE FACTOR17 Is Essential for Pollen Wall Pattern Formation in Arabidopsis

In angiosperms, pollen wall pattern formation is determined by primexine deposition on the microspores. Here, we show that AUXIN RESPONSE FACTOR17 (ARF17) is essential for primexine formation and pollen development in Arabidopsis (Arabidopsis thaliana). The arf17 mutant exhibited a male-sterile phenotype with normal vegetative growth. ARF17 was expressed in microsporocytes and microgametophytes from meiosis to the bicellular microspore stage. Transmission electron microscopy analysis showed that primexine was absent in the arf17 mutant, which leads to pollen wall-patterning defects and pollen degradation. Callose deposition was also significantly reduced in the arf17 mutant, and the expression of CALLOSE SYNTHASE5 (CalS5), the major gene for callose biosynthesis, was approximately 10% that of the wild type. Chromatin immunoprecipitation and electrophoretic mobility shift assays showed that ARF17 can directly bind to the CalS5 promoter. As indicated by the expression of DR5-driven green fluorescent protein, which is an synthetic auxin response reporter, auxin signaling appeared to be specifically impaired in arf17 anthers. Taken together, our results suggest that ARF17 is essential for pollen wall patterning in Arabidopsis by modulating primexine formation at least partially through direct regulation of CalS5 gene expression.In angiosperms, the pollen wall is the most complex plant cell wall. It consists of the inner wall, the intine, and the outer wall, the exine. The exine is further divided into sexine and nexine layers. The sculptured sexine includes three major parts: baculum, tectum, and tryphine (Heslop-Harrison, 1971; Piffanelli et al., 1998; Ariizumi and Toriyama, 2011; Fig. 1A). Production of a functional pollen wall requires the precise spatial and temporal cooperation of gametophytic and sporophytic tissues and metabolic events (Blackmore et al., 2007). The intine layer is controlled gametophytically, while the exine is regulated sporophytically. The sporophytic tapetum cells provide material for pollen wall formation, while primexine determines pollen wall patterning (Heslop-Harrison, 1968).Open in a separate windowFigure 1.Schematic representation of the pollen wall and primexine development. A, The innermost layer adjacent to the plasma membrane is the intine. The bacula (Ba), tectum (Te), and tryphine (T) make up the sexine layer. The nexine is located between the intine and the sexine layers. The exine includes the nexine and sexine layers. B, Primexine (Pr) appears between callose (Cl) and plasma membrane (Pm) at the early tetrad stage (left panel). Subsequently, the plasma membrane becomes undulated (middle panel) and sporopollenin deposits on the peak of the undulated plasma membrane to form bacula and tectum (right panel).After meiosis, four microspores were encased in callose to form a tetrad. Subsequently, the primexine develops between the callose layer and the microspore membrane (Fig. 1B), and the microspore plasma membrane becomes undulated (Fig. 1B; Fitzgerald and Knox, 1995; Southworth and Jernstedt, 1995). Sporopollenin precursors then accumulate on the peak of the undulated microspore membrane to form the bacula and tectum (Fig. 1B; Fitzgerald and Knox, 1995). After callose degradation, individual microspores are released from the tetrad, and the bacula and tectum continue to grow into exine with further sporopollenin deposition (Fitzgerald and Knox, 1995; Blackmore et al., 2007).The callose has been reported to affect primexine deposition and pollen wall pattern formation. The peripheral callose layer, secreted by the microsporocyte, acts as the mold for primexine (Waterkeyn and Bienfait, 1970; Heslop-Harrison, 1971). CALLOSE SYNTHASE5 (CalS5) is the major enzyme responsible for the biosynthesis of the callose peripheral of the tetrad (Dong et al., 2005; Nishikawa et al., 2005). Mutation of Cals5 and abnormal CalS5 pre-mRNA splicing resulted in defective peripheral callose deposition and primexine formation (Dong et al., 2005; Nishikawa et al., 2005; Huang et al., 2013). Besides CalS5, four membrane-associated proteins have also been reported to be involved in primexine formation: DEFECTIVE EXINE FORMATION1 (DEX1; Paxson-Sowders et al., 1997, 2001), NO EXINE FORMATION1 (NEF1; Ariizumi et al., 2004), RUPTURED POLLEN GRAIN1 (RPG1; Guan et al., 2008; Sun et al., 2013), and NO PRIMEXINE AND PLASMA MEMBRANE UNDULATION (NPU; Chang et al., 2012). Mutation of DEX1 results in delayed primexine formation (Paxson-Sowders et al., 2001). The primexine in nef1 is coarse compared with the wild type (Ariizumi et al., 2004). The loss-of-function rpg1 shows reduced primexine deposition (Guan et al., 2008; Sun et al., 2013), while the npu mutant does not deposit any primexine (Chang et al., 2012). Recently, it was reported that Arabidopsis (Arabidopsis thaliana) CYCLIN-DEPENDENT KINASE G1 (CDKG1) associates with the spliceosome to regulate the CalS5 pre-mRNA splicing for pollen wall formation (Huang et al., 2013). Clearly, disrupted primexine deposition leads to aberrant pollen wall patterning and ruptured pollen grains in these mutants.The plant hormone auxin has multiple roles in plant reproductive development (Aloni et al., 2006; Sundberg and Østergaard, 2009). Knocking out the two auxin biosynthesis genes, YUC2 and YUC6, caused an essentially sterile phenotype in Arabidopsis (Cheng et al., 2006). Auxin transport is essential for anther development; defects in auxin flow in anther filaments resulted in abnormal pollen mitosis and pollen development (Feng et al., 2006). Ding et al. (2012) showed that the endoplasmic reticulum-localized auxin transporter PIN8 regulates auxin homeostasis and male gametophyte development in Arabidopsis. Evidence for the localization, biosynthesis, and transport of auxin indicates that auxin regulates anther dehiscence, pollen maturation, and filament elongation during late anther development (Cecchetti et al., 2004, 2008). The role of auxin in pollen wall development has not been reported.The auxin signaling pathway requires the auxin response factor (ARF) family proteins (Quint and Gray, 2006; Guilfoyle and Hagen, 2007; Mockaitis and Estelle, 2008; Vanneste and Friml, 2009). ARF proteins can either activate or repress the expression of target genes by directly binding to auxin response elements (AuxRE; TGTCTC/GAGACA) in the promoters (Ulmasov et al., 1999; Tiwari et al., 2003). The Arabidopsis ARF family contains 23 members. A subgroup in the ARF family, ARF10, ARF16, and ARF17, are targets of miRNA160 (Okushima et al., 2005b; Wang et al., 2005). Plants expressing miR160-resistant ARF17 exhibited pleiotropic developmental defects, including abnormal stamen structure and reduced fertility (Mallory et al., 2005). This indicates a potential role for ARF17 in plant fertility, although the detailed function remains unknown. In addition, ARF17 was also proposed to negatively regulate adventitious root formation (Sorin et al., 2005; Gutierrez et al., 2009), although an ARF17 knockout mutant was not reported and its phenotype is unknown.In this work, we isolated and characterized a loss-of-function mutant of ARF17. Results from cytological observations suggest that ARF17 controls callose biosynthesis and primexine deposition. Consistent with this, the ARF17 protein is highly abundant in microsporocytes and tetrads. Furthermore, we demonstrate that the ARF17 protein is able to bind the promoter region of CalS5. Our results suggest that ARF17 regulates pollen wall pattern formation in Arabidopsis.     

