DELLA-induced early transcriptional changes during etiolated development in Arabidopsis thaliana

The transcription factor SlLBD40 regulates seed germination by inhibiting cell wall remodeling enzymes during endosperm weakening

Uniform seed germination is crucial for consistent seedling emergence and efficient seedling production. In this study, we identified a seed-expressed protein in tomato (Solanum lycopersicum), lateral organ boundaries domain 40 (SlLBD40), that regulates germination speed. CRISPR/Cas9-generated SlLBD40 knockout mutants exhibited faster germination due to enhanced seed imbibition, independent of the seed coat. The expression of SlLBD40 was induced during the imbibition process, particularly in the micropylar endosperm, suggesting its role in endosperm weakening. Gene ontology analysis of RNA-seq data indicated that differentially expressed genes were enriched in cell wall-related processes. SlLBD40 directly targeted genes encoding cell wall remodeling enzymes implicated in endosperm weakening, including expansin 6 (SlEXP6), xyloglucan endotransglucosylase/hydrolase 23 (SlXTH23), and endo-β-mannanase 1 (SlMAN1). Our findings shed light on the role of endosperm weakening in regulating seed germination and propose potential gene targets for improving germination in species constrained by endosperm strength.

Gene editing of ZmGA20ox3 improves plant architecture and drought tolerance in maize

KEY MESSAGE

Editing ZmGA20ox3 can achieve the effect similar to applying Cycocel, which can reduce maize plant height and enhance stress resistance. Drought stress, a major plant abiotic stress, is capable of suppressing crop yield performance severely. However, the trade-off between crop drought tolerance and yield performance turns out to be significantly challenging in drought-resistant crop breeding. Several phytohormones [e.g., gibberellin (GA)] have been reported to play a certain role in plant drought response, which also take on critical significance in plant growth and development. In this study, the loss-of-function mutations of GA biosynthesis enzyme ZmGA20ox3 were produced using the CRISPR-Cas9 system in maize. As indicated by the result of 2-year field trials, the above-mentioned mutants displayed semi-dwarfing phenotype with the decrease of GA1, and almost no yield loss was generated compared with wild-type (WT) plants. Interestingly, as revealed by the transcriptome analysis, differential expressed genes (DEGs) were notably enriched in abiotic stress progresses, and biochemical tests indicated the significantly increased ABA, JA, and DIMBOA levels in mutants, suggesting that ZmGA20ox3 may take on vital significance in stress response in maize. The in-depth analysis suggested that the loss function of ZmGA20ox3 can enhance drought tolerance in maize seedling, reduce Anthesis-Silking Interval (ASI) delay while decreasing the yield loss significantly in the field under drought conditions. The results of this study supported that regulating ZmGA20ox3 can improve plant height while enhancing drought resistance in maize, thus serving as a novel method for drought-resistant genetic improvement in maize.

Interaction of VvDELLA2 and VvCEB1 Mediates Expression of Expansion-Related Gene during GA-Induced Enlargement of Grape Fruit

Exogenous gibberellin treatment can promote early growth of grape fruit, but the underlying regulatory mechanisms are not well understood. Here, we show that VvDELLA2 directly regulates the activity of the VvCEB1 transcription factor, a key regulator in the control of cell expansion in grape fruit. Our results show that VvCEB1 binds directly to the promoters of cell expansion-related genes in grape fruit and acts as a transcriptional activator, while VvDELLA2 blocks VvCEB1 function by binding to its activating structural domain. The exogenous gibberellin treatment relieved this inhibition by promoting the degradation of VvDELLA2 protein, thus, allowing VvCEB1 to transcriptionally activate the expression of cell expansion-related genes. In conclusion, we conclude that exogenous GA3 treatment regulates early fruit expansion by affecting the VvDELLA-VvCEB1 interaction in grape fruit development.

DELLA proteins positively regulate seed size in Arabidopsis

Human and animal nutrition is mainly based on seeds. Seed size is a key factor affecting seed yield and has thus been one of the primary objectives of plant breeders since the domestication of crop plants. Seed size is coordinately regulated by signals of maternal and zygotic tissues that control the growth of the seed coat, endosperm and embryo. Here, we provide previously unreported evidence for the role of DELLA proteins, key repressors of gibberellin responses, in the maternal control of seed size. The gain-of-function della mutant gai-1 produces larger seeds as a result of an increase in the cell number in ovule integuments. This leads to an increase in ovule size and, in turn, to an increase in seed size. Moreover, DELLA activity promotes increased seed size by inducing the transcriptional activation of AINTEGUMENTA, a genetic factor that controls cell proliferation and organ growth, in the ovule integuments of gai-1. Overall, our results indicate that DELLA proteins are involved in the control of seed size and suggest that modulation of the DELLA-dependent pathway could be used to improve crop yield.

Gibberellins regulate ovule number through a DELLA-CUC2 complex in Arabidopsis

Ovule development is a key process for plant reproduction, helping to ensure correct seed production. Several molecular factors and plant hormones such as gibberellins are involved in ovule initiation and development. Gibberellins control ovule development by the destabilization of DELLA proteins, whereas DELLA activity has been shown to act as a positive factor for ovule primordia emergence. But the molecular mechanism by which DELLA acts in ovule primordia initiation remained unknown. In this study we report that DELLA proteins participate in ovule initiation by the formation of a protein complex with the CUC2 transcription factor. The DELLA protein GAI requires CUC2 to promote ovule primordia formation, through the direct GAI-CUC2 interaction in placental cells that would determine the boundary regions between ovules during pistil development. Analysis of GAI-CUC2 interaction and co-localization in the placenta supports this hypothesis. Moreover, molecular analysis identified a subset of the loci for which the GAI protein may act as a transcriptional co-regulator in a CUC2-dependent manner. The DELLA-CUC2 complex is a component of the gene regulatory network controlling ovule primordia initiation in Arabidopsis.

Sugar metabolism during pre- and post-fertilization events in plants under high temperature stress

High temperature challenges global crop production by limiting the growth and development of the reproductive structures and seed. It impairs the developmental stages of male and female gametogenesis, pollination, fertilization, endosperm formation and embryo development. Among these, the male reproductive processes are highly prone to abnormalities under high temperature at various stages of development. The disruption of source-sink balance is the main constraint for satisfactory growth of the reproductive structures which is disturbed at the level of sucrose import and utilization within the tissue. Seed development after fertilization is affected by modulation in the activity of enzymes involved in starch metabolism. In addition, the alteration in the seed-filling rate and its duration affects the seed weight and quality. The present review critically discusses the role of sugar metabolism in influencing the various stages of gamete and seed development under high temperature stress. It also highlights the interaction of the sugars with hormones that mediate the transport of sugars to sink tissues. The role of transcription factors for the regulation of sugar availability under high temperature has also been discussed. Further, the omics-based systematic investigation has been suggested to understand the synergistic or antagonistic interactions between sugars, hormones and reactive oxygen species at various points of sucrose flow from source to sink under high temperature stress.

