The ULTRAPETALA gene controls shoot and floral meristem size in Arabidopsis

The regulation of proper shoot and floral meristem size during plant development is mediated by a complex interaction of stem cell promoting and restricting factors. The phenotypic effects of mutations in the ULTRAPETALA gene, which is required to control shoot and floral meristem cell accumulation in Arabidopsis thaliana, are described. ultrapetala flowers contain more floral organs and whorls than wild-type plants, phenotypes that correlate with an increase in floral meristem size preceding organ initiation. ultrapetala plants also produce more floral meristems than wild-type plants, correlating with an increase in inflorescence meristem size without visible fasciation. Expression analysis indicates that ULTRAPETALA controls meristem cell accumulation partly by limiting the domain of CLAVATA1 expression. Genetic studies show that ULTRAPETALA acts independently of ERA1, but has overlapping functions with PERIANTHIA and the CLAVATA signal transduction pathway in controlling shoot and floral meristem size and meristem determinacy. Thus ULTRAPETALA defines a novel locus that restricts meristem cell accumulation in Arabidopsis shoot and floral meristems.     

Hormones talking: Does hormonal cross-talk shape the Arabidopsis gynoecium?

The proper development of fruits is important for the sexual reproduction and propagation of many plant species. The fruit of Arabidopsis derives from the fertilized gynoecium, which initiates at the center of the flower and obtains its final shape, size, and functional tissues through progressive stages of development. Hormones, specially auxins, play important roles in gynoecium and fruit patterning. Cytokinins, which act as counterparts to auxins in other plant tissues, have been studied more in the context of ovule formation and parthenocarpy. We recently studied the role of cytokinins in gynoecium and fruit patterning and found that they have more than one role during gynoecium and fruit patterning. We also compared the cytokinin response localization to the auxin response localization in these organs, and studied the effects of spraying cytokinins in young flowers of an auxin response line. In this addendum, we discuss further the implications of the observed results in the knowledge about the relationship between cytokinins and auxins at the gynoecium.     

The WOX13 homeobox gene promotes replum formation in the Arabidopsis thaliana fruit

The Arabidopsis fruit forms a seedpod that develops from the fertilized gynoecium. It is mainly comprised of an ovary in which three distinct tissues can be differentiated: the valves, the valve margins and the replum. Separation of cells at the valve margin allows for the valves to detach from the replum and thus dispersal of the seeds. Valves and valve margins are located in lateral positions whereas the replum is positioned medially and retains meristematic properties resembling the shoot apical meristem (SAM). Members of the WUSCHEL‐related homeobox family have been involved in stem cell maintenance in the SAM, and within this family, we found that WOX13 is expressed mainly in meristematic tissues including the replum. We also show that wox13 loss‐of‐function mutations reduce replum size and enhance the phenotypes of mutants affected in the replum identity gene RPL. Conversely, misexpression of WOX13 produces, independently from BP and RPL, an oversized replum and valve defects that closely resemble those of mutants in JAG/FIL activity genes. Our results suggest that WOX13 promotes replum development by likely preventing the activity of the JAG/FIL genes in medial tissues. This regulation seems to play a role in establishing the gradient of JAG/FIL activity along the medio‐lateral axis of the fruit critical for proper patterning. Our data have allowed us to incorporate the role of WOX13 into the regulatory network that orchestrates fruit patterning.     

barren inflorescence2 regulates axillary meristem development in the maize inflorescence

Organogenesis in plants is controlled by meristems. Shoot apical meristems form at the apex of the plant and produce leaf primordia on their flanks. Axillary meristems, which form in the axils of leaf primordia, give rise to branches and flowers and therefore play a critical role in plant architecture and reproduction. To understand how axillary meristems are initiated and maintained, we characterized the barren inflorescence2 mutant, which affects axillary meristems in the maize inflorescence. Scanning electron microscopy, histology and RNA in situ hybridization using knotted1 as a marker for meristematic tissue show that barren inflorescence2 mutants make fewer branches owing to a defect in branch meristem initiation. The construction of the double mutant between barren inflorescence2 and tasselsheath reveals that the function of barren inflorescence2 is specific to the formation of branch meristems rather than bract leaf primordia. Normal maize inflorescences sequentially produce three types of axillary meristem: branch meristem, spikelet meristem and floral meristem. Introgression of the barren inflorescence2 mutant into genetic backgrounds in which the phenotype was weaker illustrates additional roles of barren inflorescence2 in these axillary meristems. Branch, spikelet and floral meristems that form in these lines are defective, resulting in the production of fewer floral structures. Because the defects involve the number of organs produced at each stage of development, we conclude that barren inflorescence2 is required for maintenance of all types of axillary meristem in the inflorescence. This defect allows us to infer the sequence of events that takes place during maize inflorescence development. Furthermore, the defect in branch meristem formation provides insight into the role of knotted1 and barren inflorescence2 in axillary meristem initiation.     

