Hypoxia: A novel function for VIN3

VERNALIZATION INSENSITIVE 3 (VIN3) encodes a PHD domain chromatin remodelling protein that is induced in response to cold and is required for the establishment of the vernalization response in Arabidopsis thaliana.¹ Vernalization is the acquisition of the competence to flower after exposure to prolonged low temperatures, which in Arabidopsis is associated with the epigenetic repression of the floral repressor FLOWERING LOCUS C (FLC).^2,3 During vernalization VIN3 binds to the chromatin of the FLC locus,¹ and interacts with conserved components of Polycomb-group Repressive Complex 2 (PRC2).^4,5 This complex catalyses the tri-methylation of histone H3 lysine 27 (H3K27me3),^4,6,7 a repressive chromatin mark that increases at the FLC locus as a result of vernalization.^4,7–10 In our recent paper¹¹ we found that VIN3 is also induced by hypoxic conditions, and as is the case with low temperatures, induction occurs in a quantitative manner. Our experiments indicated that VIN3 is required for the survival of Arabidopsis seedlings exposed to low oxygen conditions. We suggested that the function of VIN3 during low oxygen conditions is likely to involve the mediation of chromatin modifications at certain loci that help the survival of Arabidopsis in response to prolonged hypoxia. Here we discuss the implications of our observations and hypotheses in terms of epigenetic mechanisms controlling gene regulation in response to hypoxia.Key words: arabidopsis, VIN3, FLC, hypoxia, vernalization, chromatin remodelling, survival     

The TRPA1 channel and oral hypoglycemic agents

Diabetes mellitus type 2 (DM2) results from the combination of insulin unresponsiveness in target tissues and the failure of pancreatic β cells to secrete enough insulin.¹ It is a highly prevalent chronic disease that is aggravated with time, leading to major complications, such as cardiovascular disease and peripheral and ocular neuropathies.² Interestingly, therapies to improve glucose homeostasis in diabetic patients usually involve the use of glibenclamide, an oral hypoglycemic drug that blocks ATP-sensitive K⁺ channels (K_ATP),^3,4 forcing β cells to release more insulin to overcome peripheral insulin resistance. However, sulfonylureas are ineffective for long-term treatments and ultimately result in the administration of insulin to control glucose levels.⁵ The mechanisms underlying β-cell failure to respond effectively with glibenclamide after long-term treatments still needs clarification. A recent study demonstrating that this drug activates TRPA1,⁶ a member of the Transient Receptor Potential (TRP) family of ion channels and a functional protein in insulin secreting cells,^7,8 has highlighted a possible role for TRPA1 as a potential mediator of sulfonylurea-induced toxicity.     

RALFs: Peptide regulators of plant growth

Peptide signaling regulates a variety of developmental processes and environmental responses in plants.^1–6 For example, the peptide systemin induces the systemic defense response in tomato⁷ and defensins are small cysteine-rich proteins that are involved in the innate immune system of plants.^8,9 The CLAVATA3 peptide regulates meristem size¹⁰ and the SCR peptide is the pollen self-incompatibility recognition factor in the Brassicaceae.^11,12 LURE peptides produced by synergid cells attract pollen tubes to the embryo sac.⁹ RALFs are a recently discovered family of plant peptides that play a role in plant cell growth.Key words: peptide, growth factor, alkalinization     

Why have chloroplasts developed a unique motility system?