The blue light receptor CRY1 interacts with GID1 and DELLA proteins to repress GA signaling during photomorphogenesis in Arabidopsis

Light is a critical environmental cue that regulates a variety of diverse plant developmental processes. Cryptochrome 1 (CRY1) is the major photoreceptor that mediates blue light-dependent photomorphogenic responses such as the inhibition of hypocotyl elongation. Gibberellin (GA) participates in the repression of photomorphogenesis and promotes hypocotyl elongation. However, the antagonistic interaction between blue light and GA is not well understood. Here, we report that blue light represses GA-induced degradation of the DELLA proteins (DELLAs), which are key negative regulators in the GA signaling pathway, via CRY1, thereby inhibiting the GA response during hypocotyl elongation. Both in vitro and in vivo biochemical analyses demonstrated that CRY1 physically interacts with GA receptors-GA-INSENSITIVE DWARF 1 proteins (GID1s)-and DELLAs in a blue light-dependent manner. Furthermore, we showed that CRY1 inhibits the association between GID1s and DELLAs. Genetically, CRY1 antagonizes the function of GID1s to repress the expression of cell elongation-related genes and thus hypocotyl elongation. Taken together, our findings demonstrate that CRY1 coordinates blue light and GA signaling for plant photomorphogenesis by stabilizing DELLAs through the binding and inactivation of GID1s, providing new insights into the mechanism by which blue light antagonizes the function of GA in photomorphogenesis.

Light contributes to salt resistance through GAI protein regulation in Arabidopsis thaliana

The role of DELLAs in response to light intensity under salt stress is largely unknown. Therefore, the effect of three light intensities-low (35), medium (60), and high (155) μmol m^-2 s^-1 on Arabidopsis plants growth under saline condition (150 mM NaCl) was evaluated. High light intensity exhibited significant growth in the number of lateral roots related to the low light. Immunoblot assay revealed increased DELLA accumulation at the seedling stage under high light intensity. High light promotes seed germination by 24-44%, whilst, lateral roots by 25-90% in wild-type ecotypes. The lateral roots increased significantly in gai (gibberellic acid insensitive mutant) as compared with gai-t6 (wild type like gibberellic acid insensitive mutant) in response to low to medium and high to medium light intensity. High light increased seedling survival rate by 67% in Col-0 (Columbia) and 60% in Ler (Landsberg erecta) and showed a 28% increase in survival rate in gai mutant under salt stress as compared to gai-t6. Furthermore, salt-stress responsive genes' expression in gai-mutant establishes the relationship of DELLA proteins with salt resistance. Together, light is a cardinal element, its optimum quantity is highly beneficial and promotes salt stress resistance through DELLA protein at seedling stage in plants.

Evolution of DELLA function and signaling in land plants

DELLA proteins are master growth regulators that repress responses to a group of plant growth hormones called gibberellins (GAs). Manipulation of DELLA function and signaling was instrumental in the development of high‐yielding crop varieties that saved millions from starvation during the “Green Revolution.” Despite decades of extensive research, it is still unclear how DELLA function and signaling mechanisms evolved within the land plant lineage. Here, we review current knowledge on DELLA protein function with reference to structure, posttranslational modifications, downstream transcriptional targets, and protein–protein interactions. Furthermore, we discuss older and recent findings regarding the evolution of DELLA signaling within the land plant lineage, with an emphasis on bryophytes, and identify future avenues of research that would enable us to shed more light on the evolution of DELLA signaling. Unraveling how DELLA function and signaling mechanisms have evolved could enable us to engineer better crops in an attempt to contribute to mitigating the effects of global warming and achieving global food security.

DELLA genes first appeared in the common ancestor of land plants and underwent two major duplications during land plant evolution.

DELLAs repress gibberellin responses in vascular plants but their function in nonvascular plants remains elusive.

The phytochrome-interacting transcription factor CsPIF8 contributes to cold tolerance in citrus by regulating superoxide dismutase expression

As one of the subtropical and tropical fruit trees, Citrus sinensis is sensitive to cold stress. However, most transcription factors (TFs) that regulate cold tolerance in citrus have not yet been reported. A phytochrome-interacting transcription factor (PIF) gene (CsPIF8) in citrus was significantly upregulated under cold stress. Overexpression of CsPIF8 increased cold tolerance in transgenic tomato plants and grapefruit callus, whereas virus-induced gene silencing-mediated suppression of PIF8 increased cold sensitivity in seedlings of Poncirus trifoliata. Superoxide dismutase (SOD) reduces the superoxide anion (O₂^-) level to enhance cold tolerance in plants. Chromatin immunoprecipitation combined with high-throughput sequencing, yeast one hybrid, electrophoretic mobility shift and dual luciferase assays showed that CsPIF8 directly bound the E-box (CANNTG) of CsSOD promoter and activated the promoter of CsSOD. Furthermore, the expression level of CsSOD and CsSOD activity were significantly increased, whereas the level of O₂^- was significantly reduced in the transgenic lines. The Poncirus trifoliata seedlings with VIGS-mediated suppression of PIF8 exhibited the opposite effects. These results have shown that CsPIF8 improved cold tolerance in citrus through regulating the expression level of SOD and SOD activity. These findings may provide novel insights into the regulation of PIF8 in the response to cold stress in citrus.

Cryptochrome-mediated hypocotyl phototropism was regulated antagonistically by gibberellic acid and sucrose in Arabidopsis

Both phototropins (phot1 and phot2) and cryptochromes (cry1 and cry2) were proven as the Arabidopsis thaliana blue light receptors. Phototropins predominately function in photomovement, and cryptochromes play a role in photomorphogenesis. Although cryptochromes have been proposed to serve as positive modulators of phototropic responses, the underlying mechanism remains unknown. Here, we report that depleting sucrose from the medium or adding gibberellic acids (GAs) can partially restore the defects in phototropic curvature of the phot1 phot2 double mutants under high-intensity blue light; this restoration does not occur in phot1 phot2 cry1 cry2 quadruple mutants and nph3 (nonphototropic hypocotyl 3) mutants which were impaired phototropic response in sucrose-containing medium. These results indicate that GAs and sucrose antagonistically regulate hypocotyl phototropism in a cryptochromes dependent manner, but it showed a crosstalk with phototropin signaling on NPH3. Furthermore, cryptochromes activation by blue light inhibit GAs synthesis, thus stabilizing DELLAs to block hypocotyl growth, which result in the higher GAs content in the shade side than the lit side of hypocotyl to support the asymmetric growth of hypocotyl. Through modulation of the abundance of DELLAs by sucrose depletion or added GAs, it revealed that cryptochromes have a function in mediating phototropic curvature.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