Axillary meristem development in the branchless Zu-0 ecotype of Arabidopsis thaliana

Axillary meristems form in the leaf axils during post-embryonic development. In order to initiate the genetic dissection of axillary meristem development, we have characterized the late-flowering branchless ecotype of Arabidopsis thaliana (L.) Heynh., Zu-0. The first-formed rosette leaves of Zu-0 plants all initiate axillary meristems, but later-formed leaves of the rosette remain branchless. Alteration in the meristem development is axillary meristem-specific because the shoot apical and floral meristems develop normally. Scanning electron microscopy, histology and RNA in situ analysis with SHOOTMERISTEMLESS ( STM), a marker for meristematic tissues, show that a mound of cells form and STM mRNA accumulates in barren leaf axils, indicating that axillary meristems initiate but arrest in their development prior to organizing a meristem proper. Expression and retention of the STM RNA in barren leaf axils further suggests that STM expression is not sufficient for the establishment of the axillary meristem proper.     

Comparative ontogeny of perfect and pistillate florets in Senecio vernalis (Asteraceae)

We studied the inflorescence, and in particular ontogeny and development of the florets in Senecio vernalis as a representative member of Asteraceae, using epi-illumination microscopy. Initiation and subsequent development of florets on the highly convex inflorescence apex occur acropetally, except for pistillate ray florets, which show a lag in initiation. Receptacular bracts derive from the receptacular surface after development of all florets. The order of whorl initiation in both disc and ray florets include corolla, androecium and finally the pappus, together with the gynoecium. Development of corolla lobes from a ring meristem occurs in bidirectional order starting from the lateral side, whereas stamens incept unidirectionally from the abaxial side. Concurrently with the inception of two median carpel primordia, a ring meristem develops at the base of the corolla from which pappus bristles differentiate in later stages. Pistillate ray florets show significant differences from perfect disc florets as reflected by the zygomorphic shape of the floral apex and a shift of floral merosity from pentamery to tetramery. Loss of stamens in ray florets occurs due to abortion of primordia after initiation.     

Early flower development in Arabidopsis.

The early development of the flower of Arabidopsis thaliana is described from initiation until the opening of the bud. The morphogenesis, growth rate, and surface structure of floral organs were recorded in detail using scanning electron microscopy. Flower development has been divided into 12 stages using a series of landmark events. Stage 1 begins with the initiation of a floral buttress on the flank of the apical meristem. Stage 2 commences when the flower primordium becomes separate from the meristem. Sepal primordia then arise (stage 3) and grow to overlie the primordium (stage 4). Petal and stamen primordia appear next (stage 5) and are soon enclosed by the sepals (stage 6). During stage 6, petal primordia grow slowly, whereas stamen primordia enlarge more rapidly. Stage 7 begins when the medial stamens become stalked. These soon develop locules (stage 8). A long stage 9 then commences with the petal primordia becoming stalked. During this stage all organs lengthen rapidly. This includes the gynoecium, which commences growth as an open-ended tube during stage 6. When the petals reach the length of the lateral stamens, stage 10 begins. Stigmatic papillae appear soon after (stage 11), and the petals rapidly reach the height of the medial stamens (stage 12). This final stage ends when the 1-millimeter-long bud opens. Under our growing conditions 1.9 buds were initiated per day on average, and they took 13.25 days to progress through the 12 stages from initiation until opening.     

Arabidopsis (Brassicaceae) flower development and gynoecium patterning in wild type and Ettin mutants

Screening for mutations that alter flower development in Arabidopsis has led to the identification of two general types of genetic loci: those affecting meristem and organ identity, and those affecting growth and development independent of identity. ettin (ett) mutants belong to the latter class and exhibit pleiotropic phenotypes distinct from previously described Arabidopsis mutants. These phenotypes include increases in sepal and petal number, decreases in stamen number and anther locule number, and gross alteration of tissue patterning in the gynoecium. To determine when and how differences in ett floral meristems originate, flower development was compared between the wild type and ett mutants. ett floral meristems exhibit increases in abaxial sepal and petal primordia number without apparent increases in meristem size. Extra sepal and petal primordia develop into normal organs. In contrast, stamen and carpel primordia exhibit alterations in shape and form, subsequent to premature elongation of the terminal floral meristem. Phenotypes are allele-strength dependent. The stigma develops precociously and style differentiation is basally and abaxially misplaced in ett gynoecia. The data are discussed in the context of a model suggesting that two concentric boundaries specify the apical-basal pattern of gynoecium differentiation.     