Spore Density Determines Infection Strategy by the Plant Pathogenic Fungus Plectosphaerella cucumerina

Necrotrophic and biotrophic pathogens are resisted by different plant defenses. While necrotrophic pathogens are sensitive to jasmonic acid (JA)-dependent resistance, biotrophic pathogens are resisted by salicylic acid (SA)- and reactive oxygen species (ROS)-dependent resistance. Although many pathogens switch from biotrophy to necrotrophy during infection, little is known about the signals triggering this transition. This study is based on the observation that the early colonization pattern and symptom development by the ascomycete pathogen Plectosphaerella cucumerina (P. cucumerina) vary between inoculation methods. Using the Arabidopsis (Arabidopsis thaliana) defense response as a proxy for infection strategy, we examined whether P. cucumerina alternates between hemibiotrophic and necrotrophic lifestyles, depending on initial spore density and distribution on the leaf surface. Untargeted metabolome analysis revealed profound differences in metabolic defense signatures upon different inoculation methods. Quantification of JA and SA, marker gene expression, and cell death confirmed that infection from high spore densities activates JA-dependent defenses with excessive cell death, while infection from low spore densities induces SA-dependent defenses with lower levels of cell death. Phenotyping of Arabidopsis mutants in JA, SA, and ROS signaling confirmed that P. cucumerina is differentially resisted by JA- and SA/ROS-dependent defenses, depending on initial spore density and distribution on the leaf. Furthermore, in situ staining for early callose deposition at the infection sites revealed that necrotrophy by P. cucumerina is associated with elevated host defense. We conclude that P. cucumerina adapts to early-acting plant defenses by switching from a hemibiotrophic to a necrotrophic infection program, thereby gaining an advantage of immunity-related cell death in the host.Plant pathogens are often classified as necrotrophic or biotrophic, depending on their infection strategy (Glazebrook, 2005; Nishimura and Dangl, 2010). Necrotrophic pathogens kill living host cells and use the decayed plant tissue as a substrate to colonize the plant, whereas biotrophic pathogens parasitize living plant cells by employing effector molecules that suppress the host immune system (Pel and Pieterse, 2013). Despite this binary classification, the majority of pathogenic microbes employ a hemibiotrophic infection strategy, which is characterized by an initial biotrophic phase followed by a necrotrophic infection strategy at later stages of infection (Perfect and Green, 2001). The pathogenic fungi Magnaporthe grisea, Sclerotinia sclerotiorum, and Mycosphaerella graminicola, the oomycete Phytophthora infestans, and the bacterial pathogen Pseudomonas syringae are examples of hemibiotrophic plant pathogens (Perfect and Green, 2001; Koeck et al., 2011; van Kan et al., 2014; Kabbage et al., 2015).Despite considerable progress in our understanding of plant resistance to necrotrophic and biotrophic pathogens (Glazebrook, 2005; Mengiste, 2012; Lai and Mengiste, 2013), recent debate highlights the dynamic and complex interplay between plant-pathogenic microbes and their hosts, which is raising concerns about the use of infection strategies as a static tool to classify plant pathogens. For instance, the fungal genus Botrytis is often labeled as an archetypal necrotroph, even though there is evidence that it can behave as an endophytic fungus with a biotrophic lifestyle (van Kan et al., 2014). The rice blast fungus Magnaporthe oryzae, which is often classified as a hemibiotrophic leaf pathogen (Perfect and Green, 2001; Koeck et al., 2011), can adopt a purely biotrophic lifestyle when infecting root tissues (Marcel et al., 2010). It remains unclear which signals are responsible for the switch from biotrophy to necrotrophy and whether these signals rely solely on the physiological state of the pathogen, or whether host-derived signals play a role as well (Kabbage et al., 2015).The plant hormones salicylic acid (SA) and jasmonic acid (JA) play a central role in the activation of plant defenses (Glazebrook, 2005; Pieterse et al., 2009, 2012). The first evidence that biotrophic and necrotrophic pathogens are resisted by different immune responses came from Thomma et al. (1998), who demonstrated that Arabidopsis (Arabidopsis thaliana) genotypes impaired in SA signaling show enhanced susceptibility to the biotrophic pathogen Hyaloperonospora arabidopsidis (formerly known as Peronospora parastitica), while JA-insensitive genotypes were more susceptible to the necrotrophic fungus Alternaria brassicicola. In subsequent years, the differential effectiveness of SA- and JA-dependent defense mechanisms has been confirmed in different plant-pathogen interactions, while additional plant hormones, such as ethylene, abscisic acid (ABA), auxins, and cytokinins, have emerged as regulators of SA- and JA-dependent defenses (Bari and Jones, 2009; Cao et al., 2011; Pieterse et al., 2012). Moreover, SA- and JA-dependent defense pathways have been shown to act antagonistically on each other, which allows plants to prioritize an appropriate defense response to attack by biotrophic pathogens, necrotrophic pathogens, or herbivores (Koornneef and Pieterse, 2008; Pieterse et al., 2009; Verhage et al., 2010).In addition to plant hormones, reactive oxygen species (ROS) play an important regulatory role in plant defenses (Torres et al., 2006; Lehmann et al., 2015). Within minutes after the perception of pathogen-associated molecular patterns, NADPH oxidases and apoplastic peroxidases generate early ROS bursts (Torres et al., 2002; Daudi et al., 2012; O’Brien et al., 2012), which activate downstream defense signaling cascades (Apel and Hirt, 2004; Torres et al., 2006; Miller et al., 2009; Mittler et al., 2011; Lehmann et al., 2015). ROS play an important regulatory role in the deposition of callose (Luna et al., 2011; Pastor et al., 2013) and can also stimulate SA-dependent defenses (Chaouch et al., 2010; Yun and Chen, 2011; Wang et al., 2014; Mammarella et al., 2015). However, the spread of SA-induced apoptosis during hyperstimulation of the plant immune system is contained by the ROS-generating NADPH oxidase RBOHD (Torres et al., 2005), presumably to allow for the sufficient generation of SA-dependent defense signals from living cells that are adjacent to apoptotic cells. Nitric oxide (NO) plays an additional role in the regulation of SA/ROS-dependent defense (Trapet et al., 2015). This gaseous molecule can stimulate ROS production and cell death in the absence of SA while preventing excessive ROS production at high cellular SA levels via S-nitrosylation of RBOHD (Yun et al., 2011). Recently, it was shown that pathogen-induced accumulation of NO and ROS promotes the production of azelaic acid, a lipid derivative that primes distal plants for SA-dependent defenses (Wang et al., 2014). Hence, NO, ROS, and SA are intertwined in a complex regulatory network to mount local and systemic resistance against biotrophic pathogens. Interestingly, pathogens with a necrotrophic lifestyle can benefit from ROS/SA-dependent defenses and associated cell death (Govrin and Levine, 2000). For instance, Kabbage et al. (2013) demonstrated that S. sclerotiorum utilizes oxalic acid to repress oxidative defense signaling during initial biotrophic colonization, but it stimulates apoptosis at later stages to advance necrotrophic colonization. Moreover, SA-induced repression of JA-dependent resistance not only benefits necrotrophic pathogens but also hemibiotrophic pathogens after having switched from biotrophy to necrotrophy (Glazebrook, 2005; Pieterse et al., 2009, 2012).Plectosphaerella cucumerina ((P. cucumerina, anamorph Plectosporum tabacinum) anamorph Plectosporum tabacinum) is a filamentous ascomycete fungus that can survive saprophytically in soil by decomposing plant material (Palm et al., 1995). The fungus can cause sudden death and blight disease in a variety of crops (Chen et al., 1999; Harrington et al., 2000). Because P. cucumerina can infect Arabidopsis leaves, the P. cucumerina-Arabidopsis interaction has emerged as a popular model system in which to study plant defense reactions to necrotrophic fungi (Berrocal-Lobo et al., 2002; Ton and Mauch-Mani, 2004; Carlucci et al., 2012; Ramos et al., 2013). Various studies have shown that Arabidopsis deploys a wide range of inducible defense strategies against P. cucumerina, including JA-, SA-, ABA-, and auxin-dependent defenses, glucosinolates (Tierens et al., 2001; Sánchez-Vallet et al., 2010; Gamir et al., 2014; Pastor et al., 2014), callose deposition (García-Andrade et al., 2011; Gamir et al., 2012, 2014; Sánchez-Vallet et al., 2012), and ROS (Tierens et al., 2002; Sánchez-Vallet et al., 2010; Barna et al., 2012; Gamir et al., 2012, 2014; Pastor et al., 2014). Recent metabolomics studies have revealed large-scale metabolic changes in P. cucumerina-infected Arabidopsis, presumably to mobilize chemical defenses (Sánchez-Vallet et al., 2010; Gamir et al., 2014; Pastor et al., 2014). Furthermore, various chemical agents have been reported to induce resistance against P. cucumerina. These chemicals include β-amino-butyric acid, which primes callose deposition and SA-dependent defenses, benzothiadiazole (BTH or Bion; Görlach et al., 1996; Ton and Mauch-Mani, 2004), which activates SA-related defenses (Lawton et al., 1996; Ton and Mauch-Mani, 2004; Gamir et al., 2014; Luna et al., 2014), JA (Ton and Mauch-Mani, 2004), and ABA, which primes ROS and callose deposition (Ton and Mauch-Mani, 2004; Pastor et al., 2013). However, among all these studies, there is increasing controversy about the exact signaling pathways and defense responses contributing to plant resistance against P. cucumerina. While it is clear that JA and ethylene contribute to basal resistance against the fungus, the exact roles of SA, ABA, and ROS in P. cucumerina resistance vary between studies (Thomma et al., 1998; Ton and Mauch-Mani, 2004; Sánchez-Vallet et al., 2012; Gamir et al., 2014).This study is based on the observation that the disease phenotype during P. cucumerina infection differs according to the inoculation method used. We provide evidence that the fungus follows a hemibiotrophic infection strategy when infecting from relatively low spore densities on the leaf surface. By contrast, when challenged by localized host defense to relatively high spore densities, the fungus switches to a necrotrophic infection program. Our study has uncovered a novel strategy by which plant-pathogenic fungi can take advantage of the early immune response in the host plant.     