The Sugar-Signaling Hub: Overview of Regulators and Interaction with the Hormonal and Metabolic Network

Plant growth and development has to be continuously adjusted to the available resources. Their optimization requires the integration of signals conveying the plant metabolic status, its hormonal balance, and its developmental stage. Many investigations have recently been conducted to provide insights into sugar signaling and its interplay with hormones and nitrogen in the fine-tuning of plant growth, development, and survival. The present review emphasizes the diversity of sugar signaling integrators, the main molecular and biochemical mechanisms related to the sugar-signaling dependent regulations, and to the regulatory hubs acting in the interplay of the sugar-hormone and sugar-nitrogen networks. It also contributes to compiling evidence likely to fill a few knowledge gaps, and raises new questions for the future.

The zinc-finger transcription factor ZAT6 is essential for hydrogen peroxide induction of anthocyanin synthesis in Arabidopsis

The accumulation of flavonoids is activated by various abiotic stresses, and the induction of reactive oxygen species (ROS) especially hydrogen peroxide (H₂O₂) is a general response to abiotic stress in plants. However, the direct link between flavonoids and H₂O₂ and underlying mechanism remain elusive. In this study, we found that the concentrations of anthocyanin and flavonoids were significantly induced by H₂O₂ treatment. Furthermore, we found that the transcript level of ZINC FINGER of ARABIDOPSIS THALIANA 6 (ZAT6) was significantly activated after exogenous H₂O₂ treatment, and modulation of AtZAT6 expression positively affected the concentrations of both anthocyanin and total flavonoids. Notably, exogenous H₂O₂-induced anthocyanin synthesis was largely alleviated in AtZAT6 knockdown plants, but showed higher level in AtZAT6 overexpressing plants. AtZAT6 directly activated the expressions of TT5, TT7, TT3, TT18, MYB12, and MYB111 through binding to their promoters with TACAAT elements of these genes, and the activation of MYB12 and MYB111 up-regulated the expressions of TT4 and TT6. Taken together, this study indicates that AtZAT6 plays important role in H₂O₂-activated anthocyanin synthesis, via directly binding to the promoters of several genes that involved in anthocyanin synthesis.

Molecular mechanisms and ecological function of far-red light signalling

Land plants possess the ability to sense and respond to far-red light (700-760 nm), which serves as an important environmental cue. Due to the nature of far-red light, it is not absorbed by chlorophyll and thus is enriched in canopy shade and will also penetrate deeper into soil than other visible wavelengths. Far-red light responses include regulation of seed germination, suppression of hypocotyl growth, induction of flowering and accumulation of anthocyanins, which depend on one member of the phytochrome photoreceptor family, phytochrome A (phyA). Here, we review the current understanding of the underlying molecular mechanisms of how plants sense far-red light through phyA and the physiological responses to this light quality. Light-activated phytochromes act on two primary pathways within the nucleus; suppression of the E3 ubiquitin ligase complex CUL4/DDB1^COP1/SPA and inactivation of the PHYTOCHROME INTERACTING FACTOR (PIF) family of bHLH transcription factors. These pathways integrate with other signal transduction pathways, including phytohormones, for tissue and developmental stage specific responses. Unlike other phytochromes that mediate red-light responses, phyA is transported from the cytoplasm to the nucleus in far-red light by the shuttle proteins FAR-RED ELONGATED HYPOCOTYL 1 (FHY1) and FHY1-LIKE (FHL). However, additional mechanisms must exist that shift the action of phyA to far-red light; current hypotheses are discussed.

A Novel RGL2-DOF6 Complex Contributes to Primary Seed Dormancy in Arabidopsis thaliana by Regulating a GATA Transcription Factor

The DELLA protein RGA-LIKE2 (RGL2) is a key transcriptional repressor of gibberellic acid (GA) signaling that regulates seed germination. We identified GATA12, a gene encoding a GATA-type zinc finger transcription factor, as one of the downstream targets of RGL2 in Arabidopsis thaliana. Our data show that freshly harvested (unstratified) seeds of GATA12 antisense suppression lines have reduced dormancy compared with the wild-type, while ectopic expression lines show enhanced seed dormancy. We show that GATA12 expression is negatively regulated by GA, and its transcript levels decline dramatically under dormancy-breaking conditions such as dry storage and cold stratification of seeds. GATA12 promoter has several GAMYB- and DOF-associated motifs that are known to be GA- and RGL2-responsive, respectively. Chromatin immunoprecipitation assay showed that a protein complex containing RGL2 can bind to GATA12 promoter and thereby regulate its expression. RGL2 lacks a DNA binding domain and requires a transcription factor to induce GATA12 expression. Our data show that this RGL2-containing protein complex includes DNA BINDING1 ZINC FINGER6 (DOF6), which is a known negative regulator of germination in freshly harvested seeds. We further show that this novel RGL2-DOF6 complex is required for activating GATA12 expression, thus revealing a molecular mechanism to enforce primary seed dormancy.

Ectopic expression of specific GA2 oxidase mutants promotes yield and stress tolerance in rice

A major challenge of modern agricultural biotechnology is the optimization of plant architecture for enhanced productivity, stress tolerance and water use efficiency (WUE). To optimize plant height and tillering that directly link to grain yield in cereals and are known to be tightly regulated by gibberellins (GAs), we attenuated the endogenous levels of GAs in rice via its degradation. GA 2-oxidase (GA2ox) is a key enzyme that inactivates endogenous GAs and their precursors. We identified three conserved domains in a unique class of C₂₀ GA2ox, GA2ox6, which is known to regulate the architecture and function of rice plants. We mutated nine specific amino acids in these conserved domains and observed a gradient of effects on plant height. Ectopic expression of some of these GA2ox6 mutants moderately lowered GA levels and reprogrammed transcriptional networks, leading to reduced plant height, more productive tillers, expanded root system, higher WUE and photosynthesis rate, and elevated abiotic and biotic stress tolerance in transgenic rice. Combinations of these beneficial traits conferred not only drought and disease tolerance but also increased grain yield by 10-30% in field trials. Our studies hold the promise of manipulating GA levels to substantially improve plant architecture, stress tolerance and grain yield in rice and possibly in other major crops.

