Molecular interactions of a soluble gibberellin receptor, GID1, with a rice DELLA protein, SLR1, and gibberellin

The suppressive function of the rice DELLA protein SLR1 is dependent on its transcriptional activation activity

When the gibberellin (GA) receptor GIBBERELLIN INSENSITIVE DWARF 1 (GID1) binds to GA, GID1 interacts with DELLA proteins, repressors of GA signaling. This interaction inhibits the suppressive function of DELLA protein and thereby activates the GA response. However, how DELLA proteins exert their suppressive function and how GID1s inhibit suppressive function of DELLA proteins is unclear. By yeast one-hybrid experiments and transient expression of the N-terminal region of rice DELLA protein (SLR1) in rice callus, we established that the N-terminal DELLA/TVHYNP motif of SLR1 possesses transactivation activity. When SLR1 proteins with various deletions were over-expressed in rice, the severity of dwarfism correlated with the transactivation activity observed in yeast, indicating that SLR1 suppresses plant growth through transactivation activity. This activity was suppressed by the GA-dependent GID1-SLR1 interaction, which may explain why GA responses are induced in the presence of GA. The C-terminal GRAS domain of SLR1 also exhibits a suppressive function on plant growth, possibly by directly or indirectly interacting with the promoter region of target genes. Our results indicate that the N-terminal region of SLR1 has two roles in GA signaling: interaction with GID1 and transactivation activity.

Convergent starvation signals and hormone crosstalk in regulating nutrient mobilization upon germination in cereals

Germination is a unique developmental transition from metabolically quiescent seed to actively growing seedling that requires an ensemble of hydrolases for coordinated nutrient mobilization to support heterotrophic growth until autotrophic photosynthesis is established. This study reveals two crucial transcription factors, MYBS1 and MYBGA, present in rice (Oryza sativa) and barley (Hordeum vulgare), that function to integrate diverse nutrient starvation and gibberellin (GA) signaling pathways during germination of cereal grains. Sugar represses but sugar starvation induces MYBS1 synthesis and its nuclear translocation. GA antagonizes sugar repression by enhancing conuclear transport of the GA-inducible MYBGA with MYBS1 and the formation of a stable bipartite MYB-DNA complex to activate the α-amylase gene. We further discovered that not only sugar but also nitrogen and phosphate starvation signals converge and interconnect with GA to promote the conuclear import of MYBS1 and MYBGA, resulting in the expression of a large set of GA-inducible but functionally distinct hydrolases, transporters, and regulators associated with mobilization of the full complement of nutrients to support active seedling growth in cereals.

Growth-induced hormone dilution can explain the dynamics of plant root cell elongation

In the elongation zone of the Arabidopsis thaliana plant root, cells undergo rapid elongation, increasing their length by ∼10-fold over 5 h while maintaining a constant radius. Although progress is being made in understanding how this growth is regulated, little consideration has been given as to how cell elongation affects the distribution of the key regulating hormones. Using a multiscale mathematical model and measurements of growth dynamics, we investigate the distribution of the hormone gibberellin in the root elongation zone. The model quantifies how rapid cell expansion causes gibberellin to dilute, creating a significant gradient in gibberellin levels. By incorporating the gibberellin signaling network, we simulate how gibberellin dilution affects the downstream components, including the growth-repressing DELLA proteins. We predict a gradient in DELLA that provides an explanation of the reduction in growth exhibited as cells move toward the end of the elongation zone. These results are validated at the molecular level by comparing predicted mRNA levels with transcriptomic data. To explore the dynamics further, we simulate perturbed systems in which gibberellin levels are reduced, considering both genetically modified and chemically treated roots. By modeling these cases, we predict how these perturbations affect gibberellin and DELLA levels and thereby provide insight into their altered growth dynamics.