CELLULOSE SYNTHASE-LIKE A2, a Glucomannan Synthase,Is Involved in Maintaining Adherent Mucilage Structure in Arabidopsis Seed

Mannans are hemicellulosic polysaccharides that are considered to have both structural and storage functions in the plant cell wall. However, it is not yet known how mannans function in Arabidopsis (Arabidopsis thaliana) seed mucilage. In this study, CELLULOSE SYNTHASE-LIKE A2 (CSLA2; At5g22740) expression was observed in several seed tissues, including the epidermal cells of developing seed coats. Disruption of CSLA2 resulted in thinner adherent mucilage halos, although the total amount of the adherent mucilage did not change compared with the wild type. This suggested that the adherent mucilage in the mutant was more compact compared with that of the wild type. In accordance with the role of CSLA2 in glucomannan synthesis, csla2-1 mucilage contained 30% less mannosyl and glucosyl content than did the wild type. No appreciable changes in the composition, structure, or macromolecular properties were observed for nonmannan polysaccharides in mutant mucilage. Biochemical analysis revealed that cellulose crystallinity was substantially reduced in csla2-1 mucilage; this was supported by the removal of most mucilage cellulose through treatment of csla2-1 seeds with endo-β-glucanase. Mutation in CSLA2 also resulted in altered spatial distribution of cellulose and an absence of birefringent cellulose microfibrils within the adherent mucilage. As with the observed changes in crystalline cellulose, the spatial distribution of pectin was also modified in csla2-1 mucilage. Taken together, our results demonstrate that glucomannans synthesized by CSLA2 are involved in modulating the structure of adherent mucilage, potentially through altering cellulose organization and crystallization.Mannan polysaccharides are a complex set of hemicellulosic cell wall polymers that are considered to have both structural and storage functions. Based on the particular chemical composition of the backbone and the side chains, mannan polysaccharides are classified into four types: pure mannan, glucomannan, galactomannan, and galactoglucomannan (Moreira and Filho, 2008; Wang et al., 2012; Pauly et al., 2013). Each of these polysaccharides is composed of a β-1,4-linked backbone containing Man or a combination of Glc and Man residues. In addition, the mannan backbone can be substituted with side chains of α-1,6-linked Gal residues. Mannan polysaccharides have been proposed to cross link with cellulose and other hemicelluloses via hydrogen bonds (Fry, 1986; Iiyama et al., 1994; Obel et al., 2007; Scheller and Ulvskov, 2010). Furthermore, it has been reported that heteromannans with different levels of substitution can interact with cellulose in diverse ways (Whitney et al., 1998). Together, these observations indicate the complexity of mannan polysaccharides in the context of cell wall architecture.CELLULOSE SYNTHASE-LIKE A (CSLA) enzymes have been shown to have mannan synthase activity in vitro. These enzymes polymerize the β-1,4-linked backbone of mannans or glucomannans, depending on the substrates (GDP-Man and/or GDP-Glc) provided (Richmond and Somerville, 2000; Liepman et al., 2005, 2007; Pauly et al., 2013). In Arabidopsis (Arabidopsis thaliana), nine CSLA genes have been identified; different CSLAs are responsible for the synthesis of different mannan types (Liepman et al., 2005, 2007). CSLA7 has mannan synthase activity in vitro (Liepman et al., 2005) and has been shown to synthesize stem glucomannan in vivo (Goubet et al., 2009). Disrupting the CSLA7 gene results in defective pollen growth and embryo lethality phenotypes in Arabidopsis, indicating structural or signaling functions of mannan polysaccharides during plant embryo development (Goubet et al., 2003). A mutation in CSLA9 results in the inhibition of Agrobacterium tumefaciens-mediated root transformation in the rat4 mutant (Zhu et al., 2003). CSLA2, CSLA3, and CSLA9 are proposed to play nonredundant roles in the biosynthesis of stem glucomannans, although mutations in CSLA2, CSLA3, or CSLA9 have no effect on stem development or strength (Goubet et al., 2009). All of the Arabidopsis CSLA proteins have been shown to be involved in the biosynthesis of mannan polysaccharides in the plant cell wall (Liepman et al., 2005, 2007), although the precise physiological functions of only CSLA7 and CSLA9 have been conclusively demonstrated.In Arabidopsis, when mature dry seeds are hydrated, gel-like mucilage is extruded to envelop the entire seed. Ruthenium red staining of Arabidopsis seeds reveals two different mucilage layers, termed the nonadherent and the adherent mucilage layers (Western et al., 2000; Macquet et al., 2007a). The outer, nonadherent mucilage is loosely attached and can be easily extracted by shaking seeds in water. Compositional and linkage analyses suggest that this layer is almost exclusively composed of unbranched rhamnogalacturonan I (RG-I) (>80% to 90%), with small amounts of branched RG-I, arabinoxylan, and high methylesterified homogalacturonan (HG). By contrast, the inner, adherent mucilage layer is tightly attached to the seed and can only be removed by strong acid or base treatment, or by enzymatic digestion (Macquet et al., 2007a; Huang et al., 2011; Walker et al., 2011). As with the nonadherent layer, adherent mucilage is also mainly composed of unbranched RG-I, but with small numbers of arabinan and galactan ramifications (Penfield et al., 2001; Willats et al., 2001; Dean et al., 2007; Macquet et al., 2007a, 2007b; Arsovski et al., 2009; Haughn and Western, 2012). There are also minor amounts of pectic HG in the adherent mucilage, with high methylesterified HG in the external domain compared with the internal domain of the adherent layer (Willats et al., 2001; Macquet et al., 2007a; Rautengarten et al., 2008; Sullivan et al., 2011; Saez-Aguayo et al., 2013). In addition, the adherent mucilage contains cellulose (Blake et al., 2006; Macquet et al., 2007a), which is entangled with RG-I and is thought to anchor the pectin-rich mucilage onto seeds (Macquet et al., 2007a; Harpaz-Saad et al., 2011, 2012; Mendu et al., 2011; Sullivan et al., 2011). As such, Arabidopsis seed mucilage is considered to be a useful model for investigating the biosynthesis of cell wall polysaccharides and how this process is regulated in vivo (Haughn and Western, 2012).Screening for altered seed coat mucilage has led to the identification of several genes encoding enzymes that are involved in the biosynthesis or modification of mucilage components. RHAMNOSE SYNTHASE2/MUCILAGE-MODIFIED4 (MUM4) is responsible for the synthesis of UDP-l-Rha (Usadel et al., 2004; Western et al., 2004; Oka et al., 2007). The putative GALACTURONSYLTRANSFERASE11 can potentially synthesize mucilage RG-I or HG pectin from UDP-d-GalUA (Caffall et al., 2009). GALACTURONSYLTRANSFERASE-LIKE5 appears to function in the regulation of the final size of the mucilage RG-I (Kong et al., 2011, 2013). Mutant seeds defective in these genes display reduced thickness of the extruded mucilage layer compared with wild-type Arabidopsis seeds.RG-I deposited in the apoplast of seed coat epidermal cells appears to be synthesized in a branched form that is subsequently modified by enzymes in the apoplast. MUM2 encodes a β-galactosidase that removes Gal residues from RG-I side chains (Dean et al., 2007; Macquet et al., 2007b). β-XYLOSIDASE1 encodes an α-l-arabinfuranosidase that removes Ara residues from RG-I side chains (Arsovski et al., 2009). Disruptions of these genes lead to defective hydration properties and affect the extrusion of mucilage. Furthermore, correct methylesterification of mucilage HG is also required for mucilage extrusion. HG is secreted into the wall in a high methylesterified form that can then be enzymatically demethylesterified by pectin methylesterases (PMEs; Bosch and Hepler, 2005). PECTIN METHYLESTERASE INHIBITOR6 (PMEI6) inhibits PME activities (Saez-Aguayo et al., 2013). The subtilisin-like Ser protease (SBT1.7) can activate other PME inhibitors, but not PMEI6 (Rautengarten et al., 2008; Saez-Aguayo et al., 2013). Disruption of either PMEI6 or SBT1.7 results in the delay of mucilage release.Although cellulose is present at low levels in adherent mucilage, it plays an important adhesive role for the attachment of mucilage pectin to the seed coat epidermal cells. The orientation and amount of pectin associated with the cellulose network is largely determined by cellulose conformation properties (Macquet et al., 2007a; Haughn and Western, 2012). Previous studies have demonstrated that CELLULOSE SYNTHASE A5 (CESA5) is required for the production of seed mucilage cellulose and the adherent mucilage in the cesa5 mutant can be easily extracted with water (Harpaz-Saad et al., 2011, 2012; Mendu et al., 2011; Sullivan et al., 2011).Despite all of these discoveries, large gaps remain in the current knowledge of the biosynthesis and functions of mucilage polysaccharides in seed coats. In this study, we show that CSLA2 is involved in the biosynthesis of mucilage glucomannan. Furthermore, we show that CSLA2 functions in the maintenance of the normal structure of the adherent mucilage layer through modifying the mucilage cellulose ultrastructure.     