Gibberellin Signaling Requires Chromatin Remodeler PICKLE to Promote Vegetative Growth and Phase Transitions

PICKLE (PKL) is an ATP-dependent chromodomain-helicase-DNA-binding domain (CHD3) chromatin remodeling enzyme in Arabidopsis (Arabidopsis thaliana). Previous studies showed that PKL promotes embryonic-to-vegetative transition by inhibiting expression of seed-specific genes during seed germination. The pkl mutants display a low penetrance of the "pickle root" phenotype, with a thick and green primary root that retains embryonic characteristics. The penetrance of this pickle root phenotype in pkl is dramatically increased in gibberellin (GA)-deficient conditions. At adult stages, the pkl mutants are semidwarfs with delayed flowering time, which resemble reduced GA-signaling mutants. These findings suggest that PKL may play a positive role in regulating GA signaling. A recent biochemical analysis further showed that PKL and GA signaling repressors DELLAs antagonistically regulate hypocotyl cell elongation genes by direct protein-protein interaction. To elucidate further the role of PKL in GA signaling and plant development, we studied the genetic interaction between PKL and DELLAs using the hextuple mutant containing pkl and della pentuple (dP) mutations. Here, we show that PKL is required for most of GA-promoted developmental processes, including vegetative growth such as hypocotyl, leaf, and inflorescence stem elongation, and phase transitions such as juvenile-to-adult leaf and vegetative-to-reproductive phase. The removal of all DELLA functions (in the dP background) cannot rescue these phenotypes in pkl RNA-sequencing analysis using the ga1 (a GA-deficient mutant), pkl, and the ga1 pkl double mutant further shows that expression of 80% of GA-responsive genes in seedlings is PKL dependent, including genes that function in cell elongation, cell division, and phase transitions. These results indicate that the CHD3 chromatin remodeler PKL is required for regulating gene expression during most of GA-regulated developmental processes.

GA signalling and cross-talk with other signalling pathways

Gibberellins (GAs) are phytohormones that regulate growth and development. DELLA proteins repress GA responses. GA binding to its receptor triggers a series of events that culminate in the destruction of DELLA proteins by the 26S proteasome, which removes the repression of GA signalling. DELLA proteins are transcription co-activators that induce the expression of genes which encode products that inhibit GA responses. In addition to repressing GA responses, DELLA proteins influence the activity of other signalling pathways and serve as a central hub from which other pathways influence GA signalling. In this role, DELLA proteins bind to and inhibit proteins, including transcription factors that act in the signalling pathways of other hormones and light. The binding of these proteins to DELLA proteins also inhibits DELLA activity. GA signalling is subject to homoeostatic regulation through GA-induced repression of GA biosynthesis gene expression, and increased production of the GA receptor and enzymes that catabolize bioactive GAs. This review also discusses the nature of mutant DELLA alleles that are used to produce high-yielding 'Green Revolution' cereal varieties, and highlights important gaps in our knowledge of GA signalling.

A Pivotal Role of DELLAs in Regulating Multiple Hormone Signals

Plant phenotypic plasticity is controlled by diverse hormone pathways, which integrate and convey information from multiple developmental and environmental signals. Moreover, in plants many processes such as growth, development, and defense are regulated in similar ways by multiple hormones. Among them, gibberellins (GAs) are phytohormones with pleiotropic actions, regulating various growth processes throughout the plant life cycle. Previous work has revealed extensive interplay between GAs and other hormones, but the molecular mechanism became apparent only recently. Molecular and physiological studies have demonstrated that DELLA proteins, considered as master negative regulators of GA signaling, integrate multiple hormone signaling pathways through physical interactions with transcription factors or regulatory proteins from different families. In this review, we summarize the latest progress in GA signaling and its direct crosstalk with the main phytohormone signaling, emphasizing the multifaceted role of DELLA proteins with key components of major hormone signaling pathways.

The Arabidopsis MYB96 Transcription Factor Is a Positive Regulator of ABSCISIC ACID-INSENSITIVE4 in the Control of Seed Germination

Seed germination is a key developmental transition that initiates the plant life cycle. The timing of germination is determined by the coordinated action of two phytohormones, gibberellin and abscisic acid (ABA). In particular, ABA plays a key role in integrating environmental information and inhibiting the germination process. The utilization of embryonic lipid reserves contributes to seed germination by acting as an energy source, and ABA suppresses lipid degradation to modulate the germination process. Here, we report that the ABA-responsive R2R3-type MYB transcription factor MYB96, which is highly expressed in embryo, regulates seed germination by controlling the expression of abscisic acid-insensitive4 (ABI4) in Arabidopsis (Arabidopsis thaliana). In the presence of ABA, germination was accelerated in MYB96-deficient myb96-1 seeds, whereas the process was significantly delayed in MYB96-overexpressing activation-tagging myb96-ox seeds. Consistently, myb96-1 seeds degraded a larger extent of lipid reserves even in the presence of ABA, while reduced lipid mobilization was observed in myb96-ox seeds. MYB96 directly regulates ABI4, which acts as a repressor of lipid breakdown, to define its spatial and temporal expression. Genetic analysis further demonstrated that ABI4 is epistatic to MYB96 in the control of seed germination. Taken together, the MYB96-ABI4 module regulates lipid mobilization specifically in the embryo to ensure proper seed germination under suboptimal conditions.

New mechanistic links between sugar and hormone signalling networks

Plant growth and development must be coordinated with metabolism, notably with the efficiency of photosynthesis and the uptake of nutrients. This coordination requires local connections between hormonal response and metabolic state, as well as long-distance connections between shoot and root tissues. Recently, several molecular mechanisms have been proposed to explain the integration of sugar signalling with hormone pathways. In this work, DELLA and PIF proteins have emerged as hubs in sugar-hormone cross-regulation networks.

GA(3) enhances root responsiveness to exogenous IAA by modulating auxin transport and signalling in Arabidopsis

We used auxin-signalling mutants, auxin transport mutants, and auxin-related marker lines to show that exogenously applied GA enhances auxin-induced root inhibition by affecting auxin signalling and transport. Variation in root elongation is valuable when studying the interactions of phytohormones. Auxins influence the biosynthesis and signalling of gibberellins (GAs), but the influence of GAs on auxins in root elongation is poorly understood. This study was conducted to investigate the effect of GA3 on Arabidopsis root elongation in the presence of auxin. Root elongation was inhibited in roots treated with both IAA and GA3, compared to IAA alone, and the effect was dose dependent. Further experiments showed that GA3 could modulate auxin signalling based on root elongation in auxin-signalling mutants and the expression of auxin-responsive reporters. The GA3-enhanced inhibition of root elongation observed in the wild type was not found in the auxin-signalling mutants tir1-1 and axr1-3. GA3 increased DR5::GUS expression in the root meristem and elongation zones, and IAA2::GUS in the columella. The DR5rev::GFP signal was enhanced in columella cells of the root caps and in the elongation zone in GA3-treated seedling roots. A reduction was observed in the stele of PAC-treated roots. We also examined the effect of GA3 on auxin transport. The enhanced responsiveness caused by GA3 was not observed in the auxin influx mutant aux1-7 or the efflux mutant eir1-1. Additional molecular data demonstrated that GA3 could promote auxin transport via AUX1 and PIN proteins. However, GA3-induced PIN gene expression did not fully explain GA-enhanced PIN protein accumulation. These results suggest that GA3 is involved in auxin-mediated primary root elongation by modulating auxin signalling and transport, and thus enhances root responsiveness to exogenous IAA.