Mathematical modeling elucidates the role of transcriptional feedback in gibberellin signaling

The hormone gibberellin (GA) is a key regulator of plant growth. Many of the components of the gibberellin signal transduction [e.g., GIBBERELLIN INSENSITIVE DWARF 1 (GID1) and DELLA], biosynthesis [e.g., GA 20-oxidase (GA20ox) and GA3ox], and deactivation pathways have been identified. Gibberellin binds its receptor, GID1, to form a complex that mediates the degradation of DELLA proteins. In this way, gibberellin relieves DELLA-dependent growth repression. However, gibberellin regulates expression of GID1, GA20ox, and GA3ox, and there is also evidence that it regulates DELLA expression. In this paper, we use integrated mathematical modeling and experiments to understand how these feedback loops interact to control gibberellin signaling. Model simulations are in good agreement with in vitro data on the signal transduction and biosynthesis pathways and in vivo data on the expression levels of gibberellin-responsive genes. We find that GA-GID1 interactions are characterized by two timescales (because of a lid on GID1 that can open and close slowly relative to GA-GID1 binding and dissociation). Furthermore, the model accurately predicts the response to exogenous gibberellin after a number of chemical and genetic perturbations. Finally, we investigate the role of the various feedback loops in gibberellin signaling. We find that regulation of GA20ox transcription plays a significant role in both modulating the level of endogenous gibberellin and generating overshoots after the removal of exogenous gibberellin. Moreover, although the contribution of other individual feedback loops seems relatively small, GID1 and DELLA transcriptional regulation acts synergistically with GA20ox feedback.

Evaluating the potential of SHI expression as a compacting tool for ornamental plants

Control of plant growth, especially elongation of stems, is important in modern plant production, and many plant species, including cereals, grasses, fruit trees and ornamentals, are regularly treated chemically to control their stature and flowering time. Chemical treatments ensure short, homogenous plants, which are more robust and easy to harvest, transport and sell. Although growth retardants are an expensive and undesirable step in plant production, it is unfortunately necessary at present. Compact growth is desirable in most ornamentals and this trait can be difficult to obtain by traditional breeding. As an alternative, biotechnology could provide plant varieties with optimized growth habits. This review is an introduction to the family of SHI transcription factors, which has recently been used to produce compact plants of very diverse species. The possible functions and regulations of the SHI proteins are discussed, and the potential of using overexpression as means to dwarf plants is assessed. In conclusion the breeding of some species, especially flowering ornamentals, could benefit from this strategy. Furthermore, detailed knowledge about the role of SHI proteins in plant growth and development could help shed more light on the interactions between plant hormone signaling pathways.

Stamen-derived bioactive gibberellin is essential for male flower development of Cucurbita maxima L

Gibberellin (GA) signalling during pumpkin male flower development is highly regulated, including biosynthetic, perception, and transduction pathways. GA 20-oxidases, 3-oxidases, and 2-oxidases catalyse the final part of GA synthesis. Additionally, 7-oxidase initiates this part of the pathway in some cucurbits including Cucurbita maxima L. (pumpkin). Expression patterns for these GA-oxidase-encoding genes were examined by competitive reverse transcription-PCR (RT-PCR) and endogenous GA levels were determined during pumpkin male flower development. In young flowers, GA20ox3 transcript levels are high in stamens, followed by high levels of the GA precursor GA(9). Later, just before flower opening, transcript levels for GA3ox3 and GA3ox4 increase in the hypanthium and stamens, respectively. In the stamen, following GA3ox4 expression, bioactive GA(4) levels rise dramatically. Accordingly, catabolic GA2ox2 and GA2ox3 transcript levels are low in developing flowers, and increase in mature flowers. Putative GA receptor GID1b and DELLA repressor GAIPb transcript levels do not change in developing flowers, but increase sharply in mature flowers. Emasculation arrests floral development completely and leads to abscission of premature flowers. Application of GA(4) (but not of its precursors GA(12)-aldehyde or GA(9)) restores normal growth of emasculated flowers. These results indicate that de novo GA(4) synthesis in the stamen is under control of GA20ox3 and GA3ox4 genes just before the rapid flower growth phase. Stamen-derived bioactive GA is essential and sufficient for male flower development, including the petal and the pedicel growth.

