Arabidopsis membrane steroid binding protein 1 is involved in inhibition of cell elongation

A putative Membrane Steroid Binding Protein (designated MSBP1) was identified and functionally characterized as a negative regulator of cell elongation in Arabidopsis thaliana. The MSBP1 gene encodes a 220-amino acid protein that can bind to progesterone, 5-dihydrotestosterone, 24-epi-brassinolide (24-eBL), and stigmasterol with different affinities in vitro. Transgenic plants overexpressing MSBP1 showed short hypocotyl phenotype and increased steroid binding capacity in membrane fractions, whereas antisense MSBP1 transgenic plants showed long hypocotyl phenotypes and reduced steroid binding capacity, indicating that MSBP1 negatively regulates hypocotyl elongation. The reduced cell elongation of MSBP1-overexpressing plants was correlated with altered expression of genes involved in cell elongation, such as expansins and extensins, indicating that enhanced MSBP1 affected a regulatory pathway for cell elongation. Suppression or overexpression of MSBP1 resulted in enhanced or reduced sensitivities, respectively, to exogenous progesterone and 24-eBL, suggesting a negative role of MSBP1 in steroid signaling. Expression of MSBP1 in hypocotyls is suppressed by darkness and activated by light, suggesting that MSBP1, as a negative regulator of cell elongation, plays a role in plant photomorphogenesis. This study demonstrates the functional roles of a steroid binding protein in growth regulation in higher plants.     

Ectopic expression of carpel-specific MADS box genes from lily and lisianthus causes similar homeotic conversion of sepal and petal in Arabidopsis

Two MADS box genes, Lily MADS Box Gene 2 (LMADS2) and Eustoma grandiflorum MADS Box Gene 1 (EgMADS1), with an extensive similarity to the petunia (Petunia hybrida) FLORAL BINDING PROTEIN 7/11 and Arabidopsis AGL11, were characterized from the lily (Lilium longiflorum) and lisianthus (Eustoma grandiflorum). The expression of LMADS2 and EgMADS1 mRNA was restricted to the carpel and was absent in the other flower organs or vegetative leaves. LMADS2 mRNA was detected mainly in ovules and weakly in style tissues of the carpel, whereas EgMADS1 mRNA was only expressed in the ovules. Transgenic Arabidopsis plants ectopically expressing LMADS2 or EgMADS1 showed similar novel phenotypes resembling 35S::AGAMOUS plants by significantly reducing plant size, flowering early, and losing inflorescence indeterminacy. Ectopic expression of these two genes also generated similar ap2-like flowers by inducing homeotic conversion of the sepals into carpel-like structures in which stigmatic papillae and ovules were observed. In addition, the petals were converted into stamen-like structures in the second whorl of 35S::LMADS2 and 35S::EgMADS1 transgenic Arabidopsis. Our data indicated that LMADS2 and EgMADS1 are putative D functional MADS box genes in lily and lisianthus with a function similar to C functional genes once ectopically expressed in Arabidopsis.     

The role of miR156/SPLs modules in Arabidopsis lateral root development

miR156 is an evolutionarily highly conserved miRNA in plants that defines an age‐dependent flowering pathway. The investigations thus far have largely, if not exclusively, confined to plant aerial organs. Root branching architecture is a major determinant of water and nutrients uptake for plants. We show here that MIR156 genes are differentially expressed in specific cells/tissues of lateral roots. Plants overexpressing miR156 produce more lateral roots whereas reducing miR156 levels leads to fewer lateral roots. We demonstrate that at least one representative from the three groups of miR156 targets SQUAMOSA PROMOTER BINDING PROTEIN‐LIKE (SPL) genes: SPL3, SPL9 and SPL10 are involved in the repression of lateral root growth, with SPL10 playing a dominant role. In addition, both MIR156 and SPLs are responsive to auxin signaling suggesting that miR156/SPL modules might be involved in the proper timing of the lateral root developmental progression. Collectively, these results unravel a role for miR156/SPLs modules in lateral root development in Arabidopsis.     

The plant-specific actin binding protein SCAB1 stabilizes actin filaments and regulates stomatal movement in Arabidopsis

Microfilament dynamics play a critical role in regulating stomatal movement; however, the molecular mechanism underlying this process is not well understood. We report here the identification and characterization of STOMATAL CLOSURE-RELATED ACTIN BINDING PROTEIN1 (SCAB1), an Arabidopsis thaliana actin binding protein. Plants lacking SCAB1 were hypersensitive to drought stress and exhibited reduced abscisic acid-, H(2)O(2)-, and CaCl(2)-regulated stomatal movement. In vitro and in vivo analyses revealed that SCAB1 binds, stabilizes, and bundles actin filaments. SCAB1 shares sequence similarity only with plant proteins and contains a previously undiscovered actin binding domain. During stomatal closure, actin filaments switched from a radial orientation in open stomata to a longitudinal orientation in closed stomata. This switch took longer in scab1 plants than in wild-type plants and was correlated with the delay in stomatal closure seen in scab1 mutants in response to drought stress. Our results suggest that SCAB1 is required for the precise regulation of actin filament reorganization during stomatal closure.     