The transcriptional regulator BBX24 impairs DELLA activity to promote shade avoidance in Arabidopsis thaliana

In response to canopy shade, plant vegetative structures elongate to gain access to light. However, the mechanism that allows a plastic transcriptional response to canopy shade light is not fully elucidated. Here we propose that the activity of PIF4, a key transcription factor in the shade signalling network, is modulated by the interplay between the BBX24 transcriptional regulator and DELLA proteins, which are negative regulators of the gibberellin (GA) signalling pathway. We show that GA-related targets are enriched among genes responsive to BBX24 under shade and that the shade-response defect in bbx24 mutants is rescued by a GA treatment that promotes DELLA degradation. BBX24 physically interacts with DELLA proteins and alleviates DELLA-mediated repression of PIF4 activity. The proposed molecular mechanism provides reversible regulation of the activity of a key transcription factor that may prove especially relevant under fluctuating light conditions.

Characterization of Arabidopsis thaliana GCN2 kinase roles in seed germination and plant development

Eukaryotic GCN2 (general control nonderepressible 2) is a serine/threonine protein kinase that plays an essential role in modulating amino acid metabolism in response to nutrient deprivation. A wide spectrum of GCN2 functions in yeast and mammals has been characterized that spans from responses to amino acid deficiency, development, differentiation and proper functions of mammalian organs to organism's life span, tumor cell survival and immune responses. Here we demonstrate that Arabidopsis thaliana GCN2 (AtGCN2) plays crucial roles in plant growth and development. We present evidence that AtGCN2 negatively regulates seed germination under diverse environmental conditions. Our genetic data supported the notion that AtGCN2 is required for leaf morphology and normal cellular physiology by controlling chlorophyll contents. Our gene expression analyses revealed that AtGCN2 negatively regulates several transcription factor genes that play important roles in plant gibberellic acid-related crosstalk. We concluded that AtGCN2 plays pivotal roles in various cellular processes essential for normal growth and development, hence expanding the functions of this general regulator beyond being merely a stress player.

Arabidopsis DELLA and two HD-ZIP transcription factors regulate GA signaling in the epidermis through the L1 box cis-element

Gibberellins (GAs) are plant hormones that affect plant growth and regulate gene expression differentially across tissues. To study the molecular mechanisms underlying GA signaling in Arabidopsis thaliana, we focused on a GDSL lipase gene (LIP1) induced by GA and repressed by DELLA proteins. LIP1 contains an L1 box promoter sequence, conserved in the promoters of epidermis-specific genes, that is bound by ATML1, an HD-ZIP transcription factor required for epidermis specification. In this study, we demonstrate that LIP1 is specifically expressed in the epidermis and that its L1 box sequence mediates GA-induced transcription. We show that this sequence is overrepresented in the upstream regulatory regions of GA-induced and DELLA-repressed transcriptomes and that blocking GA signaling in the epidermis represses the expression of L1 box-containing genes and negatively affects seed germination. We show that DELLA proteins interact directly with ATML1 and its paralogue PDF2 and that silencing of both HD-ZIP transcription factors inhibits epidermal gene expression and delays germination. Our results indicate that, upon seed imbibition, increased GA levels reduce DELLA protein abundance and release ATML1/PDF2 to activate L1 box gene expression, thus enhancing germination potential.

Connecting growth and defense: the emerging roles of brassinosteroids and gibberellins in plant innate immunity

Brassinosteroids (BRs) and gibberellins (GAs) are two groups of phytohormones that regulate many common developmental processes throughout the plant life cycle. Fueled by large-scale 'omics' technologies and the burgeoning field of plant computational biology, the past few years have witnessed paradigm-shifting advances in our understanding of how BRs and GA are perceived and their signals transduced. Accumulating evidence also implicates BR and GA in the coordination and integration of plant immune responses. Similarly to other growth regulators, BR and GA play ambiguous roles in molding pathological outcomes, the effects of which may depend not only on the pathogen's lifestyle and infection strategy, but also on specialized features of each interaction. Analysis of the underpinning molecular mechanisms points to a crucial role of GA-inhibiting DELLA proteins and the BR-regulated transcription factor BZR1. Acting at the interface of developmental and defense signaling, these proteins likely serve as central hubs for pathway crosstalk and signal integration, allowing appropriate modulation of plant growth and defense in response to various stimuli. In this review, we outline the latest discoveries dealing with BR and GA modulation of plant innate immunity and highlight interactions between BR and GA signaling, plant defense, and microbial virulence.

DELLA protein functions as a transcriptional activator through the DNA binding of the indeterminate domain family proteins

DELLA protein is a key negative regulator of gibberellin (GA) signaling. Although how DELLA regulates downstream gene expression remains unclear, DELLA has been proposed to function as a transcriptional activator. However, because DELLA lacks a DNA-binding domain, intermediate protein(s) mediating the DELLA/DNA interaction are believed to be necessary for activating DELLA target genes. Here, using yeast hybrid screenings, we identified five members of indeterminate domain (IDD) protein family which bind physically to both DELLA and the promoter sequence of the GA-positive regulator SCARECROW-LIKE 3 (SCL3), which previously was characterized as a DELLA direct target gene. Transient assays using Arabidopsis protoplasts demonstrated that a luciferase reporter controlled by the SCL3 promoter was additively transactivated by REPRESSOR of ga1-3 (RGA) and IDDs. Phenotypic analysis of transgenic plants expressing AtIDD3 (one of the 16 IDDs in the Arabidopsis genome) fused with the plant-specific repression domain (SRDX) supported the possibility that AtIDD3 is positively involved in GA signaling. In addition, we found that SCL3 protein also interacts with IDDs, resulting in the suppression of its target gene expression. In this context, DELLA and SCL3 interact competitively with IDD proteins to regulate downstream gene expression. These results suggest that the coregulators DELLA and SCL3, using IDDs as transcriptional scaffolds for DNA binding, antagonistically regulate the expression of their downstream targets to control the GA signaling pathway.