Stamen-derived bioactive gibberellin is essential for male flower development of Cucurbita maxima L

Gibberellin (GA) signalling during pumpkin male flower development is highly regulated, including biosynthetic, perception, and transduction pathways. GA 20-oxidases, 3-oxidases, and 2-oxidases catalyse the final part of GA synthesis. Additionally, 7-oxidase initiates this part of the pathway in some cucurbits including Cucurbita maxima L. (pumpkin). Expression patterns for these GA-oxidase-encoding genes were examined by competitive reverse transcription-PCR (RT-PCR) and endogenous GA levels were determined during pumpkin male flower development. In young flowers, GA20ox3 transcript levels are high in stamens, followed by high levels of the GA precursor GA(9). Later, just before flower opening, transcript levels for GA3ox3 and GA3ox4 increase in the hypanthium and stamens, respectively. In the stamen, following GA3ox4 expression, bioactive GA(4) levels rise dramatically. Accordingly, catabolic GA2ox2 and GA2ox3 transcript levels are low in developing flowers, and increase in mature flowers. Putative GA receptor GID1b and DELLA repressor GAIPb transcript levels do not change in developing flowers, but increase sharply in mature flowers. Emasculation arrests floral development completely and leads to abscission of premature flowers. Application of GA(4) (but not of its precursors GA(12)-aldehyde or GA(9)) restores normal growth of emasculated flowers. These results indicate that de novo GA(4) synthesis in the stamen is under control of GA20ox3 and GA3ox4 genes just before the rapid flower growth phase. Stamen-derived bioactive GA is essential and sufficient for male flower development, including the petal and the pedicel growth.

Brassinosteroids antagonize gibberellin- and salicylate-mediated root immunity in rice

Brassinosteroids (BRs) are a unique class of plant steroid hormones that orchestrate myriad growth and developmental processes. Although BRs have long been known to protect plants from a suite of biotic and abiotic stresses, our understanding of the underlying molecular mechanisms is still rudimentary. Aiming to further decipher the molecular logic of BR-modulated immunity, we have examined the dynamics and impact of BRs during infection of rice (Oryza sativa) with the root oomycete Pythium graminicola. Challenging the prevailing view that BRs positively regulate plant innate immunity, we show that P. graminicola exploits BRs as virulence factors and hijacks the rice BR machinery to inflict disease. Moreover, we demonstrate that this immune-suppressive effect of BRs is due, at least in part, to negative cross talk with salicylic acid (SA) and gibberellic acid (GA) pathways. BR-mediated suppression of SA defenses occurred downstream of SA biosynthesis, but upstream of the master defense regulators NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 and OsWRKY45. In contrast, BR alleviated GA-directed immune responses by interfering at multiple levels with GA metabolism, resulting in indirect stabilization of the DELLA protein and central GA repressor SLENDER RICE1 (SLR1). Collectively, these data favor a model whereby P. graminicola coopts the plant BR pathway as a decoy to antagonize effectual SA- and GA-mediated defenses. Our results highlight the importance of BRs in modulating plant immunity and uncover pathogen-mediated manipulation of plant steroid homeostasis as a core virulence strategy.

Rapid and orthogonal logic gating with a gibberellin-induced dimerization system

Using a newly synthesized gibberellin analog containing an acetoxymethyl group (GA(3)-AM) and its binding proteins, we developed an efficient chemically inducible dimerization (CID) system that is completely orthogonal to existing rapamycin-mediated protein dimerization. Combining the two systems should allow applications that have been difficult or impossible with only one CID system. By using both chemical inputs (rapamycin and GA(3)-AM), we designed and synthesized Boolean logic gates in living mammalian cells. These gates produced output signals such as fluorescence and membrane ruffling on a timescale of seconds, substantially faster than earlier intracellular logic gates. The use of two orthogonal dimerization systems in the same cell also allows for finer modulation of protein perturbations than is possible with a single dimerizer.

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.

Gibberellins – terpenoid plant hormones: Biological importance and chemical analysis

Gibberellins (GAs) are a large group of diterpenoid carboxylic acids, some members of which function as plant hormones controlling diverse aspects of growth and development. Biochemical, genetic, and genomic approaches have led to the identification of the majority of the genes that encode GA biosynthesis and deactivation enzymes. Recent studies have shown that both GA biosynthesis and deactivation pathways are tightly regulated by developmental, hormonal, and environmental signals, consistent with the role of GAs as key growth regulators. In this review, we summarize our current understanding of the GA biosynthesis and deactivation pathways in plants and fungi, and discuss methods for their qualitative and quantitative analysis. The challenges for their extraction and purification from plant tissues, which form complex matrices containing thousands of interfering substances, are discussed.

