Plant DELLAs restrain growth and promote survival of adversity by reducing the levels of reactive oxygen species

Plant growth is adaptively modulated in response to environmental change. The phytohormone gibberellin (GA) promotes growth by stimulating destruction of the nuclear growth-repressing DELLA proteins [1-7], thus providing a mechanism for environmentally responsive growth regulation [8, 9]. Furthermore, DELLAs promote survival of adverse environments [8]. However, the relationship between these survival and growth-regulatory mechanisms was previously unknown. Here, we show that both mechanisms are dependent upon control of the accumulation of reactive oxygen species (ROS). ROS are small molecules generated during development and in response to stress that play diverse roles as eukaryotic intracellular second messengers [10]. We show that Arabidopsis DELLAs cause ROS levels to remain low after either biotic or abiotic stress, thus delaying cell death and promoting tolerance. In essence, stress-induced DELLA accumulation elevates the expression of genes encoding ROS-detoxification enzymes, thus reducing ROS levels. In accord with recent demonstrations that ROS control root cell expansion [11, 12], we also show that DELLAs regulate root-hair growth via a ROS-dependent mechanism. We therefore propose that environmental variability regulates DELLA activity [8] and that DELLAs in turn couple the downstream regulation of plant growth and stress tolerance through modulation of ROS levels.     

Flexible control of plant architecture and yield via switchable expression of Arabidopsis gai

The growth of plants is repressed by DELLA proteins, nuclear regulators whose activities are opposed by the growth-promoting phytohormone gibberellin (GA). Mutations affecting DELLA protein function were previously used by plant breeders to create the high-yielding semidwarf wheat varieties of the green revolution. gai is an Arabidopsis mutant DELLA protein-encoding orthologue of the wheat semidwarfing genes. Here we describe the development of a transgene that confers ethanol-inducible gai expression. Transient induction of gai causes transient growth repression: growth prior to and after treatment is unaffected. Appropriate ethanol treatments result in dwarf plants that produce the same numbers of seeds as untreated controls. This new technology represents a substantial advance in the applicability of genes encoding mutant DELLA proteins to agricultural and horticultural improvement, enhancing the flexibity with which these genes can be used for the sustainable achievement of increased crop plant yields.     

Step-by-step acquisition of the gibberellin-DELLA growth-regulatory mechanism during land-plant evolution

Angiosperms (flowering plants) evolved relatively recently and are substantially diverged from early land plants (bryophytes, lycophytes, and others [1]). The phytohormone gibberellin (GA) adaptively regulates angiosperm growth via the GA-DELLA signaling mechanism [2-7]. GA binds to GA receptors (GID1s), thus stimulating interactions between GID1s and the growth-repressing DELLAs [8-12]. Subsequent 26S proteasome-mediated destruction of the DELLAs promotes growth [13-17]. Here we outline the evolution of the GA-DELLA mechanism. We show that the interaction between GID1 and DELLA components from Selaginella kraussiana (a lycophyte) is GA stimulated. In contrast, GID1-like (GLP1) and DELLA components from Physcomitrella patens (a bryophyte) do not interact, suggesting that GA-stimulated GID1-DELLA interactions arose in the land-plant lineage after the bryophyte divergence ( approximately 430 million years ago [1]). We further show that a DELLA-deficient P. patens mutant strain lacks the derepressed growth characteristic of DELLA-deficient angiosperms, and that both S. kraussiana and P. patens lack detectable growth responses to GA. These observations indicate that early land-plant DELLAs do not repress growth in situ. However, S. kraussiana and P. patens DELLAs function as growth-repressors when expressed in the angiosperm Arabidopsis thaliana. We conclude that the GA-DELLA growth-regulatory mechanism arose during land-plant evolution and via independent stepwise recruitment of GA-stimulated GID1-DELLA interaction and DELLA growth-repression functions.     