Organelle movement in plants is dependent on actin filaments with most of the organelles being transported along the actin cables by class XI myosins. Although chloroplast movement is also actin filament-dependent, a potential role of myosin motors in this process is poorly understood. Interestingly, chloroplasts can move in any direction and change the direction within short time periods, suggesting that chloroplasts use the newly formed actin filaments rather than preexisting actin cables. Furthermore, the data on myosin gene knockouts and knockdowns in Arabidopsis and tobacco do not support myosins'' XI role in chloroplast movement. Our recent studies revealed that chloroplast movement and positioning are mediated by the short actin filaments localized at chloroplast periphery (cp-actin filaments) rather than cytoplasmic actin cables. The accumulation of cp-actin filaments depends on kinesin-like proteins, KAC1 and KAC2, as well as on a chloroplast outer membrane protein CHUP1. We propose that plants evolved a myosin XI-independent mechanism of the actin-based chloroplast movement that is distinct from the mechanism used by other organelles.Key words: actin, Arabidopsis, blue light, kinesin, myosin, organelle movement, phototropinOrganelle movement and positioning are pivotal aspects of the intracellular dynamics in most eukaryotes. Although plants are sessile organisms, their organelles are quickly repositioned in response to fluctuating environmental conditions and certain endogenous signals. By and large, plant organelle movements and positioning are dependent on actin filaments, although microtubules play certain accessory roles in organelle dynamics.^1,2 Actin inhibitors effectively retard the movements of mitochondria,^3–6 peroxisomes,^5,7–11 Golgi stacks,^12,13 endoplasmic reticulum (ER),^14,15 and nuclei.^16–18 These organelles are co-aligned and associated with actin filaments.^{5,7,8,10–12,15,18} Recent progress in this field started to reveal the molecular motility system responsible for the organelle transport in plants.¹⁹Chloroplast movement is among the most fascinating models of organelle movement in plants because it is precisely controlled by ambient light conditions.^20,21 Weak light induces chloroplast accumulation response so that chloroplasts can capture photosynthetic light efficiently (Fig. 1A). Strong light induces chloroplast avoidance response to escape from photodamage (Fig. 1B).²² The blue light-induced chloroplast movement is mediated by the blue light receptor phototropin (phot). In some cryptogam plants, the red light-induced chloroplast movement is regulated by a chimeric phytochrome/phototropin photoreceptor neochrome.^23–25 In a model plant Arabidopsis, phot1 and phot2 function redundantly to regulate the accumulation response,²⁶ whereas phot2 alone is essential for the avoidance response.^27,28 Several additional factors regulating chloroplast movement were identified by analyses of Arabidopsis mutants deficient in chloroplast photorelocation.^29–32 In particular, identification of CHUP1 (chloroplast unusual positioning 1) revealed the connection between chloroplasts and actin filaments at the molecular level.²⁹ CHUP1 is a chloroplast outer membrane protein capable of interacting with F-actin, G-actin and profilin in vitro.^29,33,34 The chup1 mutant plants are defective in both the chloroplast movement and chloroplast anchorage to the plasma membrane,^22,29,33 suggesting that CHUP1 plays an important role in linking chloroplasts to the plasma membrane through the actin filaments. However, how chloroplasts move using the actin filaments and whether chloroplast movement utilizes the actin-based motility system similar to other organelle movements remained to be determined.Open in a separate windowFigure 1Schematic distribution patterns of chloroplasts in a palisade cell under different light conditions, weak (A) and strong (B) lights. Shown as a side view of mid-part of the cell and a top view with three different levels (i.e., top, middle and bottom of the cell). The cell was irradiated from the leaf surface shown as arrows. Weak light induces chloroplast accumulation response (A) and strong light induces the avoidance response (B).Here, we review the recent findings pointing to existence of a novel actin-based mechanisms for chloroplast movement and discuss the differences between the mechanism responsible for movement of chloroplasts and other organelles.     

Plant defensins: Defense,development and application

Plant defensins are small, highly stable, cysteine-rich peptides that constitute a part of the innate immune system primarily directed against fungal pathogens. Biological activities reported for plant defensins include antifungal activity, antibacterial activity, proteinase inhibitory activity and insect amylase inhibitory activity. Plant defensins have been shown to inhibit infectious diseases of humans and to induce apoptosis in a human pathogen. Transgenic plants overexpressing defensins are strongly resistant to fungal pathogens. Based on recent studies, some plant defensins are not merely toxic to microbes but also have roles in regulating plant growth and development.Key words: defensin, antifungal, antimicrobial peptide, development, innate immunityDefensins are diverse members of a large family of cationic host defence peptides (HDP), widely distributed throughout the plant and animal kingdoms.^1–3 Defensins and defensin-like peptides are functionally diverse, disrupting microbial membranes and acting as ligands for cellular recognition and signaling.⁴ In the early 1990s, the first members of the family of plant defensins were isolated from wheat and barley grains.^5,6 Those proteins were originally called γ-thionins because their size (∼5 kDa, 45 to 54 amino acids) and cysteine content (typically 4, 6 or 8 cysteine residues) were found to be similar to the thionins.⁷ Subsequent “γ-thionins” homologous proteins were indentified and cDNAs were cloned from various monocot or dicot seeds.⁸ Terras and his colleagues⁹ isolated two antifungal peptides, Rs-AFP1 and Rs-AFP2, noticed that the plant peptides'' structural and functional properties resemble those of insect and mammalian defensins, and therefore termed the family of peptides “plant defensins” in 1995. Sequences of more than 80 different plant defensin genes from different plant species were analyzed.¹⁰ A query of the UniProt database (www.uniprot.org/) currently reveals publications of 371 plant defensins available for review. The Arabidopsis genome alone contains more than 300 defensin-like (DEFL) peptides, 78% of which have a cysteine-stabilized α-helix β-sheet (CSαβ) motif common to plant and invertebrate defensins.¹¹ In addition, over 1,000 DEFL genes have been identified from plant EST projects.¹²Unlike the insect and mammalian defensins, which are mainly active against bacteria,^2,3,10,13 plant defensins, with a few exceptions, do not have antibacterial activity.¹⁴ Most plant defensins are involved in defense against a broad range of fungi.^2,3,10,15 They are not only active against phytopathogenic fungi (such as Fusarium culmorum and Botrytis cinerea), but also against baker''s yeast and human pathogenic fungi (such as Candida albicans).² Plant defensins have also been shown to inhibit the growth of roots and root hairs in Arabidopsis thaliana¹⁶ and alter growth of various tomato organs which can assume multiple functions related to defense and development.⁴     