An unusual spliced variant of DELLA protein, a negative regulator of gibberellin signaling, in lettuce

DELLA proteins are negative regulators of the signaling of gibberellin (GA), a phytohormone regulating plant growth. DELLA degradation is triggered by its interaction with GID1, a soluble GA receptor, in the presence of bioactive GA. We isolated cDNA from a spliced variant of LsDELLA1 mRNA in lettuce, and named it LsDELLA1sv. It was deduced that LsDELLA1sv encodes truncated LsDELLA1, which has DELLA and VHYNP motifs at the N terminus but lacks part of the C-terminal GRAS domain. The recombinant LsDELLA1sv protein interacted with both Arabidopsis GID1 and lettuce GID1s in the presence of GA. A yeast two-hybrid assay suggested that LsDELLA1sv interacted with LsDELLA1. The ratio of LsDELLA1sv to LsDELLA1 transcripts was higher in flower samples at the late reproductive stage and seed samples (dry seeds and imbibed seeds) than in the other organ samples examined. This study suggests that LsDELLA1sv is a possible modulator of GA signaling in lettuce.

WRKY transcription factors: key components in abscisic acid signalling

WRKY transcription factors (TFs) are key regulators of many plant processes, including the responses to biotic and abiotic stresses, senescence, seed dormancy and seed germination. For over 15 years, limited evidence has been available suggesting that WRKY TFs may play roles in regulating plant responses to the phytohormone abscisic acid (ABA), notably some WRKY TFs are ABA-inducible repressors of seed germination. However, the roles of WRKY TFs in other aspects of ABA signalling, and the mechanisms involved, have remained unclear. Recent significant progress in ABA research has now placed specific WRKY TFs firmly in ABA-responsive signalling pathways, where they act at multiple levels. In Arabidopsis, WRKY TFs appear to act downstream of at least two ABA receptors: the cytoplasmic PYR/PYL/RCAR-protein phosphatase 2C-ABA complex and the chloroplast envelope-located ABAR-ABA complex. In vivo and in vitro promoter-binding studies show that the target genes for WRKY TFs that are involved in ABA signalling include well-known ABA-responsive genes such as ABF2, ABF4, ABI4, ABI5, MYB2, DREB1a, DREB2a and RAB18. Additional well-characterized stress-inducible genes such as RD29A and COR47 are also found in signalling pathways downstream of WRKY TFs. These new insights also reveal that some WRKY TFs are positive regulators of ABA-mediated stomatal closure and hence drought responses. Conversely, many WRKY TFs are negative regulators of seed germination, and controlling seed germination appears a common function of a subset of WRKY TFs in flowering plants. Taken together, these new data demonstrate that WRKY TFs are key nodes in ABA-responsive signalling networks.

Strigolactone and karrikin signal perception: receptors, enzymes, or both?

The signaling molecules strigolactone (SL) and karrikin are involved in seed germination, development of axillary meristems, senescence of leaves, and interactions with arbuscular mycorrhizal fungi. The signal transduction pathways for both SLs and karrikins require the same F-box protein (MAX2) and closely related α/β hydrolase fold proteins (DAD2 and KAI2). The crystal structure of DAD2 has been solved revealing an α/β hydrolase fold protein with an internal cavity capable of accommodating SLs. DAD2 responds to the SL analog GR24 by changing conformation and binding to MAX2 in a GR24 concentration-dependent manner. DAD2 can also catalyze hydrolysis of GR24. Structure activity relationships of analogs indicate that the butenolide ring common to both SLs and karrikins is essential for biological activity, but the remainder of the molecules can be significantly modified without loss of activity. The combination of data from the study of DAD2, KAI2, and chemical analogs of SLs and karrikins suggests a model for binding that requires nucleophilic attack by the active site serine of the hydrolase at the carbonyl atom of the butenolide ring. A conformational change occurs in the hydrolase that results in interaction with the F-box protein MAX2. Downstream signal transduction is then likely to occur via SCF (Skp-Cullin-F-box) complex-mediated ubiquitination of target proteins and their subsequent degradation. The role of the catalytic activity of the hydrolase is unclear but it may be integral in binding as well as possibly allowing the signal to be cleared from the receptor. The α/β hydrolase fold family consists mostly of active enzymes, with a few notable exceptions. We suggest that DAD2 and KAI2 represent an intermediate stage where some catalytic activity is retained at the same time as a receptor role has evolved.