The 'Green Revolution' dwarfing genes play a role in disease resistance in Triticum aestivum and Hordeum vulgare

The Green Revolution dwarfing genes, Rht-B1b and Rht-D1b, encode mutant forms of DELLA proteins and are present in most modern wheat varieties. DELLA proteins have been implicated in the response to biotic stress in the model plant, Arabidopsis thaliana. Using defined wheat Rht near-isogenic lines and barley Sln1 gain of function (GoF) and loss of function (LoF) lines, the role of DELLA in response to biotic stress was investigated in pathosystems representing contrasting trophic styles (biotrophic, hemibiotrophic, and necrotrophic). GoF mutant alleles in wheat and barley confer a resistance trade-off with increased susceptibility to biotrophic pathogens and increased resistance to necrotrophic pathogens whilst the converse was conferred by a LoF mutant allele. The polyploid nature of the wheat genome buffered the effect of single Rht GoF mutations relative to barley (diploid), particularly in respect of increased susceptibility to biotrophic pathogens. A role for DELLA in controlling cell death responses is proposed. Similar to Arabidopsis, a resistance trade-off to pathogens with contrasting pathogenic lifestyles has been identified in monocotyledonous cereal species. Appreciation of the pleiotropic role of DELLA in biotic stress responses in cereals has implications for plant breeding.     

DELLA protein function in growth responses to canopy signals

Plants can sense neighbour competitors through light-quality signals and respond with shade-avoidance responses. These include increased shoot elongation, which enhances light capture and thus competitive power. Such plant-plant interactions therefore profoundly affect plant development in crowded populations. Shade-avoidance responses are tightly coordinated by interactions between light signals and hormones, with essential roles for the phytochrome B photoreceptor [sensing the red:far red (R:FR) ratio] and the hormone gibberellin (GA). The family of growth-suppressing DELLA proteins are targets for GA signalling and are proposed to integrate signals from other hormones. However, the importance of these regulators has not been studied in the ecologically relevant, complex realm of plant canopies. Here we show that DELLA abundance is regulated during growth responses to neighbours in dense Arabidopsis stands. This occurs in a R:FR-dependent manner in petioles, depends on GA, and matches the induction kinetics of petiole elongation. Similar interactions were observed in the growth response of seedling hypocotyls and are general for a second canopy signal, reduced blue light. Enhanced DELLA stability in the gai mutant inhibits shade-avoidance responses, indicating that DELLA proteins constrain shade-avoidance. However, using multiple DELLA knockout mutants, we show that the observed DELLA breakdown is not sufficient to induce shade-avoidance in petioles, but plays a more central role in hypocotyls. These data provide novel information on the regulation of shade-avoidance under ecologically important conditions, defining the importance of DELLA proteins and GA and unravelling the existence of GA- and DELLA-independent mechanisms.     

The interplay between MAMP and SA signaling

There are two major modes for plant recognition of biotrophic microbial pathogens. In one mode, plant pattern recognition receptors (PRRs) recognize microbe associated molecular patterns (MAMPs, also called PAMPs), which are molecules such as flg22, a fragment of bacterial flagellin. In the other mode, the products of plant resistance (R) genes recognize pathogen effectors or host proteins modified by effectors. Salicylic acid (SA) -mediated defense responses are an important part of R gene-mediated resistance. It was not clear how these two signaling mechanisms interact with each other. Recently, we reported that treatment with flg22 triggered SA accumulation in Arabidopsis leaves. Disruptions of SA signaling components strongly affected MAMP-triggered gene expression responses. Flg22-triggered resistance to Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) was partly dependent on SA signaling. Our results demonstrated the importance of SA signaling in flg22-triggered resistance and, at the same time, the importance of some other signaling mechanism(s) in this resistance. Here we discuss potential signaling components of flg22-triggered SA accumulation and other signaling mechanisms potentially contributing to flg22-triggered resistance to Pst DC3000.Key words: arabidopsis, expression profiling, MAMP, PAD4, PAMP, salicylic acid (SA), SID2     