Gibberellins and DELLAs: central nodes in growth regulatory networks

Gibberellins (GAs) are growth-promoting phytohormones that were crucial in breeding improved semi-dwarf varieties during the green revolution. However, the molecular basis for GA-induced growth stimulation is poorly understood. In this review, we use light-regulated hypocotyl elongation as a case study, combined with a meta-analysis of available transcriptome data, to discuss the role of GAs as central nodes in networks connecting environmental inputs to growth. These networks are highly tissue-specific, with dynamic and rapid regulation that mostly occurs at the protein level, directly affecting the activity and transcription of effectors. New systems biology approaches addressing the role of GAs in growth should take these properties into account, combining tissue-specific interactomics, transcriptomics and modeling, to provide essential knowledge to fuel a second green revolution.

DELLA proteins modulate Arabidopsis defences induced in response to caterpillar herbivory

Upon insect herbivory, many plant species change the direction of metabolic flux from growth into defence. Two key pathways modulating these processes are the gibberellin (GA)/DELLA pathway and the jasmonate pathway. In this study, the effect of caterpillar herbivory on plant-induced responses was compared between wild-type Arabidopsis thaliana (L.) Heynh. and quad-della mutants that have constitutively elevated GA responses. The labial saliva (LS) of caterpillars of the beet armyworm, Spodoptera exigua, is known to influence induced plant defence responses. To determine the role of this herbivore cue in determining metabolic shifts, plants were subject to herbivory by caterpillars with intact or impaired LS secretions. In both wild-type and quad-della plants, a jasmonate burst is an early response to caterpillar herbivory. Negative growth regulator DELLA proteins are required for the LS-mediated suppression of hormone levels. Jasmonate-dependent marker genes are induced in response to herbivory independently of LS, with the exception of AtPDF1.2 that showed LS-dependent expression in the quad-della mutant. Early expression of the salicylic acid (SA)-marker gene, AtPR1, was not affected by herbivory which also reflected SA hormone levels; however, this gene showed LS-dependent expression in the quad-della mutant. DELLA proteins may positively regulate glucosinolate levels and suppress laccase-like multicopper oxidase activity in response to herbivory. The present results show a link between DELLA proteins and early, induced plant defences in response to insect herbivory; in particular, these proteins are necessary for caterpillar LS-associated attenuation of defence hormones.

Phototropism: growing towards an understanding of plant movement

Phototropism, or the differential cell elongation exhibited by a plant organ in response to directional blue light, provides the plant with a means to optimize photosynthetic light capture in the aerial portion and water and nutrient acquisition in the roots. Tremendous advances have been made in our understanding of the molecular, biochemical, and cellular bases of phototropism in recent years. Six photoreceptors and their associated signaling pathways have been linked to phototropic responses under various conditions. Primary detection of directional light occurs at the plasma membrane, whereas secondary modulatory photoreception occurs in the cytoplasm and nucleus. Intracellular responses to light cues are processed to regulate cell-to-cell movement of auxin to allow establishment of a trans-organ gradient of the hormone. Photosignaling also impinges on the transcriptional regulation response established as a result of changes in local auxin concentrations. Three additional phytohormone signaling pathways have also been shown to influence phototropic responsiveness, and these pathways are influenced by the photoreceptor signaling as well. Here, we will discuss this complex dance of intra- and intercellular responses that are regulated by these many systems to give rise to a rapid and robust adaptation response observed as organ bending.

ABA-insensitive3, ABA-insensitive5, and DELLAs Interact to activate the expression of SOMNUS and other high-temperature-inducible genes in imbibed seeds in Arabidopsis

Seeds monitor the environment to germinate at the proper time, but different species respond differently to environmental conditions, particularly light and temperature. In Arabidopsis thaliana, light promotes germination but high temperature suppresses germination. We previously reported that light promotes germination by repressing SOMNUS (SOM). Here, we examined whether high temperature also regulates germination through SOM and found that high temperature activates SOM expression. Consistent with this, som mutants germinated more frequently than the wild type at high temperature. The induction of SOM mRNA at high temperature required abscisic acid (ABA) and gibberellic acid biosynthesis, and ABA-insensitive3 (ABI3), ABI5, and DELLAs positively regulated SOM expression. Chromatin immunoprecipitation assays indicated that ABI3, ABI5, and DELLAs all target the SOM promoter. At the protein level, ABI3, ABI5, and DELLAs all interact with each other, suggesting that they form a complex on the SOM promoter to activate SOM expression at high temperature. We found that high-temperature-inducible genes frequently have RY motifs and ABA-responsive elements in their promoters, some of which are targeted by ABI3, ABI5, and DELLAs in vivo. Taken together, our data indicate that ABI3, ABI5, and DELLAs mediate high-temperature signaling to activate the expression of SOM and other high-temperature-inducible genes, thereby inhibiting seed germination.

ABA-insensitive3, ABA-insensitive5, and DELLAs Interact to activate the expression of SOMNUS and other high-temperature-inducible genes in imbibed seeds in Arabidopsis

Seeds monitor the environment to germinate at the proper time, but different species respond differently to environmental conditions, particularly light and temperature. In Arabidopsis thaliana, light promotes germination but high temperature suppresses germination. We previously reported that light promotes germination by repressing SOMNUS (SOM). Here, we examined whether high temperature also regulates germination through SOM and found that high temperature activates SOM expression. Consistent with this, som mutants germinated more frequently than the wild type at high temperature. The induction of SOM mRNA at high temperature required abscisic acid (ABA) and gibberellic acid biosynthesis, and ABA-insensitive3 (ABI3), ABI5, and DELLAs positively regulated SOM expression. Chromatin immunoprecipitation assays indicated that ABI3, ABI5, and DELLAs all target the SOM promoter. At the protein level, ABI3, ABI5, and DELLAs all interact with each other, suggesting that they form a complex on the SOM promoter to activate SOM expression at high temperature. We found that high-temperature-inducible genes frequently have RY motifs and ABA-responsive elements in their promoters, some of which are targeted by ABI3, ABI5, and DELLAs in vivo. Taken together, our data indicate that ABI3, ABI5, and DELLAs mediate high-temperature signaling to activate the expression of SOM and other high-temperature-inducible genes, thereby inhibiting seed germination.