Polar Auxin Transport Is Essential for Medial versus Lateral Tissue Specification and Vascular-Mediated Valve Outgrowth in Arabidopsis Gynoecia

Although it is generally accepted that auxin is important for the patterning of the female reproductive organ, the gynoecium, the flow as well as the temporal and spatial actions of auxin have been difficult to show during early gynoecial development. The primordium of the Arabidopsis (Arabidopsis thaliana) gynoecium is composed of two congenitally fused, laterally positioned carpel primordia bisected by two medially positioned meristematic regions that give rise to apical and internal tissues, including the ovules. This organization makes the gynoecium one of the most complex plant structures, and as such, the regulation of its development has remained largely elusive. By determining the spatiotemporal expression of auxin response reporters and localization of PINFORMED (PIN) auxin efflux carriers, we have been able to create a map of the auxin flow during the earliest stages of gynoecial primordium initiation and outgrowth. We show that transient disruption of polar auxin transport (PAT) results in ectopic auxin responses, broadened expression domains of medial tissue markers, and disturbed lateral preprocambium initiation. Based on these results, we propose a new model of auxin-mediated gynoecial patterning, suggesting that valve outgrowth depends on PIN1-mediated lateral auxin maxima as well as subsequent internal auxin drainage and provascular formation, whereas the growth of the medial domains is less dependent on correct PAT. In addition, PAT is required to prevent the lateral domains, at least in the apical portion of the gynoecial primordium, from obtaining medial fates.The gynoecium is a highly complex assembly comprised of different tissues that work together to support female reproductive competence in angiosperms. As such, studies of the regulatory networks controlling gynoecial development are essential to not only understand plant reproduction, but also increase our knowledge about intertissue-specific cross talk and coordinated development. A gynoecium is composed of one or more carpels, which may have evolved by the invagination of an ancestral leaf-like structure carrying spores along its edges (for review, see Hawkins and Liu, 2014). The Arabidopsis (Arabidopsis thaliana) gynoecium is a bilateral structure composed of two congenitally fused carpels likely derived from the fusion of two leaf-like structures in which the central domains became the lateral valves and the peripheral meristematic margins carrying the ovules became the medial tissues (Hawkins and Liu, 2014). It has been suggested that the medial domains of the Arabidopsis gynoecial primordium are partially differentiated quasi-meristems with maintained meristematic characteristics allowing for prolonged proliferation (Girin et al., 2009). Accordingly, many lateral domain-specific genes are associated with leaf development, while several genes active in the medial domains are related to meristematic activity (Dinneny et al., 2005; Alonso-Cantabrana et al., 2007; González-Reig et al., 2012).Arabidopsis gynoecium development has been described and reviewed extensively (Sessions, 1997; Bowman et al., 1999; Ferrándiz et al., 1999; Balanzá et al., 2006; Østergaard, 2009; Sundberg and Ferrandíz, 2009) and is summarized in Figure 1. Briefly, at early floral stage 5 (stages according to Smyth et al., 1990), after the initiation of outer floral organs, the remaining floral meristem becomes dome shaped. Although the meristem still appears radially symmetric, it is considered to have a medial plane (black dashed lines in Fig. 1) facing the inflorescence meristem and a lateral plane (white dashed lines in Fig. 1) perpendicular to the medial plane. The terminal floral meristem subsequently broadens in the lateral plane, resulting in a bilateral flattened plate (late floral stage 5). At floral stage 6, differential growth has resulted in a central invagination positioned along the lateral plane, and differential gene expressions indicate that initial patterning events distinguishing medial and lateral domains as well as inner (adaxial) and outer (abaxial) tissues have initiated (Bowman et al., 1999). By the end of floral stage 7, the adaxial medial tissues grow toward each other, forming two medial ridges, also called carpel margin meristems (CMMs). The CMMs will give rise to placentae and subsequently ovule primordia at floral stage 8 (Schneitz et al., 1995). By stage 9, the major tissue types of the mature gynoecium become morphologically distinct as the style and stigmatic papillae start to differentiate. Cell differentiation and cell expansion continue during stages 10 to 12, and the gynoecium is fully mature and ready to accept pollen approximately 10 d after it started to initiate from the terminal floral meristem.Open in a separate windowFigure 1.Arabidopsis gynoecium development. Transmitted light confocal images of the remaining floral meristem (stage [st] 5) and the first stages of gynoecial primordia development (stages 6 and 7), and false-colored DIC images of floral stages 8 to 12 gynoecia along a developmental time scale showing the time in days after floral initiation at the end of each stage. Early and late stage 5 as well as upper images at floral stages 6 and 7 are viewed from above. Lower floral stage 6 image is viewed from the lateral side. Lower images of floral stages 7 to 12 gynoecia are viewed from the medial side. Upper images of floral stages 8 to 12 gynoecia show transverse sections. Stages and time scale are adapted after Smyth et al. (1990) and Sessions (1997). White dashed lines indicate lateral plane, black dashed lines indicate medial plane, arrowheads indicate lateral crease, and asterisks indicate CMM. Bars = 10 µm (stages 5–7), 25 µm (stages 8–10), and 50 µm (stages 11 and 12).It is commonly accepted that the plant hormone auxin is important for gynoecium development, and several models have been put forward to explain this on a mechanistic level (Nemhauser et al., 2000; Østergaard, 2009; Sundberg and Østergaard, 2009; Nole-Wilson et al., 2010; Marsch-Martínez et al., 2012; Hawkins and Liu, 2014). However, we still lack a clear picture of the auxin dynamics and response sites during the earliest developmental stages when the major patterning decisions are made. During lateral organ development, instructive auxin peaks or gradients are formed by site-specific auxin biosynthesis and polar auxin transport (PAT), which results in procambium formation, organ outgrowth, and tissue differentiation (Sachs, 1969; Benková et al., 2003; Mattsson et al., 2003; Heisler et al., 2005; Scarpella et al., 2006; Furutani et al., 2014). The plasma membrane-bound PINFORMED (PIN) proteins as well as at least four members of the ATP-binding cassette subfamily B (ABCB)/MULTI-DRUG RESISTANT/P-GLYCOPROTEIN (PGP) protein family show auxin efflux capacity (for review, see Habets and Offringa, 2014). The PIN proteins are often polarly localized at the plasma membrane, whereas the ABCB/PGP proteins are generally localized apolarly. Therefore, the PINs are largely responsible for the net directional flow of auxin, while the ABCB proteins most likely contribute to PAT by regulating the effective cellular auxin available for polar transport (Mravec et al., 2008; Wang et al., 2013). The phytotropin 1-N-naphtylphthalamic acid (NPA) is a well established and widely used PAT inhibitor, although its exact mode of action is obscure (Petrásek et al., 2003). NPA treatment mimics the pin-like shoot phenotype of pin1 loss-of-function mutants (Okada et al., 1991), and even though NPA appears not to directly interact with PIN proteins, it may influence subcellular dynamics and has been shown to bind to ABCB family members, thereby blocking their transport capacity (Noh et al., 2001; Murphy et al., 2002; Geisler et al., 2003; Nagashima et al., 2008; Kim et al., 2010). This suggests that NPA may reduce PAT in part by restricting the amount of auxin available for PIN-mediated polar transport.Although the pin1-1 knockout mutant rarely produces flowers (Okada et al., 1991), gynoecia of the hypomorphic pin1-5 mutant form elongated styles and reduced or even missing carpels (Bennett et al., 1995; Sohlberg et al., 2006). Auxin biosynthesis mutants also produce disproportionate gynoecial tissues (Cheng et al., 2006; Stepanova et al., 2008), suggesting that auxin peaks and fluxes are important for the coordinated development of gynoecial domains. However, because the gynoecium is the last organ to initiate from the floral meristem, the abnormal gynoecial development in auxin-related mutants may result from developmental defects that occurred prior to gynoecium formation. By transiently treating inflorescences with NPA, Nemhauser et al. (2000) showed that PAT in the gynoecial primordia is important for differential development. However, the whole gynoecium was regarded as one entity with apical-basal polarity, and the possibility that the lateral carpels and the medial meristematic tissues could respond differently to the treatment was never discussed. Thus, where and how NPA affects PAT-regulated development has remained elusive.To understand how local auxin activities influence the outgrowth and patterning events of young gynoecial primordia, we determined the localization of PIN and PGP auxin efflux proteins and the resulting auxin response domains. This allowed us to map the directional flow and auxin response peaks during the earliest stages of gynoecium development. In addition, we induced transient disruptions in PAT and assessed the response of auxin signaling and domain-specific markers to establish how auxin signaling and vascular, lateral, and medial domains are affected by alterations in PAT. Based on our data, we propose a new model for auxin-regulated gynoecial patterning in which the medial versus lateral identity is dependent on correct auxin localization, and subsequent carpel valve outgrowth is dependent on transport-mediated apical auxin drainage.     

Cold stress signalling in female reproductive tissues

Cold stress is a significant threat for plant productivity and impacts on plant distribution and crop production, particularly so when it occurs during the growth phase. A developmental stage at risk is that of flowering, since a single stress event during sensitive stages, such as the full‐bloom stage of fruit trees can be fatal for reproductive success. Although pollen development and fertilization are widely viewed as the most critical reproductive phases, the development and function of female reproductive tissues, which in Angiosperms are embedded in the gynoecium, are also affected by cold stress. Today however, we have essentially no understanding of the cold stress response pathways that act during floral organogenesis. In this review, we briefly summarize our current knowledge of cold stress signalling modules active in vegetative tissues that may provide a framework of general principles also transferable to female reproductive tissues. We then align these signalling cascades with those that govern gynoecium development to identify factors that may act in both processes and could thereby contribute to cold stress responses in female reproductive tissues.