Matrix metalloproteinase (MMP) and TGFβ1-stimulated cell migration in skin and cornea wound healing

Cell migration during wound healing is a complex process that involves the expression of a number of growth factors and cytokines. One of these factors, transforming growth factor-beta (TGFβ) controls many aspects of normal and pathological cell behavior. It induces migration of keratinocytes in wounded skin and of epithelial cells in damaged cornea. Furthermore, this TGFβ-induced cell migration is correlated with the production of components of the extracellular matrix (ECM) proteins and expression of integrins and matrix metalloproteinases (MMPs). MMP digests ECMs and integrins during cell migration, but the mechanisms regulating their expression and the consequences of their induction remain unclear. It has been suggested that MMP-14 activates cellular signaling processes involved in the expression of MMPs and other molecules associated with cell migration. Because of the manifold effects of MMP-14, it is important to understand the roles of MMP-14 not only the cleavage of ECM but also in the activation of signaling pathways.Key words: wound healing, migration, matrix metalloproteinase, transforming growth factor, skin, corneaWound healing is a well-ordered but complex process involving many cellular activities including inflammation, growth factor or cytokine secretion, cell migration and proliferation. Migration of skin keratinocytes and corneal epithelial cells requires the coordinated expression of various growth factors such as platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), transforming growth factor (TGF), keratinocyte growth factor (KGF), hepatocyte growth factor (HGF), insulin-like growth factor (IGF), epidermal growth factor (EGF), small GTPases, and macrophage stimulating protein (reviewed in refs. ¹ and ²). The epithelial cells in turn regulate the expression of matrix metalloproteinases (MMPs), extracellular matrix (ECM) proteins and integrins during cell migration.^1,3,4 TGF-β is a well-known cytokine involved in processes such as cell growth inhibition, embryogenesis, morphogenesis, tumorigenesis, differentiation, wound healing, senescence and apoptosis (reviewed in refs. ⁵ and ⁶). It is also one of the most important cytokines responsible for promoting the migration of skin keratinocytes and corneal epithelial cells.^3,6,7TGFβ has two quite different effects on skin keratinocytes: it suppresses their multiplication and promotes their migration. The TGFβ-induced cell growth inhibition is usually mediated by Smad signaling, which upregulates expression of the cell cycle inhibitor p21^WAF1/Cip1 or p12^CDK2-AP1 in HaCaT skin keratinocyte cells and human primary foreskin keratinocytes.^8,9 Keratinocyte migration in wounded skin is associated with strong expression of TGFβ and MMPs,¹ and TGFβ stimulates the migration of manually scratched wounded HaCaT cells.¹⁰ TGFβ also induces cell migration and inhibits proliferation of injured corneal epithelial cells, whereas it stimulates proliferation of normal corneal epithelial cells via effects on the MAPK family and Smad signaling.^2,7 Indeed, skin keratinocytes and corneal epithelial cells display the same two physiological responses to TGFβ during wound healing; cell migration and growth inhibition. However as mentioned above, TGFβ has a different effect on normal cells. For example, it induces the epithelial to mesenchymal transition (EMT) of normal mammary cells and lens epithelial cells.^11,12 It also promotes the differentiation of corneal epithelial cells, and induces the fibrosis of various tissues.^2,6The MMPs are a family of structurally related zinc-dependent endopeptidases that are secreted into the extracellular environment.