MUCILAGE-RELATED10 Produces Galactoglucomannan That Maintains Pectin and Cellulose Architecture in Arabidopsis Seed Mucilage

Plants invest a lot of their resources into the production of an extracellular matrix built of polysaccharides. While the composition of the cell wall is relatively well characterized, the functions of the individual polymers and the enzymes that catalyze their biosynthesis remain poorly understood. We exploited the Arabidopsis (Arabidopsis thaliana) seed coat epidermis (SCE) to study cell wall synthesis. SCE cells produce mucilage, a specialized secondary wall that is rich in pectin, at a precise stage of development. A coexpression search for MUCILAGE-RELATED (MUCI) genes identified MUCI10 as a key determinant of mucilage properties. MUCI10 is closely related to a fenugreek (Trigonella foenumgraecum) enzyme that has in vitro galactomannan α-1,6-galactosyltransferase activity. Our detailed analysis of the muci10 mutants demonstrates that mucilage contains highly branched galactoglucomannan (GGM) rather than unbranched glucomannan. MUCI10 likely decorates glucomannan, synthesized by CELLULOSE SYNTHASE-LIKE A2, with galactose residues in vivo. The degree of galactosylation is essential for the synthesis of the GGM backbone, the structure of cellulose, mucilage density, as well as the adherence of pectin. We propose that GGM scaffolds control mucilage architecture along with cellulosic rays and show that Arabidopsis SCE cells represent an excellent model in which to study the synthesis and function of GGM. Arabidopsis natural varieties with defects similar to muci10 mutants may reveal additional genes involved in GGM synthesis. Since GGM is the most abundant hemicellulose in the secondary walls of gymnosperms, understanding its biosynthesis may facilitate improvements in the production of valuable commodities from softwoods.The plant cell wall is the key determinant of plant growth (Cosgrove, 2005) and represents the most abundant source of biopolymers on the planet (Pauly and Keegstra, 2010). Consequently, plants invest a lot of their resources into the production of this extracellular structure. Thus, it is not surprising that approximately 15% of Arabidopsis (Arabidopsis thaliana) genes are likely dedicated to the biosynthesis and modification of cell wall polymers (Carpita et al., 2001). Plant walls consist mainly of polysaccharides (cellulose, hemicellulose, and pectin) but also contain lignin and glycoproteins. While the biochemical structure of each wall component has been relatively well characterized, the molecular players involved in their biogenesis remain poorly understood (Keegstra, 2010). The functions of the individual polymers, and how they are assembled into a three-dimensional matrix, are also largely unknown (Burton et al., 2010; Burton and Fincher, 2012).Significant breakthroughs in cell wall research have been achieved through the examination of specialized plant tissues that contain elevated levels of a single polysaccharide (Pauly and Keegstra, 2010). Some species, particularly legumes, accumulate large amounts of the hemicellulose galactomannan during secondary wall thickening of the seed (Srivastava and Kapoor, 2005). Analysis of the developing fenugreek (Trigonella foenumgraecum) endosperm led to the purification of a GALACTOMANNAN GALACTOSYLTRANSFERASE (TfGMGT), the first glycosyltransferase (GT) whose activity in plant cell wall synthesis was demonstrated in vitro (Scheller and Ulvskov, 2010). TfGMGT catalyzes the decoration of mannan chains with single α-1,6-galactosyl residues (Edwards et al., 1999). A similar approach in guar (Cyamopsis tetragonoloba) seeds revealed that the β-1,4-linked mannan backbone is synthesized by a member of the CELLULOSE SYNTHASE-LIKE A (CSLA) protein family (Dhugga et al., 2004).Galactomannan functions as a storage polymer in the endosperm of the aforementioned seeds, analogous to starch in cereal grains (Dhugga et al., 2004), but it also has important rheological properties in the cell wall that have been exploited to produce valuable stabilizers and gelling agents for human consumption (Srivastava and Kapoor, 2005). The Man-to-Gal ratio is essential for the application of galactomannan gums in the food industry (Edwards et al., 1992). This is because unsubstituted mannan chains can interact via hydrogen bonds to produce crystalline microfibrils similar to cellulose (Millane and Hendrixson, 1994). Indeed, some algae that lack cellulose employ mannan fibrils as a structural material (Preston, 1968). The addition of Gal branches to the smooth, ribbon-like mannan chains creates hairy regions that limit self-association and promote gelation (Dea et al., 1977). All mannans are likely synthesized as highly substituted polymers that are trimmed in the cell wall (Scheller and Ulvskov, 2010).Generally, polysaccharides containing backbones of β-1,4-linked Man units can be classified as heteromannan (HM). Galactoglucomannan (GGM) is the main hemicellulose in gymnosperm secondary walls and, in contrast to galactomannan, has a backbone that contains both Glc and Man units (Pauly et al., 2013). HM is detected in most Arabidopsis cell types (Handford et al., 2003) and facilitates embryogenesis (Goubet et al., 2009), germination (Rodríguez-Gacio et al., 2012), tip growth (Bernal et al., 2008), and vascular development (Benová-Kákosová et al., 2006; Yin et al., 2011). In the last 10 years, in vitro mannan synthase activity has been demonstrated for recombinant CSLA proteins from many land plants (Liepman et al., 2005, 2007; Suzuki et al., 2006; Gille et al., 2011; Wang et al., 2012). HM synthesis may also involve CELLULOSE SYNTHASE-LIKE D (CSLD) enzymes and MANNAN SYNTHESIS-RELATED (MSR) accessory proteins (Yin et al., 2011; Wang et al., 2013), but their precise roles in relation to the CSLAs have not been established. Arabidopsis CSLA2, like most other isoforms, can use both GDP-Man and GDP-Glc as substrates in vitro (Liepman et al., 2005, 2007) and is responsible for stem glucomannan synthesis in vivo along with CSLA3 and CSLA7 (Goubet et al., 2009). CSLA2 also participates in the synthesis of glucomannan present in mucilage produced by seed coat epidermal (SCE) cells (Yu et al., 2014).Arabidopsis SCE cells represent an excellent genetic model in which to study the synthesis, polar secretion, and modification of polysaccharides, since these processes dominate a precise stage of seed coat development but are not essential for seed viability in laboratory conditions (Haughn and Western, 2012; North et al., 2014; Voiniciuc et al., 2015). Hydration of mature seeds in water releases a large gelatinous capsule, rich in the pectic polymer rhamnogalacturonan I, which can be easily stained or extracted (Macquet et al., 2007). Biochemical and cytological experiments indicate that Arabidopsis seed mucilage is more than just pectin and, in addition to cellulose, is likely to contain glycoproteins and at least two hemicellulosic polymers (Voiniciuc et al., 2015). There is mounting evidence that, despite their low abundance, these components play critical functions in seed mucilage architecture. The structure of homogalacturonan (HG), the major pectin in primary cell walls but a minor mucilage component, appears to be a key determinant of gelling properties and mucilage extrusion (Rautengarten et al., 2008; Saez-Aguayo et al., 2013; Voiniciuc et al., 2013). Mucilage attachment to seeds is maintained by the SALT OVERLY SENSITIVE5 glycoprotein and cellulose synthesized by multiple CELLULOSE SYNTHASE (CESA) isoforms (Harpaz-Saad et al., 2011; Mendu et al., 2011; Sullivan et al., 2011; Griffiths et al., 2014, 2015). From more than 35 genes that are reported to affect Arabidopsis seed mucilage properties (Voiniciuc et al., 2015), only CSLA2, CESA3, CESA5, GALACTURONOSYLTRANSFERASE11 (GAUT11; Caffall et al., 2009), and GAUT-LIKE5 (GATL5; Kong et al., 2013) are predicted to encode GTs. This highlights that, despite many detailed studies about mucilage production in SCE cells, the synthesis of its components remains poorly understood.To address this issue, we conducted a reverse genetic search for MUCILAGE-RELATED (MUCI) genes that may be required for polysaccharide biosynthesis. One of these, MUCI10, encodes a member of the Carbohydrate Active Enzymes family, GT34 (Lombard et al., 2014), which includes at least two enzymatic activities and seven Arabidopsis proteins (Keegstra and Cavalier, 2010). Five of them function as XYLOGLUCAN XYLOSYLTRANSFERASES (XXT1–XXT5) in vivo and/or in vitro (Faik et al., 2002; Cavalier et al., 2008; Vuttipongchaikij et al., 2012). MUCI10/GT7 (At2g22900) and its paralog GT6 (At4g37690) do not function as XXTs (Vuttipongchaikij et al., 2012) and are more closely related to the TfGMGT enzyme (Faik et al., 2002; Keegstra and Cavalier, 2010). MUCI10, also called GALACTOSYLTRANSFERASE-LIKE6 (GTL6), served as a Golgi marker in multiple proteomic studies of Arabidopsis callus cultures (Dunkley et al., 2004, 2006; Nikolovski et al., 2012, 2014). Nevertheless, the role of TfGMGT orthologs in Arabidopsis remained unknown. We show that MUCI10 is responsible for the extensive galactosylation of glucomannan in mucilage and influences glucomannan backbone synthesis, cellulose structure, and the distribution of pectin.     