The single functional blast resistance gene Pi54 activates a complex defence mechanism in rice

The Pi54 gene (Pi-k(h)) confers a high degree of resistance to diverse strains of the fungus Magnaporthe oryzae. In order to understand the genome-wide co-expression of genes in the transgenic rice plant Taipei 309 (TP) containing the Pi54 gene, microarray analysis was performed at 72 h post-inoculation of the M. oryzae strain PLP-1. A total of 1154 differentially expressing genes were identified in TP-Pi54 plants. Of these, 587 were up-regulated, whereas 567 genes were found to be down-regulated. 107 genes were found that were exclusively up-regulated and 58 genes that were down- regulated in the case of TP-Pi54. Various defence response genes, such as callose, laccase, PAL, and peroxidase, and genes related to transcription factors like NAC6, Dof zinc finger, MAD box, bZIP, and WRKY were found to be up-regulated in the transgenic line. The enzymatic activities of six plant defence response enzymes, such as peroxidase, polyphenol oxidase, phenylalanine ammonia lyase, β-glucosidase, β-1,3-glucanase, and chitinase, were found to be significantly high in TP-Pi54 at different stages of inoculation by M. oryzae. The total phenol content also increased significantly in resistant transgenic plants after pathogen inoculation. This study suggests the activation of defence response and transcription factor-related genes and a higher expression of key enzymes involved in the defence response pathway in the rice line TP-Pi54, thus leading to incompatible host-pathogen interaction.

Dominant and pleiotropic effects of a GAI gene in wheat results from a lack of interaction between DELLA and GID1

Dominance, semidominance, and recessiveness are important modes of Mendelian inheritance. The phytohormone gibberellin (GA) regulates many plant growth and developmental processes. The previously cloned semidominant GA-insensitive (GAI) genes Reduced height1 (Rht1) and Rht2 in wheat (Triticum aestivum) were the basis of the Green Revolution. However, no completely dominant GAI gene has been cloned. Here, we report the molecular characterization of Rht-B1c, a dominant GAI allele in wheat that confers more extreme characteristics than its incompletely dominant alleles. Rht-B1c is caused by a terminal repeat retrotransposons in miniature insertion in the DELLA domain. Yeast two-hybrid assays showed that Rht-B1c protein fails to interact with GA-INSENSITIVE DWARF1 (GID1), thereby blocking GA responses and resulting in extreme dwarfism and pleiotropic effects. By contrast, Rht-B1b protein only reduces interaction with GID1. Furthermore, we analyzed its functions using near-isogenic lines and examined its molecular mechanisms in transgenic rice. These results indicated that the affinity between GID1 and DELLA proteins is key to regulation of the stability of DELLA proteins, and differential interactions determine dominant and semidominant gene responses to GA.

The Gibberellin perception system evolved to regulate a pre-existing GAMYB-mediated system during land plant evolution

Gibberellin (GA) controls pollen development in flowering plants via the GAMYB transcription factor. Here we show that GAMYB is conserved in Selaginella moellendorffii (lycophyte) and Physcomitrella patens (moss), although the former contains the GA signalling pathway, the latter does not. In the lycophyte, GA treatment promotes the outer wall development on microspores, whereas treatment with GA biosynthesis inhibitors disturbs its development. Contrary, in the moss, GAMYB homologue knockouts also produce abnormal spores that resemble Selaginella microspores treated with GA biosynthesis inhibitors and pollen grains of rice gamyb mutant. Moreover, the knockouts fail to develop male organs, instead ectopically forming female organs. Thus, before the establishment of the GA signalling pathway, basal land plants, including mosses, contained a GAMYB-based system for spore and sexual organ development. Subsequently, during the evolution from mosses to basal vascular plants including lycophytes, GA signalling might have merged to regulate this pre-existing GAMYB-based system.