¹³ Members of the MMP family have been classified into gelatinases, stromelysins, collagenases and membrane type-MMPs (MT-MMPs) depending on their substrate specificity and structural properties. Like TGFβ, MMPs influence normal physiological processes including wound healing, tissue remodeling, angiogenesis and embryonic development, as well as pathological conditions such as rheumatoid arthritis, atherosclerosis and tumor invasion.^13,14The expression patterns of MMPs during skin and cornea wound healing are well studied. In rats, MMP-2, -3, -9, -11, -13 and -14 are expressed,¹⁵ and in mice, MMP-1, -2, -3, -9, -10 and -14 are expressed during skin wound healing.¹ MMP-1, -3, -7 and -12 are increased in corneal epithelial cells during Wnt 7a-induced rat cornea wound healing.¹⁶ Wound repair after excimer laser keratectomy is characterized by increased expression of MMP-1, -2, -3 and -9 in the rabbit cornea, and MMP-2, -9 in the rat cornea.^17,18 The expression of MMP-2 and -9 during skin keratinocyte and corneal epithelial cell migration has been the most thoroughly investigated, and it has been shown that their expression generally depends on the activity of MMP-14. MMP-14 (MT1-MMP) is constitutively anchored to the cell membrane; it activates other MMPs such as MMP-2, and also cleaves various types of ECM molecules including collagens, laminins, fibronectin as well as its ligands, the integrins.¹³ The latent forms of some cytokines are also cleaved and activated by MMP-14.¹⁹ Overexpression of MMP-14 protein was found to stimulate HT1080 human fibrosarcoma cell migration.²⁰ In contrast, the attenuation of MMP-14 expression using siRNA method decreased fibroblast invasiveness,²¹ angiogenesis of human microvascular endothelial cells,²² and human skin keratinocyte migration.¹⁰ The latter effect was shown to result from lowering MMP-9 expression. Other studies have shown that EGF has a critical role in MMP-9 expression during keratinocyte tumorigenesis and migration.^23,24 On the other hand, TGFβ modulates MMP-9 production through the Ras/MAPK pathway in transformed mouse keratinocytes and NFκB induces cell migration by binding to the MMP-9 promoter in human skin primary cultures.^25,26 Enhanced levels of pro-MMP-9 and active MMP-9 have also been noted in scratched corneal epithelia of diabetic rats.²⁷There is evidence that MMP-14 activates a number of intracellular signaling pathways including the MAPK family pathway, focal adhesion kinase (FAK), Src family, Rac and CD44, during cell migration and tumor invasion.^19,20,28 In COS-7 cells, ERK activation is stimulated by overexpression of MMP-14 and is essential for cell migration.²⁹ These observations all indicate that MMP-14 plays an important role in cell migration, not only by regulating the activity or expression of downstream MMPs but also by processing and activating migration-associated molecules such as integrins, ECMs and a variety of intracellular signaling pathays.³⁰Cell migration during wound healing is a remarkably complex phenomenon. TGFβ is just one small component of the overall process of wound healing and yet it triggers a multitude of reactions needed for cell migration. It is important to know what kinds of molecules are expressed when cell migration is initiated, but it is equally important to investigate the roles of these molecules and how their expression is regulated. Despite the availability of some information about how MMPs and signaling molecules can influence each other, much remains to be discovered in this area. It will be especially important to clarify how MMP-14 influences other signaling pathways since its role in cell migration is not restricted to digesting ECM molecules but also includes direct or indirect activation of cellular signaling pathways.     