DELLA蛋白在被子植物有性生殖中的作用

DELLA蛋白是植物生长发育过程中响应赤霉素(GA)应答途径的关键调控因子, 主要行使转录调控因子的功能, 几乎参与了植物生长发育的各个重要过程。已有的研究表明, DELLA蛋白在被子植物的雄性生殖器官、雌性生殖器官和胚胎等组织中均有表达, 在植物有性生殖过程中起着极其重要的作用。该文综述了DELLA蛋白的分子结构、特性及其在植物有性生殖过程中的表达与功能, 并讨论了现存的问题及研究思路。     

The Arabidopsis mutant sleepy1gar2-1 protein promotes plant growth by increasing the affinity of the SCFSLY1 E3 ubiquitin ligase for DELLA protein substrates

DELLA proteins restrain the cell proliferation and enlargement that characterizes the growth of plant organs. Gibberellin stimulates growth via 26S proteasome-dependent destruction of DELLAs, thus relieving DELLA-mediated growth restraint. Here, we show that the Arabidopsis thaliana sleepy1gar2-1 (sly1gar2-1) mutant allele encodes a mutant subunit (sly1gar2-1) of an SCF(SLY1) E3 ubiquitin ligase complex. SLY1 (the wild-type form) and sly1gar2-1 both confer substrate specificity on this complex via specific binding to the DELLA proteins. However, sly1gar2-1 interacts more strongly with the DELLA target than does SLY1. In addition, the strength of the SCFSLY1-DELLA interaction is increased by target phosphorylation. Growth-promoting DELLA destruction is dependent on SLY1 availability, on the strength of the interaction between SLY1 and the DELLA target, and on promotion of the SCFSLY1-DELLA interaction by DELLA phosphorylation.     

The Pseudomonas syringae type III effector tyrosine phosphatase HopAO1 suppresses innate immunity in Arabidopsis thaliana

The bacterial pathogen Pseudomonas syringae pv. tomato (Pst) strain DC3000 infects tomato and Arabidopsis plants, and is a model for studying the molecular basis of bacterial disease. Pst DC3000 secretes a battery of largely uncharacterized effector proteins into host cells via a type-III secretion system (TTSS). Little is currently known about the molecular mechanisms by which individual TTSS effectors promote virulence. The effector HopAO1 has similarity to protein tyrosine phosphatases, including a conserved catalytic site, and suppresses the hypersensitive response (HR) in some non-host plants. Whether HopAO1 has a similar effect in the host Arabidopsis is not clear. Here, we show that transgenic expression of HopAO1 in Arabidopsis suppresses callose deposition elicited by the Pst DC3000 hrpA mutant, and allows the normally non-pathogenic hrpA mutant to multiply within the leaf tissue. HopAO1 also suppresses resistance to Pst DC3000 induced by flg22, a pathogen-associated molecular pattern (PAMP). However, HopAO1 does not suppress the HR triggered by several classical avirulence genes. These results suggest that HopAO1 targets primarily PAMP-induced innate immunity in Arabidopsis. The virulence function of HopAO1 is dependent on an intact phosphatase catalytic site, as transgenic plants expressing a catalytically inactive derivative do not show these effects. Intriguingly, expression of the catalytically inactive HopAO1 has a dominant-negative effect on the function of the wild-type HopAO1. Analysis of mitogen-activated protein kinase (MAPK) activity suggests that HopAO1 targets a step downstream or independent of MAPK activation. Genome-wide expression analysis revealed that expression of several well-known defense genes was suppressed in hrpA mutant-infected HopAO1 transgenic plants.     