Grain setting defect1, Encoding a Remorin Protein,Affects the Grain Setting in Rice through Regulating Plasmodesmatal Conductance

Effective grain filling is one of the key determinants of grain setting in rice (Oryza sativa). Grain setting defect1 (GSD1), which encodes a putative remorin protein, was found to affect grain setting in rice. Investigation of the phenotype of a transfer DNA insertion mutant (gsd1-Dominant) with enhanced GSD1 expression revealed abnormalities including a reduced grain setting rate, accumulation of carbohydrates in leaves, and lower soluble sugar content in the phloem exudates. GSD1 was found to be specifically expressed in the plasma membrane and plasmodesmata (PD) of phloem companion cells. Experimental evidence suggests that the phenotype of the gsd1-Dominant mutant is caused by defects in the grain-filling process as a result of the impaired transport of carbohydrates from the photosynthetic site to the phloem. GSD1 functioned in affecting PD conductance by interacting with rice ACTIN1 in association with the PD callose binding protein1. Together, our results suggest that GSD1 may play a role in regulating photoassimilate translocation through the symplastic pathway to impact grain setting in rice.Grain filling, a key determinant of grain yield in rice (Oryza sativa), hinges on the successful translocation of photoassimilates from the leaves to the fertilized reproductive organs through the phloem transport system. Symplastic phloem loading, which is one of the main pathways responsible for the transport of photoassimilates in rice, is mediated by plasmodesmata (PD) that connect phloem companion cells with sieve elements and surrounding parenchyma cells (Kaneko et al., 1980; Chonan et al., 1981; Eom et al., 2012). PD are transverse cell wall channels structured with the cytoplasmic sleeve and the modified endoplasmic reticulum desmotubule between neighboring cells (Maule, 2008). A number of proteins affect the structure and functional performance of the PD, which in turn impacts the cell-to-cell transport of small and large molecules through the PD during plant growth, development, and defense (Cilia and Jackson, 2004; Sagi et al., 2005; Lucas et al., 2009; Simpson et al., 2009; Stonebloom et al., 2009). For example, actin and myosin, which link the desmotubule to the plasma membrane (PM) at the neck region of PD, are believed to play a role in regulating PD permeability by controlling PD aperture (White et al., 1994; Ding et al., 1996; Reichelt et al., 1999). Callose deposition can also impact the size of the PD aperture at the neck region (Radford et al., 1998; Levy et al., 2007) and callose synthase genes such as Glucan Synthase-Like7 (GSL7, also named CalS7), GSL8, and GSL12 have been shown to play a role in regulating symplastic trafficking (Guseman et al., 2010; Barratt et al., 2011; Vatén et al., 2011; Xie et al., 2011). Other proteins that have been shown to impact the structure and function of the PD include glycosylphosphatidylinositol (GPI)-anchored proteins, PD callose binding protein1 (PDCB1), which is also associated with callose deposition (Simpson et al., 2009), and LYSIN MOTIF DOMAIN-CONTAINING GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEIN2, which limits the molecular flux through the PD by chitin perception (Faulkner et al., 2013). Changes in PD permeability can have major consequences for the translocation of photoassimilates needed for grain filling in rice. However, the genes and molecular mechanisms underlying the symplastic transport of photoassimilates remain poorly characterized.Remorins are a diverse family of plant-specific proteins with conserved C-terminal sequences and highly variable N-terminal sequences. Remorins can be classified into six distinct phylogenetic groups (Raffaele et al., 2007). The functions of most remorins are unknown, but some members of the family have been shown to be involved in immune response through controlling the cell-to-cell spread of microbes. StREM1.3, a remorin that is located in PM rafts and the PD, was shown to impair the cell-to-cell movement of a plant virus X by binding to Triple Gene Block protein1 (Raffaele et al., 2009). Medicago truncatula symbiotic remorin1 (MtSYMREM1), a remorin located at the PM in Medicago truncatula, was shown to facilitate infection and the release of rhizobial bacteria into the host cytoplasm (Lefebvre et al., 2010). Overexpression of LjSYMREM1, the ortholog of MtSYMREM1 in Lotus japonicus, resulted in increased root nodulation (Lefebvre et al., 2010; Tóth et al., 2012). Although a potential association between remorins and PD permeability has been proposed (Raffaele et al., 2009), the diversity observed across remorins, plus the fact that remorin mutants generated through different approaches fail to show obvious phenotypes (Reymond et al., 1996; Bariola et al., 2004), have made it challenging to characterize the function of remorins in cell-to-cell transport.In this study, we identified a rice transfer DNA (T-DNA) insertion mutant (grain setting defect1-Dominant [gsd1-D]), with a grain setting-deficient phenotype caused by overexpression of GSD1, a remorin gene with unknown function. GSD1 is expressed specifically in phloem companion cells and is localized in the PD and PM. We provide evidence to show that overexpression of GSD1 leads to deficient grain setting in rice, likely as a consequence of reduced sugar transport resulting from decreased PD permeability in phloem companion cells.     