Cell Migration from Baby to Mother

Fetal cells migrate into the mother during pregnancy. Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades. Microchimeric fetal cells are found in various maternal tissues and organs including blood, bone marrow, skin and liver. In mice, fetal cells have also been found in the brain. The fetal cells also appear to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants. Further understanding of the ability of fetal cells to cross both the placental and blood-brain barriers, to migrate into diverse tissues, and to differentiate into multiple cell types may also advance strategies for intravenous transplantation of stem cells for cytotherapeutic repair. Here we discuss hypotheses for how fetal cells cross the placental and blood-brain barriers and the persistence and distribution of fetal cells in the mother.Key Words: fetomaternal microchimerism, stem cells, progenitor cells, placental barrier, blood-brain barrier, adhesion, migrationMicrochimerism is the presence of a small population of genetically distinct and separately derived cells within an individual. This commonly occurs following transfusion or transplantation.^1–3 Microchimerism can also occur between mother and fetus. Small numbers of cells traffic across the placenta during pregnancy. This exchange occurs both from the fetus to the mother (fetomaternal)^4–7 and from the mother to the fetus.^8–10 Similar exchange may also occur between monochorionic twins in utero.^11–13 There is increasing evidence that fetomaternal microchimerism persists lifelong in many child-bearing women.^7,14 The significance of fetomaternal microchimerism remains unclear. It could be that fetomaternal microchimerism is an epiphenomenon of pregnancy. Alternatively, it could be a mechanism by which the fetus ensures maternal fitness in order to enhance its own chances of survival. In either case, the occurrence of pregnancy-acquired microchimerism in women may have implications for graft survival and autoimmunity. More detailed understanding of the biology of microchimeric fetal cells may also advance progress towards cytotherapeutic repair via intravenous transplantation of stem or progenitor cells.Trophoblasts were the first zygote-derived cell type found to cross into the mother. In 1893, Schmorl reported the appearance of trophoblasts in the maternal pulmonary vasculature.¹⁵ Later, trophoblasts were also observed in the maternal circulation.^16–20 Subsequently various other fetal cell types derived from fetal blood were also found in the maternal circulation.^21,22 These fetal cell types included lymphocytes,²³ erythroblasts or nucleated red blood cells,^24,25 haematopoietic progenitors^7,26,27 and putative mesenchymal progenitors.^14,28 While it has been suggested that small numbers of fetal cells traffic across the placenta in every human pregnancy,^29–31 trophoblast release does not appear to occur in all pregnancies.³² Likewise, in mice, fetal cells have also been reported in maternal blood.^33,34 In the mouse, fetomaternal transfer also appears to occur during all pregnancies.³⁵     

Tuba and N-WASP function cooperatively to position the central lumen during epithelial cyst morphogenesis