Physical association of Arabidopsis hypersensitive induced reaction proteins (HIRs) with the immune receptor RPS2

Arabidopsis RPS2 is a typical nucleotide-binding leucine-rich repeat resistance protein, which indirectly recognizes the bacterial effector protein AvrRpt2 and thereby activates effector-triggered immunity (ETI). Previously, we identified two hypersensitive induced reaction (AtHIR) proteins, AtHIR1 (At1g09840) and AtHIR2 (At3g01290), as potential RPS2 complex components. AtHIR proteins contain the stomatin/prohibitin/flotillin/HflK/C domain (also known as the prohibitin domain or band 7 domain). In this study, we confirmed that AtHIR1 and AtHIR2 form complexes with RPS2 in Arabidopsis and Nicotiana benthamiana using a pulldown assay and fluorescence resonance energy transfer (FRET) analysis. Arabidopsis has four HIR family genes (AtHIR1-4). All AtHIR proteins could form homo- and hetero-oligomers in vivo and were enriched in membrane microdomains of the plasma membrane. The mRNA levels of all except AtHIR4 were significantly induced by microbe-associated molecular patterns, such as the bacterial flagellin fragment flg22. Athir2-1 and Athir3-1 mutants allowed more growth of Pto DC3000 AvrRpt2, but not Pto DC3000, indicating that these mutations reduce RPS2-mediated ETI but do not affect basal resistance to the virulent strain. Overexpression of AtHIR1 and AtHIR2 reduced growth of Pto DC3000. Taken together, the results show that the AtHIR proteins are physically associated with RPS2, are localized in membrane microdomains, and quantitatively contribute to RPS2-mediated ETI.     

The Arabidopsis F-box protein SLEEPY1 targets gibberellin signaling repressors for gibberellin-induced degradation

The nuclear DELLA proteins are highly conserved repressors of hormone gibberellin (GA) signaling in plants. In Arabidopsis thaliana, GA derepresses its signaling pathway by inducing proteolysis of the DELLA protein REPRESSOR OF ga1-3 (RGA). SLEEPY1 (SLY1) encodes an F-box-containing protein, and the loss-of-function sly1 mutant has a GA-insensitive dwarf phenotype and accumulates a high level of RGA. These findings suggested that SLY1 recruits RGA to the SCFSLY1 E3 ligase complex for ubiquitination and subsequent degradation by the 26S proteasome. In this report, we provide new insight into the molecular mechanism of how SLY1 interacts with the DELLA proteins for controlling GA response. By yeast two-hybrid and in vitro pull-down assays, we demonstrated that SLY1 interacts directly with RGA and GA INSENSITIVE (GAI, a closely related DELLA protein) via their C-terminal GRAS domain. The rga and gai null mutations additively suppressed the recessive sly1 mutant phenotype, further supporting the model that SCFSLY1 targets both RGA and GAI for degradation. The N-terminal DELLA domain of RGA previously was shown to be essential for GA-induced degradation. However, we found that this DELLA domain is not required for protein-protein interaction with SLY1 in yeast (Saccharomyces cerevisiae), suggesting that its role is in a GA-triggered conformational change of the DELLA proteins. We also identified a novel gain-of-function sly1-d mutation that increased GA signaling by reducing the levels of the DELLA protein in plants. This effect of sly1-d appears to be caused by an enhanced interaction between sly1-d and the DELLA proteins.     

Loss of Arabidopsis thaliana Dynamin-Related Protein 2B Reveals Separation of Innate Immune Signaling Pathways