The Arabidopsis Callose Synthase Gene GSL8 Is Required for Cytokinesis and Cell Patterning

Cytokinesis is the division of the cytoplasm and its separation into two daughter cells. Cell plate growth and cytokinesis appear to require callose, but direct functional evidence is still lacking. To determine the role of callose and its synthesis during cytokinesis, we identified and characterized mutants in many members of the GLUCAN SYNTHASE-LIKE (GSL; or CALLOSE SYNTHASE) gene family in Arabidopsis (Arabidopsis thaliana). Most gsl mutants (gsl1–gsl7, gsl9, gsl11, and gsl12) exhibited roughly normal seedling growth and development. However, mutations in GSL8, which were previously reported to be gametophytic lethal, were found to produce seedlings with pleiotropic defects during embryogenesis and early vegetative growth. We found cell wall stubs, two nuclei in one cell, and other defects in cell division in homozygous gsl8 insertional alleles. In addition, gsl8 mutants and inducible RNA interference lines of GSL8 showed reduced callose deposition at cell plates and/or new cell walls. Together, these data show that the GSL8 gene encodes a putative callose synthase required for cytokinesis and seedling maturation. In addition, gsl8 mutants disrupt cellular and tissue-level patterning, as shown by the presence of clusters of stomata in direct contact and by islands of excessive cell proliferation in the developing epidermis. Thus, GSL8 is required for patterning as well as cytokinesis during Arabidopsis development.Cytokinesis divides the cytoplasm of a plant cell by the deposition of plasma membrane and a cell wall during late mitosis. This process requires the phragmoplast, a dynamic, plant-specific cytoskeletal and membranous array, which delivers vesicles containing lipids, proteins, and cell wall components to the division plane to construct the cell plate. Cell plate formation involves several stages: initiation through vesicle fusion, the formation of a tubular-vesicular network, a transition to a solely tubular phase, and then further fusion to form a fenestrated sheet (Samuels et al., 1995). The outward growth of the cell plate leads to its fusion with the parental cell wall (Jürgens, 2005a, 2005b; Backues et al., 2007).Key regulators of cytokinesis include KNOLLE, KEULE, KORRIGAN, and HINKEL, which when defective induce pleiotropic phenotypes and seedling lethality (Lukowitz et al., 1996; Nicol et al., 1998; Zuo et al., 2000; Assaad et al., 2001; Strompen et al., 2002). KNOLLE, a syntaxin homolog, is required for the fusion of exocytic vesicles via a SNARE/SNAP33 complex (Lukowitz et al., 1996; Heese et al., 2001). KEULE, a homolog of yeast Sec1p, regulates syntaxin function by interacting with KNOLLE (Waizenegger et al., 2000; Assaad et al., 2001). KORRIGAN is an endo-1,4-β-glucanase required for cell wall biogenesis during cytokinesis (Zuo et al., 2000). And HINKEL is a kinesin-related protein required for the reorganization of phragmoplast microtubules during cytokinesis (Strompen et al., 2002).Additional regulators include Formin5, TWO-IN-ONE (TIO), and Arabidopsis (Arabidopsis thaliana) dynamin-like proteins (ADLs; Kang et al., 2001, 2003; Hong et al., 2003; Collings et al., 2005; Ingouff et al., 2005; Oh et al., 2005). Formin5 localizes to the cell plate and is an actin-organizing protein involved in cytokinesis and cell polarity. TIO, a Ser/Thr protein kinase, functions in cytokinesis in plant meristems and in gametogenesis (Oh et al., 2005). Members of the Arabidopsis DRP family associate with the developing cell plate, whereas DRP1a (ADL1A) locally constricts tubular membranes, interacts with callose synthase, and may facilitate callose deposition into the lumen.Callose, a β-1,3-glucan polymer with β-1,6-branches (Stone and Clarke, 1992), is synthesized in both sporophytic and gametophytic tissues and appears to play various roles. Callose accumulates at the cell plate during cytokinesis, in plasmodesmata, where it regulates cell-to-cell communication, and in dormant phloem, where it seals sieve plates after mechanical injury, pathogen attack, and metal toxicity (Stone and Clarke, 1992; Samuels et al., 1995; Lucas and Lee, 2004).Twelve GLUCAN SYNYHASE-LIKE (GSL) genes (also known as CALLOSE SYNTHASE [CalS]) have been identified in the Arabidopsis genome based on sequence homology (Richmond and Somerville, 2000; Hong et al., 2001; Enns et al., 2005). A GSL that functions in callose deposition after injury and pathogen treatment is GSL5 (Jacobs et al., 2003). Five other members of the Arabidopsis GSL family are required for microgametogenesis. GSL1 and GSL5 act redundantly to produce a callosic wall that prevents microspore degeneration, and both are needed for fertilization (Enns et al., 2005). GSL2 is required for the callosic wall around pollen mother cells, for the patterning of the pollen exine (Dong et al., 2005), and for callose deposition in the wall and plugs of pollen tubes (Nishikawa et al., 2005). GSL8 and GSL10 are independently required for the asymmetric division of microspores and for the entry of microspores into mitosis (Töller et al., 2008; Huang et al., 2009).Callose is a major component of the cell plate, especially during later plate development (Kakimoto and Shibaoka, 1992; Samuels et al., 1995; Hong et al., 2001). Callose appears to structurally reinforce the developing cell plate after the breakdown of the phragmoplast microtubule array and during plate consolidation (Samuels and Staehelin, 1996; Rensing et al., 2002). It is likely that callose is synthesized at the cell plate rather than in the endoplasmic reticulum and in the Golgi (Kakimoto and Shibaoka, 1988). GSL6 (CalS1) appears to be involved in callose synthesis at the cell plate, since a 35S∷GFP-GSL6 fusion in transgenic BY-2 tobacco (Nicotiana tabacum) cells increases callose accumulation, and GFP fluorescence was found specifically at the cell plate (Hong et al., 2001). However, functional and genetic data on the role of any GSL in Arabidopsis sporophytic cytokinesis are still lacking.Here, we report that GSL8 (CalS10) is required for normal cytokinesis. In addition, gsl8 mutants exhibit excessive cell proliferation and abnormal cell patterning, phenotypes not previously reported for cytokinesis-defective mutants.     

PLASTID MOVEMENT IMPAIRED1 and PLASTID MOVEMENT IMPAIRED1-RELATED1 Mediate Photorelocation Movements of Both Chloroplasts and Nuclei