The process of epithelial lumenogenesis requires coordination of a network of signaling machinery communicated to each cell through subsequent cell divisions. Formation of a single hollow lumen has previously been shown to require Tuba, a Cdc42 GEF, for Cdc42 activation and correct spindle orientation. Using a Caco-2 model of lumenogenesis, we show that knockdown (KD) of the actin regulator N-WASP, causes a multilumen phenotype similar to Tuba KD. Defects in lumenogenesis in Tuba KD and N-WASP KD cells are observed at the two-cell stage with inappropriate marking of the pre-apical patch (PAP )—the precursor to lumen formation. Strikingly, both Tuba and N-WASP depend on each other for localization to the PAP. We conclude that N-WASP functions cooperatively with Tuba to facilitate lumenogenesis and this requires the polyproline region of N-WASP.Key words: lumen, N-WASP, tuba, E-cadherin, pre-apical patchMany epithelial tissues are organized as hollow tubes whose open lumina connect the body with its external environment.^1,2 These tubes consist of a monolayer of polarized cells that envelope the central lumen. Lumen formation is thus a key process in epithelial morphogenesis that depends upon cell polarity to establish three cell surface domains: a basal surface adherent to the extracellular matrix, a lateral surface between cells, and an apical surface that is exposed to the luminal fluids. Of note, the apical membrane is biochemically and morphologically distinct from the baso-lateral surfaces and effectively defines the luminal surface.^3,4For a lumen to form, cells must first mark the site at which apical membrane is to be inserted, something that is achieved at the first cell division.⁵ Targeted trafficking of apical membrane constituents defines a pre-apical patch (PAP), the precursor to the definitive lumen.⁵ Such insertion of apical membrane must presumably be coordinated with the assembly of apical junctions to segregate nascent apical from lateral membrane domains.² Subsequent cell divisions direct apical membrane and protein constituents to this point of initial apical membrane placement.⁶ Coordinated luminal positioning enables the initial formation of a single hollow lumen that subsequently expands through polarized fluid secretion to separate apical membranes, such as occurs in the embryonic gastrointestinal tract,⁷ or by apoptosis or autophagy of the central cells as is observed in mammary gland development.^8,9 Failure to establish initial luminal positioning causes defective lumenogenesis, often resulting in multiple, morphologically abnormal lumina.^5,6Crucial to lumenal morphogenesis is then the mechanism(s) that mark the site where the PAP will form. Cdc42 signaling is increasingly implicated in this process,^2,10 with downstream consequences that include control of mitotic spindle orientation,⁵ which itself influences PAP placement⁵ and potentially regulation of cell-cell junctions. Like other Rho family GTPases, the subcellular location of Cdc42 signaling is determined by the action of upstream proteins, notably guanine nucleotide exchange factors (GEFs).^11,12 Of these, Tuba, a Cdc42-specific GEF,¹³ has emerged as a regulator of lumenal morphogenesis that controls PAP placement through mitotic spindle orientation.¹⁰Tuba is also a scaffolding protein¹³ capable of linking the actin assembly machinery with trafficking pathways. Not only is Tuba required for Cdc42 activation to direct spindle orientation,⁵ it also has the potential to interact with phosphoinositides that define the PAP.¹⁴ Additionally, Tuba binds directly to the actin regulator N-WASP, a key molecule in the organization of actin and itself a Cdc42 effector.¹⁵ Further, Tuba and N-WASP cooperate in various forms of actin-driven cellular motility, such as vesicle propulsion and cell invasive behavior.¹⁶ Interestingly, in epithelial cells N-WASP is also found at cadherin-based cell-cell junctions.¹⁷ In fact it has been proposed that N-WASP functions downstream of Tuba in the maintenance of epithelial junctional homeostasis as N-WASP overexpression was capable of rescuing a Tuba KD phenotype.¹⁸ Therefore, Tuba has the potential to play a central role in coordinating the molecular complexes required for productive polarization of epithelial cells and placement of the PAP during lumenogenesis. However, whether other protein interactions contribute to the morphogenetic impact of Tuba remain to be assessed.Three-dimensional cell culture systems are being utilized to identify critical components in lumen formation. In particular, Madin-Darby canine kidney cells (MDCK) and Caco-2 gastrointestinal cells are commonly used to study cyst and/or tubule formation. MDCK cells undergo both cyst and tubule growth, apoptosis being primarily responsible for the final step in lumen formation,¹⁹ while Caco-2 cells primarily utilize fluid influx to expand cysts.⁵ Cyst culture systems replicate aspects of in vivo organogenesis²⁰ providing tangible, powerful models to analyze and dissect the coordinated cellular mechanisms and processes that occur during epithelial morphogenesis.In this study we examined the relationship between Tuba and N-WASP in early epithelial lumenogenesis using Caco-2 three dimensional cyst cultures. Both Tuba and N-WASP RNAi cell lines result in mature cysts with multiple lumina, and at the two-cell stage, formed multiple PAPs. Interestingly, N-WASP KD perturbed Tuba localization at the PAP, however, N-WASP localization to the PAP was not affected to the same extent by Tuba KD. Taken together, these results suggest a complex interrelationship between Tuba and N-WASP for the coordinated formation of a single hollow lumen.     

Strigolactones: Internal and external signals in plant symbioses?

As the newest plant hormone, strigolactone research is undergoing an exciting expansion. In less than five years, roles for strigolactones have been defined in shoot branching, secondary growth, root growth and nodulation, to add to the growing understanding of their role in arbuscular mycorrhizae and parasitic weed interactions.¹ Strigolactones are particularly fascinating as signaling molecules as they can act both inside the plant as an endogenous hormone and in the soil as a rhizosphere signal.^2-4 Our recent research has highlighted such a dual role for strigolactones, potentially acting as both an endogenous and exogenous signal for arbuscular mycorrhizal development.⁵ There is also significant interest in examining strigolactones as putative regulators of responses to environmental stimuli, especially the response to nutrient availability, given the strong regulation of strigolactone production by nitrate and phosphate observed in many species.^5,6 In particular, the potential for strigolactones to mediate the ecologically important response of mycorrhizal colonization to phosphate has been widely discussed. However, using a mutant approach we found that strigolactones are not essential for phosphate regulation of mycorrhizal colonization or nodulation.⁵ This is consistent with the relatively mild impairment of phosphate control of seedling root growth observed in Arabidopsis strigolactone mutants.⁷ This contrasts with the major role for strigolactones in phosphate control of shoot branching of rice and Arabidopsis^8,9 and indicates that the integration of strigolactones into our understanding of nutrient response will be complex. New data presented here, along with the recent discovery of phosphate specific CLE peptides,¹⁰ indicates a potential role for PsNARK, a component of the autoregulation of nodulation pathway, in phosphate control of nodulation.     