Vesicular trafficking has emerged as an important means by which eukaryotes modulate responses to microbial pathogens, likely by contributing to the correct localization and levels of host components necessary for effective immunity. However, considering the complexity of membrane trafficking in plants, relatively few vesicular trafficking components with functions in plant immunity are known. Here we demonstrate that Arabidopsis thaliana Dynamin-Related Protein 2B (DRP2B), which has been previously implicated in constitutive clathrin-mediated endocytosis (CME), functions in responses to flg22 (the active peptide derivative of bacterial flagellin) and immunity against flagellated bacteria Pseudomonas syringae pv. tomato (Pto) DC3000. Consistent with a role of DRP2B in Pattern-Triggered Immunity (PTI), drp2b null mutant plants also showed increased susceptibility to Pto DC3000 hrcC ⁻, which lacks a functional Type 3 Secretion System, thus is unable to deliver effectors into host cells to suppress PTI. Importantly, analysis of drp2b mutant plants revealed three distinct branches of the flg22-signaling network that differed in their requirement for RESPIRATORY BURST OXIDASE HOMOLOGUE D (RBOHD), the NADPH oxidase responsible for flg22-induced apoplastic reactive oxygen species production. Furthermore, in drp2b, normal MAPK signaling and increased immune responses via the RbohD/Ca²⁺-branch were not sufficient for promoting robust PR1 mRNA expression nor immunity against Pto DC3000 and Pto DC3000 hrcC⁻. Based on live-cell imaging studies, flg22-elicited internalization of the plant flagellin-receptor, FLAGELLIN SENSING 2 (FLS2), was found to be partially dependent on DRP2B, but not the closely related protein DRP2A, thus providing genetic evidence for a component, implicated in CME, in ligand-induced endocytosis of FLS2. Reduced trafficking of FLS2 in response to flg22 may contribute in part to the non-canonical combination of immune signaling defects observed in drp2b. In conclusion, this study adds DRP2B to the relatively short list of known vesicular trafficking proteins with roles in flg22-signaling and PTI in plants.     

Gibberellin regulates Arabidopsis floral development via suppression of DELLA protein function

The phytohormone gibberellin (GA) regulates the development and fertility of Arabidopsis flowers. The mature flowers of GA-deficient mutant plants typically exhibit reduced elongation growth of petals and stamens. In addition, GA-deficiency blocks anther development, resulting in male sterility. Previous analyses have shown that GA promotes the elongation of plant organs by opposing the function of the DELLA proteins, a family of nuclear growth repressors. However, it was not clear that the DELLA proteins are involved in the GA-regulation of stamen and anther development. We show that GA regulates cell elongation rather than cell division during Arabidopsis stamen filament elongation. In addition, GA regulates the cellular developmental pathway of anthers leading from microspore to mature pollen grain. Genetic analysis shows that the Arabidopsis DELLA proteins RGA and RGL2 jointly repress petal, stamen and anther development in GA-deficient plants, and that this function is enhanced by RGL1 activity. GA thus promotes Arabidopsis petal, stamen and anther development by opposing the function of the DELLA proteins RGA, RGL1 and RGL2.     

DELLA signaling mediates stress-induced cell differentiation in Arabidopsis leaves through modulation of anaphase-promoting complex/cyclosome activity

Drought is responsible for considerable yield losses in agriculture due to its detrimental effects on growth. Drought responses have been extensively studied, but mostly on the level of complete plants or mature tissues. However, stress responses were shown to be highly tissue and developmental stage specific, and dividing tissues have developed unique mechanisms to respond to stress. Previously, we studied the effects of osmotic stress on dividing leaf cells in Arabidopsis (Arabidopsis thaliana) and found that stress causes early mitotic exit, in which cells end their mitotic division and start endoreduplication earlier. In this study, we analyzed this phenomenon in more detail. Osmotic stress induces changes in gibberellin metabolism, resulting in the stabilization of DELLAs, which are responsible for mitotic exit and earlier onset of endoreduplication. Consequently, this response is absent in mutants with altered gibberellin levels or DELLA activity. Mitotic exit and onset of endoreduplication do not correlate with an up-regulation of known cell cycle inhibitors but are the result of reduced levels of DP-E2F-LIKE1/E2Fe and UV-B-INSENSITIVE4, both inhibitors of the developmental transition from mitosis to endoreduplication by modulating anaphase-promoting complex/cyclosome activity, which are down-regulated rapidly after DELLA stabilization. This work fits into an emerging view of DELLAs as regulators of cell division by regulating the transition to endoreduplication and differentiation.