Brassinosteroid-induced exaggerated growth in hydroponically grown Arabidopsis plants

The effects of root application of brassinolide (BL) on the growth and development of Arabidopsis plants ( Arabidopsis thaliana ecotype Columbia [L.] Heynh) were evaluated. Initially, all leaves were evaluated on plants 18, 22, 26 and 29 days old. The younger leaves were found to exhibit maximal petiole elongation and upward leaf bending in response to BL treatment. Therefore, based on these results leaves 6, 7 and 8 on 22–24-day-old plants were selected for all subsequent studies. Elongation along the length of the petiole in response to BL treatment was uniform with the exception of an approximately 4 mm region next to the leaf where upward curvature was observed. Both BL and 24-epibrassinolide (24-epiBL) were evaluated, with BL being more effective at lower concentrations than 24-epiBL. The exaggerated growth induced by 0.1 μ M BL was not observed in plants treated with 1 000-fold higher concentrations of GA₃, IAA, NAA or 2,4-D (100 μ M ). In addition, no exaggerated growth effects were observed when plants were treated with 200 ppm ethylene or 1 m M ACC. All treatments with BL, NAA, 2,4-D, IAA or ACC promoted ethylene and ACC production in wild type Arabidopsis plants, but only BL triggered exaggerated plant growth. BL also promoted exaggerated growth and elevated levels of ACC and ethylene in the ethylene insensitive mutant etr1-3 , showing that the effect of BR on growth is independent of ethylene. This work provides evidence that BR-induced exaggerated growth of Arabidopsis plants is independent of gibberellins, auxins and ethylene.     

Photobiology: Light signal transduction and photomorphogenesis

正Light is crucial for plants, not only because of photosynthesis, but also because of photomorphogenesis. As one of the most important environmental cues, light influences multiple responses in plants,including seed germination, seedling de-etiolation,shade avoidance, phototropism, stomata and chloroplast movement, circadian rhythms, and flowering     

GIGANTEA acts in blue light signaling and has biochemically separable roles in circadian clock and flowering time regulation

Circadian clocks are widespread in nature. In higher plants, they confer a selective advantage, providing information regarding not only time of day but also time of year. Forward genetic screens in Arabidopsis (Arabidopsis thaliana) have led to the identification of many clock components, but the functions of most of these genes remain obscure. To identify both new constituents of the circadian clock and new alleles of known clock-associated genes, we performed a mutant screen. Using a clock-regulated luciferase reporter, we isolated new alleles of ZEITLUPE, LATE ELONGATED HYPOCOTYL, and GIGANTEA (GI). GI has previously been reported to function in red light signaling, central clock function, and flowering time regulation. Characterization of this and other GI alleles has helped us to further define GI function in the circadian system. We found that GI acts in photomorphogenic and circadian blue light signaling pathways and is differentially required for clock function in constant red versus blue light. Gene expression and epistasis analyses show that TIMING OF CHLOROPHYLL A/B BINDING PROTEIN1 (TOC1) expression is not solely dependent upon GI and that GI expression is only indirectly affected by TOC1, suggesting that GI acts both in series with and in parallel to TOC1 within the central circadian oscillator. Finally, we found that the GI-dependent promotion of CONSTANS expression and flowering is intact in a gi mutant with altered circadian regulation. Thus GI function in the regulation of a clock output can be biochemically separated from its role within the circadian clock.     

Conditional repression of AUXIN BINDING PROTEIN1 reveals that it coordinates cell division and cell expansion during postembryonic shoot development in Arabidopsis and tobacco

AUXIN BINDING PROTEIN1 (ABP1) has long been characterized as a potentially important mediator of auxin action in plants. Analysis of the functional requirement for ABP1 during development was hampered because of embryo lethality of the null mutant in Arabidopsis thaliana. Here, we used conditional repression of ABP1 to investigate its function during vegetative shoot development. Using an inducible cellular immunization approach and an inducible antisense construct, we showed that decreased ABP1 activity leads to a severe retardation of leaf growth involving an alteration in cell division frequency, an altered pattern of endocycle induction, a decrease in cell expansion, and a change in expression of early auxin responsive genes. In addition, local repression of ABP1 activity in the shoot apical meristem revealed an additional role for ABP1 in cell plate formation and cell shape. Moreover, cells at the site of presumptive leaf initiation were more sensitive to ABP1 repression than other regions of the meristem. This spatial context-dependent response of the meristem to ABP1 inactivation and the other data presented here are consistent with a model in which ABP1 acts as a coordinator of cell division and expansion, with local auxin levels influencing ABP1 effectiveness.     