Gibberellin control of stamen development: a fertile field

Stamen development is governed by a conserved genetic pathway, within which the role of hormones has been the subject of considerable recent research. Our understanding of the involvement of gibberellin (GA) signalling in this developmental process is further advanced than for the other phytohormones, and here we review recent experimental results in rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana) that have provided insight into the timing and mechanisms of GA regulation of stamen development, identifying the tapetum and developing pollen as major targets. GA signalling governs both tapetum secretory functions and entry into programmed cell death via the GAMYB class of transcription factor, the targets of which integrate with the established genetic framework for the regulation of tapetum function at multiple hierarchical levels.

Inter- and intra-molecular interactions of Arabidopsis thaliana DELLA protein RGL1

The phytohormone gibberellin and the DELLA proteins act together to control key aspects of plant development. Gibberellin induces degradation of DELLA proteins by recruitment of an F-box protein using a molecular switch: a gibberellin-bound nuclear receptor interacts with the N-terminal domain of DELLA proteins, and this event primes the DELLA C-terminal domain for interaction with the F-box protein. However, the mechanism of signalling between the N- and C-terminal domains of DELLA proteins is unresolved. In the present study, we used in vivo and in vitro approaches to characterize di- and tri-partite interactions of the DELLA protein RGL1 (REPRESSOR OF GA1-3-LIKE 1) of Arabidopsis thaliana with the gibberellin receptor GID1A (GIBBERELLIC ACID-INSENSITIVE DWARF-1A) and the F-box protein SLY1 (SLEEPY1). Deuterium-exchange MS unequivocally showed that the entire N-terminal domain of RGL1 is disordered prior to interaction with the GID1A; furthermore, association/dissociation kinetics, determined by surface plasmon resonance, predicts a two-state conformational change of the RGL1 N-terminal domain upon interaction with GID1A. Additionally, competition assays with monoclonal antibodies revealed that contacts mediated by the short helix Asp-Glu-Leu-Leu of the hallmark DELLA motif are not essential for the GID1A–RGL1 N-terminal domain interaction. Finally, yeast two- and three-hybrid experiments determined that unabated communication between N- and C-terminal domains of RGL1 is required for recruitment of the F-box protein SLY1.

Gibberellin biosynthesis and signalling during development of the strawberry receptacle

The enlargement of receptacle cells during strawberry (Fragaria × ananassa) fruit development is a critical factor determining fruit size, with the increase in cell expansion being one of the most important physiological processes regulated by the phytohormone gibberellin (GA). Here, we studied the role of GA during strawberry fruit development by analyzing the endogenous content of bioactive GAs and the expression of key components of GA signalling and metabolism. Bioactive GA(1) , GA(3) and GA(4) were monitored during fruit development, with the content of GA(4) being extremely high in the receptacle, peaking at the white stage of development. •Genes with high homology to genes encoding GA pathway components, including receptors (FaGID1(GIBBERELLIN-INSENSITIVE DWARF1)b and FaGID1c), DELLA (FaRGA(REPRESSOR OF GA) and FaGAI(GA-INSENSITIVE)), and enzymes involved in GA biosynthesis (FaGA3ox) and catabolism (FaGA2ox), were identified, and their expression in different tissues and developmental stages of strawberry fruit was studied in detail. The expression of all of these genes showed a stage-specific pattern during fruit development and was highest in the receptacle. FaGID1c bound GA in vitro, interacted with FaRGA in vitro and in vivo, and increased GA responses when ectopically expressed in Arabidopsis. This study thus reveals key elements of GA responses in strawberry and points to a critical role for GA in the development of the receptacle.

An updated GA signaling 'relief of repression' regulatory model

Gibberellic acid (GA) regulates many aspects of plant growth and development. The DELLA proteins act to restrain plant growth, and GA relieves this repression by promoting their degradation via the 26S proteasome pathway. The elucidation of the crystalline structure of the GA soluble receptor GID1 protein represents an important breakthrough for understanding the way in which GA is perceived and how it induces the destabilization of the DELLA proteins. Recent advances have revealed that the DELLA proteins are involved in protein-protein interactions within various environmental and hormone signaling pathways. In this review, we highlight our current understanding of the 'relief of repression' model that aims to explain the role of GA and the function of the DELLA proteins, incorporating the many aspects of cross-talk shown to exist in the control of plant development and the response to stress.