A DELLA gene, RhGAI1, is a direct target of EIN3 and mediates ethylene-regulated rose petal cell expansion via repressing the expression of RhCesA2

Ethylene plays an important role in organ growth. In Arabidopsis, ethylene can inhibit root elongation by stabilizing DELLA proteins. In previous work, it was found that ethylene suppressed cell expansion in rose petals, and five unisequences of DELLA genes are induced by ethylene. However, the mechanism of transcriptional regulation of DELLA genes by ethylene is still not clear. The results showed that the expression of RhGAI1 was induced in both ethylene-treated and ETR gene-silenced rose petals, and the promoter activity of RhGAI1 was strongly induced by RhEIN3-3 in Arabidopsis protoplasts. What is more, RhEIN3-3 could bind to the promoter of RhGAI1 directly in an electrophoretic mobility shift assay (EMSA). Cell expansion was suppressed in RhGAI1-Δ17-overexpressed Arabidopsis petals and promoted in RhGAI1-silenced rose petals. Moreover, in RhGAI1-silenced petals, the expression of nine cell expansion-related genes was clearly changed, and RhGAI1 can bind to the promoter of RhCesA2 in an EMSA. These results suggested that RhGAI1 was regulated by ethylene at the transcriptional level, and RhGAI1 was a direct target of RhEIN3-3. Also, RhGAI1 was shown to be involved in cell expansion partially through regulating the expression of cell expansion-related genes. Furthermore, RhCesA2 was a direct target of RhGAI1. This work uncovers the transcriptional regulation of RhGAI1 by ethylene and provides a better understanding of how ethylene regulates petal expansion in roses.

Gibberellic acid in plant: still a mystery unresolved

Gibberellic acid (GA), a plant hormone stimulating plant growth and development, is a tetracyclic di-terpenoid compound. GAs stimulate seed germination, trigger transitions from meristem to shoot growth, juvenile to adult leaf stage, vegetative to flowering, determines sex expression and grain development along with an interaction of different environmental factors viz., light, temperature and water. The major site of bioactive GA is stamens that influence male flower production and pedicel growth. However, this opens up the question of how female flowers regulate growth and development, since regulatory mechanisms/organs other than those in male flowers are mandatory. Although GAs are thought to act occasionally like paracrine signals do, it is still a mystery to understand the GA biosynthesis and its movement. It has not yet confirmed the appropriate site of bioactive GA in plants or which tissues targeted by bioactive GAs to initiate their action. Presently, it is a great challenge for scientific community to understand the appropriate mechanism of GA movement in plant's growth, floral development, sex expression, grain development and seed germination. The appropriate elucidation of GA transport mechanism is essential for the survival of plant species and successful crop production.

Lifting della repression of Arabidopsis seed germination by nonproteolytic gibberellin signaling

DELLA repression of Arabidopsis (Arabidopsis thaliana) seed germination can be lifted either through DELLA proteolysis by the ubiquitin-proteasome pathway or through proteolysis-independent gibberellin (GA) hormone signaling. GA binding to the GIBBERELLIN-INSENSITIVE DWARF1 (GID1) GA receptors stimulates GID1-GA-DELLA complex formation, which in turn triggers DELLA protein ubiquitination and proteolysis via the SCF(SLY1) E3 ubiquitin ligase and 26S proteasome. Although DELLA cannot be destroyed in the sleepy1-2 (sly1-2) F-box mutant, long dry after-ripening and GID1 overexpression can relieve the strong sly1-2 seed dormancy phenotype. It appears that sly1-2 seed dormancy results from abscisic acid (ABA) signaling downstream of DELLA, since dormant sly1-2 seeds accumulate high levels of ABA hormone and loss of ABA sensitivity rescues sly1-2 seed germination. DELLA positively regulates the expression of XERICO, an inducer of ABA biosynthesis. GID1b overexpression rescues sly1-2 germination through proteolysis-independent DELLA down-regulation associated with increased expression of GA-inducible genes and decreased ABA accumulation, apparently as a result of decreased XERICO messenger RNA levels. Higher levels of GID1 overexpression are associated with more efficient sly1 germination and increased GID1-GA-DELLA complex formation, suggesting that GID1 down-regulates DELLA through protein binding. After-ripening results in increased GA accumulation and GID1a-dependent GA signaling, suggesting that after-ripening triggers GA-stimulated GID1-GA-DELLA protein complex formation, which in turn blocks DELLA transcriptional activation of the XERICO inhibitor of seed germination.

Genomic analysis of DELLA protein activity

Changes in gene expression are the main outcome of hormone signaling cascades that widely control plant physiology. In the case of the hormones gibberellins, the transcriptional control is exerted through the activity of the DELLA proteins, which act as negative regulators in the signaling pathway. This review focuses on recent transcriptomic approaches in the context of gibberellin signaling, which have provided useful information on new processes regulated by these hormones such as the regulation of photosynthesis and gravitropism. Moreover, the enrichment of specific cis-elements among DELLA primary targets has also helped extend the view that DELLA proteins regulate gene expression through the interaction with multiple transcription factors from different families.

Dynamic regulation of cortical microtubule organization through prefoldin-DELLA interaction

Plant morphogenesis relies on specific patterns of cell division and expansion. It is well established that cortical microtubules influence the direction of cell expansion, but less is known about the molecular mechanisms that regulate microtubule arrangement. Here we show that the phytohormones gibberellins (GAs) regulate microtubule orientation through physical interaction between the nuclear-localized DELLA proteins and the prefoldin complex, a cochaperone required for tubulin folding. In the presence of GA, DELLA proteins are degraded, and the prefoldin complex stays in the cytoplasm and is functional. In the absence of GA, the prefoldin complex is localized to the nucleus, which severely compromises α/β-tubulin heterodimer availability, affecting microtubule organization. The physiological relevance of this molecular mechanism was confirmed by the observation that the daily rhythm of plant growth was accompanied by coordinated oscillation of DELLA accumulation, prefoldin subcellular localization, and cortical microtubule reorientation.

BRAHMA ATPase of the SWI/SNF chromatin remodeling complex acts as a positive regulator of gibberellin-mediated responses in arabidopsis

SWI/SNF chromatin remodeling complexes perform a pivotal function in the regulation of eukaryotic gene expression. Arabidopsis (Arabidopsis thaliana) mutants in major SWI/SNF subunits display embryo-lethal or dwarf phenotypes, indicating their critical role in molecular pathways controlling development and growth. As gibberellins (GA) are major positive regulators of plant growth, we wanted to establish whether there is a link between SWI/SNF and GA signaling in Arabidopsis. This study revealed that in brm-1 plants, depleted in SWI/SNF BRAHMA (BRM) ATPase, a number of GA-related phenotypic traits are GA-sensitive and that the loss of BRM results in markedly decreased level of endogenous bioactive GA. Transcriptional profiling of brm-1 and the GA biosynthesis mutant ga1-3, as well as the ga1-3/brm-1 double mutant demonstrated that BRM affects the expression of a large set of GA-responsive genes including genes responsible for GA biosynthesis and signaling. Furthermore, we found that BRM acts as an activator and directly associates with promoters of GA3ox1, a GA biosynthetic gene, and SCL3, implicated in positive regulation of the GA pathway. Many GA-responsive gene expression alterations in the brm-1 mutant are likely due to depleted levels of active GAs. However, the analysis of genetic interactions between BRM and the DELLA GA pathway repressors, revealed that BRM also acts on GA-responsive genes independently of its effect on GA level. Given the central position occupied by SWI/SNF complexes within regulatory networks controlling fundamental biological processes, the identification of diverse functional intersections of BRM with GA-dependent processes in this study suggests a role for SWI/SNF in facilitating crosstalk between GA-mediated regulation and other cellular pathways.