Nitric oxide (NO)-dependent and NO-independent signaling pathways act in ABA-inhibition of stomatal opening

We investigated the role of nitric oxide (NO) in ABA-inhibition of stomatal opening in Vicia faba L. in different size dishes. When a large dish (9 cm diameter) was used, ABA induced NO synthesis and the NO scavenger reduced ABA-inhibition of stomatal opening. When a small dish (6 cm diameter) was used, ABA induced stomatal closure and inhibited stomatal opening. The NO scavenger was able to reduce ABA-induced stomatal closure, but unable to reverse ABA-inhibition of stomatal opening. Furthermore, NO was not synthesized in response to ABA, indicating that NO is not required for ABA-inhibition of stomatal opening in the small dish. These results indicated that an NO-dependent and an NO-independent signaling pathway participate in ABA signaling pathway. An NO-dependent pathway is the major player in ABA-induced stomatal closure. However, in ABA-inhibition of stomatal opening, an NO-dependent and an NO-independent pathway act: different signaling molecules participate in ABA-signaling cascade under different environmental condition.Key words: ABA, environmental condition, nitric oxide, stomata, Vicia faba LNitric oxide (NO) is a key signaling molecule in plants.^1,2 It functions in disease resistance and programmed cell death,^3,4 root development,^5,6 and plant responses to various abiotic stresses.^1,2,7,8 In addition, NO is required for stomatal closure in response to ABA in several species including Arabidopsis, Vicia faba, pea, tomato, barley, and wheat.^9–11 ABA-inhibition of stomatal opening is a distinct process from ABA-induced stomatal closure.^12,13 In V. faba, these two processes employ a similar signaling pathway; NO is also a second messenger molecule for ABA-inhibition of stomatal opening in a large dish.¹⁴ In this study, we examined the role of NO in ABA-inhibition of stomatal opening using different dish sizes. In a small dish, NO is not involved in ABA-inhibition of stomatal opening: the NO-independent signaling pathway is the major player in it.     

Complete Genome Sequence of Croceibacter atlanticus HTCC2559T

Here we announce the complete genome sequence of Croceibacter atlanticus HTCC2559^T, which was isolated by high-throughput dilution-to-extinction culturing from the Bermuda Atlantic Time Series station in the Western Sargasso Sea. Strain HTCC2559^T contained genes for carotenoid biosynthesis, flavonoid biosynthesis, and several macromolecule-degrading enzymes. The genome confirmed physiological observations of cultivated Croceibacter atlanticus strain HTCC2559^T, which identified it as an obligate chemoheterotroph.The phylum Bacteroidetes comprises 6 to ∼30% of total bacterial communities in the ocean by fluorescence in situ hybridization (8-10). Most marine Bacteroidetes are in the family Flavobacteriaceae, most of which are aerobic respiratory heterotrophs that form a well-defined clade by 16S rRNA phylogenetic analyses (4). The members of this family are well known for degrading macromolecules, including chitin, DNA, cellulose, starch, and pectin (17), suggesting their environmental roles as detritus decomposers in the ocean (6). Marine Polaribacter and Dokdonia species in the Flavobacteriaceae have also shown to have photoheterotrophic metabolism mediated by proteorhodopsins (11, 12).Several strains of the family Flavobacteriaceae were isolated from the Sargasso Sea and Oregon coast, using high-throughput culturing approaches (7). Croceibacter atlanticus HTCC2559^T was cultivated from seawater collected at a depth of 250 m from the Sargasso Sea and was identified as a new genus in the family Flavobacteriaceae based on its 16S rRNA gene sequence similarities (6). Strain HTCC2559^T met the minimal standards for genera of the family Flavobacteriaceae (3) on the basis of phenotypic characteristics (6).Here we report the complete genome sequence of Croceibacter atlanticus HTCC2559^T. The genome sequencing was initiated by the J. Craig Venter Institute as a part of the Moore Foundation Microbial Genome Sequencing Project and completed in the current announcement. Gaps among contigs were closed by Genotech Co., Ltd. (Daejeon, Korea), using direct sequencing of combinatorial PCR products (16). The HTCC2559^T genome was analyzed with a genome annotation system based on GenDB (14) at Oregon State University and with the NCBI Prokaryotic Genomes Automatic Annotation Pipeline (15, 16).The HTCC2559^T genome is 2,952,962 bp long, with 33.9 mol% G+C content, and there was no evidence of plasmids. The number of protein-coding genes was 2,715; there were two copies of the 16S-23S-5S rRNA operon and 36 tRNA genes. The HTCC2559^T genome contained genes for a complete tricarboxylic acid cycle, glycolysis, and a pentose phosphate pathway. The genome also contained sets of genes for metabolic enzymes involved in carotenoid biosynthesis and also a serine/glycine hydroxymethyltransferase, which is often associated with the assimilatory serine cycle (13). The potential for HTCC2559^T to use bacterial type III polyketide synthase (PKS) needs to be confirmed because this organism had a naringenin-chalcone synthase (CHS) or chalcone synthase (EC 2.3.1.74), a key enzyme in flavonoid biosynthesis. CHS initiates the addition of three molecules of malonyl coenzyme A (malonyl-CoA) to a starter CoA ester (e.g., 4-coumaroyl-CoA) (1) and takes part in a few bacterial type III polyketide synthase systems (1, 2, 5, 18).The complete genome sequence confirmed that strain HTCC2559^T is an obligate chemoheterotroph because no genes for phototrophy were found. As expected from physiological characteristics (6), the HTCC2559^T genome contained a set of genes coding for enzymes required to degrade high-molecular-weight compounds, including peptidases, metallo-/serine proteases, pectinase, alginate lyases, and α-amylase.     