The root-colonizing endophyte Pirifomospora indica confers drought tolerance in Arabidopsis by stimulating the expression of drought stress-related genes in leaves

Piriformospora indica is an endophytic fungus that colonizes the roots of many plant species, including Arabidopsis. We exposed 18-day-old Arabidopsis seedlings, which were either cocultivated with the fungus or mock-treated for the last 9 days, to mild drought stress for 84 h. During the first 36 to 48 h, seedlings cocultivated with the fungus continued to grow, while the uncolonized controls did not. This results in a threefold difference in the fresh weight and a more than twofold difference in the chlorophyll content. The photosynthetic efficiency was only slightly reduced in the colonized (F variable/F maximum [Fv/Fm] at t(0 h) = 0.82 and t(36 h) = 0.79) and was severely impaired in the uncolonized (Fv/Fm at t(0 h) = 0.81 and (t)(36 h) = 0.49) seedlings, which also showed symptoms of withering. When seedlings exposed to drought stress for 72 or 84 h were transferred to soil, 10% (72 h) and none (84 h) of uncolonized seedlings reached the flowering stage and produced seeds, while 59% (72 h) and 47% (84 h) of the colonized seedlings flowered and produced seeds. After exposure to drought stress for 3 h, the message levels for RESPONSE TO DEHYDRATION 29A, EARLY RESPONSE TO DEHYDRATION1, ANAC072, DEHYDRATION-RESPONSE ELEMENT BINDING PROTEIN2A, SALT-, AND DROUGHT-INDUCED RING FINGER1, phospholipase Ddelta, CALCINEURIN B-LIKE PROTEIN (CBL)1, CBL-INTERACTING PROTEIN KINASE3, and the histone acetyltransferase (HAT) were upregulated in the leaves of P. indica-colonized seedlings. Uncolonized seedlings responded 3 to 6 h later, and the message levels increased much less. We identified an Arabidopsis ethylmethane-sulfonate mutant that is less resistant to drought stress and in which the stress-related genes were not upregulated in the presence of P. indica. Thus, P. indica confers drought-stress tolerance to Arabidopsis, and this is associated with the priming of the expression of a quite diverse set of stress-related genes in the leaves. Transfer to soil was again associated with a faster and stronger upregulation of the message levels for phospholipase Ddelta, CBL1, and HAT in P. indica-colonized seedlings, indicating that this response might also contribute to better survival on soil.     

Suppression of RICE TELOMERE BINDING PROTEIN 1 results in severe and gradual developmental defects accompanied by genome instability in rice

Although several potential telomere binding proteins have been identified in higher plants, their in vivo functions are still unknown at the plant level. Both knockout and antisense mutants of RICE TELOMERE BINDING PROTEIN1 (RTBP1) exhibited markedly longer telomeres relative to those of the wild type, indicating that the amount of functional RTBP1 is inversely correlated with telomere length. rtbp1 plants displayed progressive and severe developmental abnormalities in both germination and postgermination growth of vegetative organs over four generations (G1 to G4). Reproductive organ formation, including panicles, stamens, and spikelets, was also gradually and severely impaired in G1 to G4 mutants. Up to 11.4, 17.2, and 26.7% of anaphases in G2, G3, and G4 mutant pollen mother cells, respectively, exhibited one or more chromosomal fusions, and this progressively increasing aberrant morphology was correlated with an increased frequency of anaphase bridges containing telomeric repeat DNA. Furthermore, 35S:anti-RTBP1 plants expressing lower levels of RTBP1 mRNA exhibited developmental phenotypes intermediate between the wild type and mutants in all aspects examined, including telomere length, vegetative and reproductive growth, and degree of genomic anomaly. These results suggest that RTBP1 plays dual roles in rice (Oryza sativa), as both a negative regulator of telomere length and one of positive and functional components for proper architecture of telomeres.     

Growth promotion of Chinese cabbage and Arabidopsis by Piriformospora indica is not stimulated by mycelium-synthesized auxin

Piriformospora indica, an endophytic fungus of the order Sebacinales, interacts with the roots of a large variety of plant species. We compared the interaction of this fungus with Chinese cabbage (Brassica campestris subsp. chinensis) and Arabidopsis seedlings. The development of shoots and roots of Chinese cabbage seedlings was strongly promoted by P. indica and the fresh weight of the seedlings increased approximately twofold. The strong stimulation of root hair development resulted in a bushy root phenotype. The auxin level in the infected Chinese cabbage roots was twofold higher compared with the uncolonized controls. Three classes of auxin-related genes, which were upregulated by P. indica in Chinese cabbage roots, were isolated from a double-subtractive expressed sequence tag library: genes for proteins related to cell wall acidification, intercellular auxin transport carrier proteins such as AUX1, and auxin signal proteins. Overexpression of B. campestris BcAUX1 in Arabidopsis strongly promoted growth and biomass production of Arabidopsis seedlings and plants; the roots were highly branched but not bushy when compared with colonized Chinese cabbage roots. This suggests that BcAUX1 is a target of P. indica in Chinese cabbage. P. indica also promoted growth of Arabidopsis seedlings but the auxin levels were not higher and auxin genes were not upregulated, implying that auxin signaling is a more important target of P. indica in Chinese cabbage than in Arabidopsis. The fungus also stimulated growth of Arabidopsis aux1 and aux1/axr4 and rhd6 seedlings. Furthermore, a component in an exudate fraction from P. indica but not auxin stimulated growth of Chinese cabbage and Arabidopsis seedlings. We propose that activation of auxin biosynthesis and signaling in the roots might be the cause for the P. indica-mediated growth phenotype in Chinese cabbage.