Organelle movement and positioning play important roles in fundamental cellular activities and adaptive responses to environmental stress in plants. To optimize photosynthetic light utilization, chloroplasts move toward weak blue light (the accumulation response) and escape from strong blue light (the avoidance response). Nuclei also move in response to strong blue light by utilizing the light-induced movement of attached plastids in leaf cells. Blue light receptor phototropins and several factors for chloroplast photorelocation movement have been identified through molecular genetic analysis of Arabidopsis (Arabidopsis thaliana). PLASTID MOVEMENT IMPAIRED1 (PMI1) is a plant-specific C2-domain protein that is required for efficient chloroplast photorelocation movement. There are two PLASTID MOVEMENT IMPAIRED1-RELATED (PMIR) genes, PMIR1 and PMIR2, in the Arabidopsis genome. However, the mechanism in which PMI1 regulates chloroplast and nuclear photorelocation movements and the involvement of PMIR1 and PMIR2 in these organelle movements remained unknown. Here, we analyzed chloroplast and nuclear photorelocation movements in mutant lines of PMI1, PMIR1, and PMIR2. In mesophyll cells, the pmi1 single mutant showed severe defects in both chloroplast and nuclear photorelocation movements resulting from the impaired regulation of chloroplast-actin filaments. In pavement cells, pmi1 mutant plants were partially defective in both plastid and nuclear photorelocation movements, but pmi1pmir1 and pmi1pmir1pmir2 mutant lines lacked the blue light-induced movement responses of plastids and nuclei completely. These results indicated that PMI1 is essential for chloroplast and nuclear photorelocation movements in mesophyll cells and that both PMI1 and PMIR1 are indispensable for photorelocation movements of plastids and thus, nuclei in pavement cells.In plants, organelles move within the cell and become appropriately positioned to accomplish their functions and adapt to the environment (for review, see Wada and Suetsugu, 2004). Light-induced chloroplast movement (chloroplast photorelocation movement) is one of the best characterized organelle movements in plants (Suetsugu and Wada, 2012). Under weak light conditions, chloroplasts move toward light to capture light efficiently (the accumulation response; Zurzycki, 1955). Under strong light conditions, chloroplasts escape from light to avoid photodamage (the avoidance response; Kasahara et al., 2002; Sztatelman et al., 2010; Davis and Hangarter, 2012; Cazzaniga et al., 2013). In most green plant species, these responses are induced primarily by the blue light receptor phototropin (phot) in response to a range of wavelengths from UVA to blue light (approximately 320–500 nm; for review, see Suetsugu and Wada, 2012; Wada and Suetsugu, 2013; Kong and Wada, 2014). Phot-mediated chloroplast movement has been shown in land plants, such as Arabidopsis (Arabidopsis thaliana; Jarillo et al., 2001; Kagawa et al., 2001; Sakai et al., 2001), the fern Adiantum capillus-veneris (Kagawa et al., 2004), the moss Physcomitrella patens (Kasahara et al., 2004), and the liverwort Marchantia polymorpha (Komatsu et al., 2014). Two phots in Arabidopsis, phot1 and phot2, redundantly mediate the accumulation response (Sakai et al., 2001), whereas phot2 primarily regulates the avoidance response (Jarillo et al., 2001; Kagawa et al., 2001; Luesse et al., 2010). M. polymorpha has only one phot that mediates both the accumulation and avoidance responses (Komatsu et al., 2014), although two or more phots mediate chloroplast photorelocation movement in A. capillus-veneris (Kagawa et al., 2004) and P. patens (Kasahara et al., 2004). Thus, duplication and functional diversification of PHOT genes have occurred during land plant evolution, and plants have gained a sophisticated light sensing system for chloroplast photorelocation movement.In general, movements of plant organelles, including chloroplasts, are dependent on actin filaments (for review, see Wada and Suetsugu, 2004). Most organelles common in eukaryotes, such as mitochondria, peroxisomes, and Golgi bodies, use the myosin motor for their movements, but there is no clear evidence that chloroplast movement is myosin dependent (for review, see Suetsugu et al., 2010a). Land plants have innovated a novel actin-based motility system that is specialized for chloroplast movement as well as a photoreceptor system (for review, see Suetsugu et al., 2010a; Wada and Suetsugu, 2013; Kong and Wada, 2014). Chloroplast-actin (cp-actin) filaments, which were first found in Arabidopsis, are short actin filaments specifically localized around the chloroplast periphery at the interface between the chloroplast and the plasma membrane (Kadota et al., 2009). Strong blue light induces the rapid disappearance of cp-actin filaments and then, their subsequent reappearance preferentially at the front region of the moving chloroplasts. This asymmetric distribution of cp-actin filaments is essential for directional chloroplast movement (Kadota et al., 2009; Kong et al., 2013a). The greater the difference in the amount of cp-actin filaments between the front and rear regions of chloroplasts becomes, the faster the chloroplasts move, in which the magnitude of the difference is determined by fluence rate (Kagawa and Wada, 2004; Kadota et al., 2009; Kong et al., 2013a). Strong blue light-induced disappearance of cp-actin filaments is regulated in a phot2-dependent manner before the intensive polymerization of cp-actin filaments at the front region occurs (Kadota et al., 2009; Ichikawa et al., 2011; Kong et al., 2013a). This phot2-dependent response contributes to the greater difference in the amount of cp-actin filaments between the front and rear regions of chloroplasts. Similar behavior of cp-actin filaments has also been observed in A. capillus-veneris (Tsuboi and Wada, 2012) and P. patens (Yamashita et al., 2011).Like chloroplasts, nuclei also show light-mediated movement and positioning (nuclear photorelocation movement) in land plants (for review, see Higa et al., 2014b). In gametophytic cells of A. capillus-veneris, weak light induced the accumulation responses of both chloroplasts and nuclei, whereas strong light induced avoidance responses (Kagawa and Wada, 1993, 1995; Tsuboi et al., 2007). However, in mesophyll cells of Arabidopsis, strong blue light induced both chloroplast and nuclear avoidance responses, but weak blue light induced only the chloroplast accumulation response (Iwabuchi et al., 2007, 2010; Higa et al., 2014a). In Arabidopsis pavement cells, small numbers of tiny plastids were found and showed autofluorescence under the confocal laser-scanning microscopy (Iwabuchi et al., 2010; Higa et al., 2014a). Hereafter, the plastid in the pavement cells is called the pavement cell plastid. Strong blue light-induced avoidance responses of pavement cell plastids and nuclei were induced in a phot2-dependent manner, but the accumulation response was not detected for either organelle (Iwabuchi et al., 2007, 2010; Higa et al., 2014a). In both Arabidopsis and A. capillus-veneris, phots mediate nuclear photorelocation movement, and phot2 mediates the nuclear avoidance response (Iwabuchi et al., 2007, 2010; Tsuboi et al., 2007). The nuclear avoidance response is dependent on actin filaments in both mesophyll and pavement cells of Arabidopsis (Iwabuchi et al., 2010). Recently, it was shown that the nuclear avoidance response relies on cp-actin-dependent movement of pavement cell plastids, where nuclei are associated with pavement cell plastids of Arabidopsis (Higa et al., 2014a). In mesophyll cells, nuclear avoidance response is likely dependent on cp-actin filament-mediated chloroplast movement, because the mutants deficient in chloroplast movement were also defective in nuclear avoidance response (Higa et al., 2014a). Thus, phots mediate both chloroplast (and pavement cell plastid) and nuclear photorelocation movement by regulating cp-actin filaments.Molecular genetic analyses of Arabidopsis mutants deficient in chloroplast photorelocation movement have identified many molecular factors involved in signal transduction and/or motility systems as well as those involved in the photoreceptor system for chloroplast photorelocation movement (and thus, nuclear photorelocation movement; for review, see Suetsugu and Wada, 2012; Wada and Suetsugu, 2013; Kong and Wada, 2014). CHLOROPLAST UNUSUAL POSITIONING1 (CHUP1; Oikawa et al., 2003) and KINESIN-LIKE PROTEIN FOR ACTIN-BASED CHLOROPLAST MOVEMENT (KAC; Suetsugu et al., 2010b) are key factors for generating and/or maintaining cp-actin filaments. Both proteins are highly conserved in land plants and essential for the movement and attachment of chloroplasts to the plasma membrane in Arabidopsis (Oikawa et al., 2003, 2008; Suetsugu et al., 2010b), A. capillus-veneris (Suetsugu et al., 2012), and P. patens (Suetsugu et al., 2012; Usami et al., 2012). CHUP1 is localized on the chloroplast outer membrane and binds to globular and filamentous actins and profilin in vitro (Oikawa et al., 2003, 2008; Schmidt von Braun and Schleiff, 2008). Although KAC is a kinesin-like protein, it lacks microtubule-dependent motor activity but has filamentous actin binding activity (Suetsugu et al., 2010b). An actin-bundling protein THRUMIN1 (THRUM1) is required for efficient chloroplast photorelocation movement (Whippo et al., 2011) and interacts with cp-actin filaments (Kong et al., 2013a). chup1 and kac mutant plants were shown to lack detectable cp-actin filaments (Kadota et al., 2009; Suetsugu et al., 2010b; Ichikawa et al., 2011; Kong et al., 2013a). Similarly, cp-actin filaments were rarely detected in thrum1 mutant plants (Kong et al., 2013a), indicating that THRUM1 also plays an important role in maintaining cp-actin filaments.Other proteins J-DOMAIN PROTEIN REQUIRED FOR CHLOROPLAST ACCUMULATION RESPONSE1 (JAC1; Suetsugu et al., 2005), WEAK CHLOROPLAST MOVEMENT UNDER BLUE LIGHT1 (WEB1; Kodama et al., 2010), and PLASTID MOVEMENT IMPAIRED2 (PMI2; Luesse et al., 2006; Kodama et al., 2010) are involved in the light regulation of cp-actin filaments and chloroplast photorelocation movement. JAC1 is an auxilin-like J-domain protein that mediates the chloroplast accumulation response through its J-domain function (Suetsugu et al., 2005; Takano et al., 2010). WEB1 and PMI2 are coiled-coil proteins that interact with each other (Kodama et al., 2010). Although web1 and pmi2 were partially defective in the avoidance response, the jac1 mutation completely suppressed the phenotype of web1 and pmi2, suggesting that the WEB1/PMI2 complex suppresses JAC1 function (i.e. the accumulation response) under strong light conditions (Kodama et al., 2010). Both web1 and pmi2 showed impaired disappearance of cp-actin filaments in response to strong blue light (Kodama et al., 2010). However, the exact molecular functions of these proteins are unknown.In this study, we characterized mutant plants deficient in the PMI1 gene and two homologous genes PLASTID MOVEMENT IMPAIRED1-RELATED1 (PMIR1) and PMIR2. PMI1 was identified through molecular genetic analyses of pmi1 mutants that showed severe defects in chloroplast accumulation and avoidance responses (DeBlasio et al., 2005). PMI1 is a plant-specific C2-domain protein (DeBlasio et al., 2005; Zhang and Aravind, 2010), but its roles and those of PMIRs in cp-actin-mediated chloroplast and nuclear photorelocation movements remained unclear. Thus, we analyzed chloroplast and nuclear photorelocation movements in the single, double, and triple mutants of pmi1, pmir1, and pmir2.     