DELLA proteins and their interacting RING Finger proteins repress gibberellin responses by binding to the promoters of a subset of gibberellin-responsive genes in Arabidopsis

DELLA proteins, consisting of GA INSENSITIVE, REPRESSOR OF GA1-3, RGA-LIKE1 (RGL1), RGL2, and RGL3, are central repressors of gibberellin (GA) responses, but their molecular functions are not fully understood. We isolated four DELLA-interacting RING domain proteins, previously designated as BOTRYTIS SUSCEPTIBLE1 INTERACTOR (BOI), BOI-RELATED GENE1 (BRG1), BRG2, and BRG3 (collectively referred to as BOIs). Single mutants of each BOI gene failed to significantly alter GA responses, but the boi quadruple mutant (boiQ) showed a higher seed germination frequency in the presence of paclobutrazol, precocious juvenile-to-adult phase transition, and early flowering, all of which are consistent with enhanced GA signaling. By contrast, BOI overexpression lines displayed phenotypes consistent with reduced GA signaling. Analysis of a gai-1 boiQ pentuple mutant further indicated that the GAI protein requires BOIs to inhibit a subset of GA responses. At the molecular level, BOIs did not significantly alter the stability of a DELLA protein. Instead, BOI and DELLA proteins are targeted to the promoters of a subset of GA-responsive genes and repress their expression. Taken together, our results indicate that the DELLA and BOI proteins inhibit GA responses by interacting with each other, binding to the same promoters of GA-responsive genes, and repressing these genes.

Medicago truncatula ERN transcription factors: regulatory interplay with NSP1/NSP2 GRAS factors and expression dynamics throughout rhizobial infection

Rhizobial nodulation factors (NFs) activate a specific signaling pathway in Medicago truncatula root hairs that involves the complex interplay of Nodulation Signaling Pathway1 (NSP1)/NSP2 GRAS and Ethylene Response Factor Required for Nodulation1 (ERN1) transcription factors (TFs) to achieve full ENOD11 transcription. ERN1 acts as a direct transcriptional regulator of ENOD11 through the activation of the NF-responsive "NF box." Here, we show that NSP1, when combined with NSP2, can act as a strong positive regulator of ERN1 and ENOD11 transcription. Although ERN1 and NSP1/NSP2 both activate ENOD11, two separate promoter regions are involved that regulate expression during consecutive symbiotic stages. Our findings indicate that ERN1 is required to activate NF-elicited ENOD11 expression exclusively during early preinfection, while NSP1/NSP2 mediates ENOD11 expression during subsequent rhizobial infection. The relative contributions of ERN1 and the closely related ERN2 to the rhizobial symbiosis were then evaluated by comparing their regulation and in vivo dynamics. ERN1 and ERN2 exhibit expression profiles compatible with roles during NF signaling and subsequent infection. However, differences in expression levels and spatiotemporal profiles suggest specialized functions for these two TFs, ERN1 being involved in stages preceding and accompanying infection thread progression while ERN2 is only involved in certain stages of infection. By cross complementation, we show that ERN2, when expressed under the control of the ERN1 promoter, can restore both NF-elicited ENOD11 expression and nodule formation in an ern1 mutant background. This indicates that ERN1 and ERN2 possess similar biological activities and that functional diversification of these closely related TFs relies primarily on changes in tissue-specific expression patterns.

Overexpression of RoDELLA impacts the height, branching, and flowering behaviour of Pelargonium × domesticum transgenic plants

KEY MESSAGE : We reported the cloning of a rose DELLA gene. We obtained transgenic Pelargonium lines overexpressing this gene which presented several phenotypes in plant growth, root growth, flowering time and number of inflorescences. Control of development is an important issue for production of ornamental plant. The plant growth regulator, gibberellins (GAs), plays a pivotal role in regulating plant growth and development. DELLA proteins are nuclear negative regulator of GA signalling. Our objective was to study the role of GA in the plant architecture and in the blooming of ornamentals. We cloned a rose DELLA homologous gene, RoDELLA, and studied its function by genetic transformation of pelargonium. Several transgenic pelargonium (Pelargonium × domesticum 'Autum Haze') lines were produced that ectopically expressed RoDELLA under the control of the 35S promoter. These transgenic plants exhibited a range of phenotypes which could be related to the reduction in GA response. Most of transgenic plants showed reduced growth associated to an increase of the node and branch number. Moreover, overexpression of RoDELLA blocked or delayed flowering in transgenic pelargonium and exhibited defects in the root formation. We demonstrated that pelargonium could be used to validate ornamental gene as the rose DELLA gene. RoDELLA overexpression modified many aspects of plant developmental pathways, as the plant growth, the transition of vegetative to floral stage and the ability of rooting.

Molecular mechanism for the interaction between gibberellin and brassinosteroid signaling pathways in Arabidopsis

Plant development is modulated by the convergence of multiple environmental and endogenous signals, and the mechanisms that allow the integration of different signaling pathways is currently being unveiled. A paradigmatic case is the concurrence of brassinosteroid (BR) and gibberellin (GA) signaling in the control of cell expansion during photomorphogenesis, which is supported by physiological observations in several plants but for which no molecular mechanism has been proposed. In this work, we show that the integration of these two signaling pathways occurs through the physical interaction between the DELLA protein GAI, which is a major negative regulator of the GA pathway, and BRASSINAZOLE RESISTANT1 (BZR1), a transcription factor that broadly regulates gene expression in response to BRs. We provide biochemical evidence, both in vitro and in vivo, indicating that GAI inactivates the transcriptional regulatory activity of BZR1 upon their interaction by inhibiting the ability of BZR1 to bind to target promoters. The physiological relevance of this interaction was confirmed by the observation that the dominant gai-1 allele interferes with BR-regulated gene expression, whereas the bzr1-1D allele displays enhanced resistance to DELLA accumulation during hypocotyl elongation. Because DELLA proteins mediate the response to multiple environmental signals, our results provide an initial molecular framework for the integration with BRs of additional pathways that control plant development.