Novel protein-protein interaction family proteins involved in chloroplast movement response

To optimize photosynthetic activity, chloroplasts change their intracellular location in response to ambient light conditions; chloroplasts move toward low intensity light to maximize light capture and away from high intensity light to avoid photodamage. Although several proteins have been reported to be involved in chloroplast photorelocation movement response, any physical interaction among them was not found so far. We recently found a physical interaction between two plant-specific coiled-coil proteins, WEB1 (Weak Chloroplast Movement under Blue Light 1) and PMI2 (Plastid Movement Impaired 2), that were indentified to regulate chloroplast movement velocity. Since the both coiled-coil regions of WEB1 and PMI2 were classified into an uncharacterized protein family having DUF827 (DUF: Domain of Unknown Function) domain, it was the first report that DUF827 proteins could mediate protein-protein interaction. In this mini-review article, we discuss regarding molecular function of WEB1 and PMI2, and also define a novel protein family composed of WEB1, PMI2 and WEB1/PMI2-like proteins for protein-protein interaction in land plants.Key words: Arabidopsis, blue light, chloroplast velocity, coiled-coil region, organelle movement, phototropin, protein-protein interactionIntracellular locations of chloroplasts change in response to different light conditions to capture sunlight efficiently for energy production through photosynthesis. Chloroplasts move toward weak light to maximize light capture (the accumulation response),^1,2 and away from strong light to reduce photodamage (the avoidance response).³ In higher plants such as Arabidopsis thaliana, the responses are induced by blue light-dependent manner.^1,2 Recently, chloroplast actin (cp-actin) filaments were found to be involved in chloroplast photorelocation movement and positioning.^4,5 The cp-actin filaments are localized at the interface between the chloroplast and the plasma membrane to anchor the chloroplast to the plasma membrane, and are relocalized to the leading edge of chloroplasts before and during the movement.^4,5 The difference of cp-actin filament amounts between the front and the rear halves of chloroplasts determines the chloroplast movement velocity; as the difference increases, chloroplast velocity also increases.^4,5Several proteins have been reported to be involved in chloroplast movement. The blue light receptors, phototropin 1 (phot1) and phot2, mediate the accumulation response,⁶ and phot2 solely mediates the avoidance response.^7,8 Chloroplast Unusual Positioning 1 (CHUP1), Kinesin-like Protein for Actin-Based Chloroplast Movement 1 (KAC1) and KAC2 are involved in the cp-actin filament formation.^4,9–11 Other proteins with unknown molecular function involved in the chloroplast movement responses have also been reported. They are J-domain Protein Required for Chloroplast Accumulation Response 1 (JAC1),^12,13 Plastid Movement Impaired 1 (PMI1),¹⁴ a long coiled-coil protein Plastid Movement Impaired 2 (PMI2), a PMI2-homologous protein PMI15,¹⁵ and THRUMIN1.¹⁶Recently, we characterized two plant-specific coiled-coil proteins, Weak Chloroplast Movement under Blue Light 1 (WEB1) and PMI2, which regulate the velocity of chloroplast photorelocation movement.¹⁷ In this mini-review article, we discuss about molecular function of WEB1 and PMI2 in chloroplast photorelocation movement, and also define the WEB1/PMI2-related (WPR) protein family as a new protein family for protein-protein interaction.