Endosidin 7 Specifically Arrests Late Cytokinesis and Inhibits Callose Biosynthesis,Revealing Distinct Trafficking Events during Cell Plate Maturation

Although cytokinesis is vital for plant growth and development, our mechanistic understanding of the highly regulated membrane and cargo transport mechanisms in relation to polysaccharide deposition during this process is limited. Here, we present an in-depth characterization of the small molecule endosidin 7 (ES7) inhibiting callose synthase activity and arresting late cytokinesis both in vitro and in vivo in Arabidopsis (Arabidopsis thaliana). ES7 is a specific inhibitor for plant callose deposition during cytokinesis that does not affect endomembrane trafficking during interphase or cytoskeletal organization. The specificity of ES7 was demonstrated (1) by comparing its action with that of known inhibitors such as caffeine, flufenacet, and concanamycin A and (2) across kingdoms with a comparison in yeast. The interplay between cell plate-specific post-Golgi vesicle traffic and callose accumulation was analyzed using ES7, and it revealed unique and temporal contributions of secretory and endosomal vesicles in cell plate maturation. While RABA2A-labeled vesicles, which accumulate at the early stage of cell plate formation, were not affected by ES7, KNOLLE was differentially altered by the small molecule. In addition, the presence of clathrin-coated vesicles in cells containing elevated levels of callose and their reduction under ES7 treatment further support the role of endocytic membrane remodeling in the maturing cell plate while the plate is stabilized by callose. Taken together, these data show the essential role of callose during the late stages of cell plate maturation and establish the temporal relationship between vesicles and regulatory proteins at the cell plate assembly matrix during polysaccharide deposition.During plant cytokinesis, the de novo formation of a new cell wall partitions the cytoplasm of the dividing cell (Staehelin and Hepler, 1996; Jürgens, 2005). The formation of the transient cell plate structure is a complex multistep process (Samuels et al., 1995; Jürgens, 2005). At the end of late anaphase, vesicle delivery is guided by the phragmoplast to the center of the dividing cell, the cell plate assembly matrix (CPAM; Samuels et al., 1995). Vesicles at the CPAM undergo homotypic fusion and fission, contributing to the formation of the incipient cell plate (Jürgens, 2005). The initial vesicular fusion and fission events (fusion of Golgi-derived vesicles stage [FVS]) lead to the formation of a tubulovesicular network (TVN), which undergoes a morphological change to form a tubular network (TN). Callose deposition starts during this stage (Supplemental Fig. S1), which is thought to provide mechanical support to the membrane network that ultimately results in the planar fenestrated sheet (PFS). The cell plate expands centrifugally by the accumulation and fusion of newly arriving vesicles at its leading edge. This process is accompanied by the accumulation of new polysaccharides and the removal of excess material maturing at the center. Separation of the daughter cells concludes by fusion of the cell plate with the parental plasma membrane (Samuels et al., 1995).A vast amount of proteins including those involved in vesicle trafficking participate in cell plate formation (McMichael and Bednarek, 2013). Vesicle fusion with the target membrane is mediated by the formation of Soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor (SNARE) complexes (Bassham and Blatt, 2008). The well-characterized SNARE complex at the cell plate comprises the Q-SNARE KNOLLE and the functionally redundant R-SNARES, the vesicle-associated membrane proteins VAMP721 and VAMP722 (Lauber et al., 1997; Zhang et al., 2011; El Kasmi et al., 2013). The SEC1/Munc18 protein KEULLE, the Soluble N-ethylmaleimide-sensitive factor adaptor protein33, and the novel plant-specific SNARE11 (Assaad et al., 2001; Heese et al., 2001; Zheng et al., 2002) play a role in this SNARE complex formation. Of all the SNAREs required for vesicle fusion at the cell plate, only KNOLLE has been shown to function exclusively in cytokinesis.The formation of the cell plate requires specific amounts of vesicle-delivered membrane and other secretory products. The GTPase RABA2A is necessary for the delivery of trans-Golgi network (TGN)-derived vesicles to the cell plate leading edge (Chow et al., 2008). However, due to the excess delivery of material arriving at the cell plate formation site, it is estimated that 70% is recycled (Samuels et al., 1995; Otegui et al., 2001). Electron microscopy observations indicate the role of clathrin-coated vesicles (CCVs) in the removal and/or recycling of excess membranes from the cell plate (Samuels et al., 1995; Otegui and Staehelin, 2004; Seguí-Simarro et al., 2004). Specifically, clathrin light chain (CLC), dynamin-related proteins (DRPs), the adaptin-like TPLATE, and AP180 amino-terminal homology/epsin amino-terminal homology domain-containing protein have been identified at the cell plate, providing evidence that clathrin-mediated endocytosis facilitates this membrane recycling (Konopka et al., 2008; Konopka and Bednarek, 2008; Fujimoto et al., 2010; Van Damme et al., 2011; Ito et al., 2012; Song et al., 2012; McMichael and Bednarek, 2013). In addition, it has been suggested that plasma membrane endocytosis contributes material toward de novo cell plate formation (Dhonukshe et al., 2006). However, the level of endocytosis involvement remains questionable, as pharmacological inhibition of endocytosis does not interfere with cytokinesis (Reichardt et al., 2007). The temporal association of different vesicle populations at the CPAM might provide further insights into their contribution to the forming cell plate.Despite the large number of studies investigating membrane dynamics, relatively few studies exist on polysaccharide deposition during cell plate maturation. It has been suggested that callose, a (1,3)-β-glucan, stabilizes the delicate tubular network during the initial cell plate formation stage, until the deposition of additional polysaccharides increases its rigidity (Samuels et al., 1995). Callose accumulation is transient, with the polymer being removed once other polysaccharides such as hemicelluloses, pectins, and cellulose are deposited at the cell plate (Supplemental Fig. S1; Samuels et al., 1995; Albersheim et al., 2010). The timing of callose deposition at the cell plate in relation to that of vesicle trafficking that contributes to cell plate formation is unknown.Genetic studies have indicated a role of callose accumulation at the cell plate (Chen et al., 2009; Thiele et al., 2009; Guseman et al., 2010). However, the lethality of mutant alleles for the callose synthase/glucan synthase-like family (GSL) has hampered the detailed examination of the role of callose synthase and its product in cell plate maturation (Verma and Hong, 2001; Chen et al., 2009; Thiele et al., 2009; Guseman et al., 2010). The ability to transiently perturb callose deposition at the cell plate is key to understanding callose’s contribution to the separation of the daughter cells compared with other polysaccharides.Here, we used pharmacological inhibitors to overcome the challenges of the lethality of callose synthase mutants. In a high-throughput confocal microscopy-based screen for small molecules affecting endosomal trafficking (Drakakaki et al., 2011), endosidin 7 (ES7) was identified as an inhibitor of cell plate formation. ES7 induces characteristic cell plate gaps, observable by the mislocalization of KNOLLE and RABA2A, while it does not affect the localization of endomembrane compartment markers in interphase cells. The potential of ES7 to inhibit callose deposition at the cell plate (Drakakaki et al., 2011) provides avenues to study cell plate maturation. We have characterized the activity of ES7 using both in vitro and in vivo studies establishing its inhibitory effects on callose biosynthesis. We have exploited the properties of ES7 to characterize in detail callose deposition at the cell plate, thereby providing further insight into the overall cell plate formation process. Our results conclusively show that callose is essential for the later stages of cell plate maturation and lay out the temporal association and interplay of TGN and endosomal vesicles during polysaccharide deposition.