Gibberellin regulates PIN-FORMED abundance and is required for auxin transport-dependent growth and development in Arabidopsis thaliana

Plants integrate different regulatory signals to control their growth and development. Although a number of physiological observations suggest that there is crosstalk between the phytohormone gibberellin (GA) and auxin, as well as with auxin transport, the molecular basis for this hormonal crosstalk remains largely unexplained. Here, we show that auxin transport is reduced in the inflorescences of Arabidopsis thaliana mutants deficient in GA biosynthesis and signaling. We further show that this reduced auxin transport correlates with a reduction in the abundance of PIN-FORMED (PIN) auxin efflux facilitators in GA-deficient plants and that PIN protein levels recover to wild-type levels following GA treatment. We also demonstrate that the regulation of PIN protein levels cannot be explained by a transcriptional regulation of the PIN genes but that GA deficiency promotes, at least in the case of PIN2, the targeting of PIN proteins for vacuolar degradation. In genetic studies, we reveal that the reduced auxin transport of GA mutants correlates with an impairment in two PIN-dependent growth processes, namely, cotyledon differentiation and root gravitropic responses. Our study thus presents evidence for a role of GA in these growth responses and for a GA-dependent modulation of PIN turnover that may be causative for these differential growth responses.

Thermodynamic characterization of OsGID1-gibberellin binding using calorimetry and docking simulations

Gibberellins (GAs) are phytohormones regulating various developmental processes in plants. In rice, the initial GA-signaling events involve the binding of a GA to the soluble GA receptor protein, GID1. Although X-ray structures for certain GID1/GA complexes have recently been determined, an examination of the complexes does not fully clarify how GID1s discriminate among different GAs. Herein, we present a study aimed at defining the types of forces important to binding via a combination of isothermal titration calorimetry (ITC) and computational docking studies that employed rice GID1 (OsGID1), OsGID1 mutants, which were designed to have a decreased possible number of hydrogen bonds with bound GA, and GA variants. We find that, in general, GA binding is enthalpically driven and that a hydrogen bond between the phenolic hydroxyl of OsGID1 Tyr134 and the C-3 hydroxyl of a GA is a defining structural element. A hydrogen-bond network that involves the C-6 carboxyl of a GA that directly hydrogen bonds the hydroxyl of Ser198 and indirectly, via a two-water-molecule network, the phenolic hydroxyl of Tyr329 and the NH of the amide side-chain of Asn255 is also important for GA binding. The binding of OsGID1 by GA(1) is the most enthalpically driven association found for the biologically active GAs evaluated in this study. This observation might be a consequence of a hydrogen bond formed between the hydroxyl at the C-13 position of GA(1) and the main chain carbonyl of OsGID1 Phe245. Our results demonstrate that by combining ITC experiments and computational methods much can be learned about the thermodynamics of ligand/protein binding.

Hormone crosstalk in plant disease and defense: more than just jasmonate-salicylate antagonism

Until recently, most studies on the role of hormones in plant-pathogen interactions focused on salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). It is now clear that pathogen-induced modulation of signaling via other hormones contributes to virulence. A picture is emerging of complex crosstalk and induced hormonal changes that modulate disease and resistance, with outcomes dependent on pathogen lifestyles and the genetic constitution of the host. Recent progress has revealed intriguing similarities between hormone signaling mechanisms, with gene induction responses often achieved by derepression. Here, we report on recent advances, updating current knowledge on classical defense hormones SA, JA, and ET, and the roles of auxin, abscisic acid (ABA), cytokinins (CKs), and brassinosteroids in molding plant-pathogen interactions. We highlight an emerging theme that positive and negative regulators of these disparate hormone signaling pathways are crucial regulatory targets of hormonal crosstalk in disease and defense.

Maize DELLA proteins dwarf plant8 and dwarf plant9 as modulators of plant development

DELLA proteins are nuclear-localized negative regulators of gibberellin signaling found ubiquitously throughout higher plants. Dominant dwarfing mutations of DELLA proteins have been primarily responsible for the dramatic increases in harvest index of the 'green revolution'. Maize contains two genetic loci encoding DELLA proteins, dwarf plant8 (d8) and dwarf plant 9 (d9). The d8 gene and three of its dominant dwarfing alleles have been previously characterized at the molecular level. Almost 20 years after the initial description of the mutant, this investigation represents the first molecular characterization of d9 and its gibberellin-insensitive mutant, D9-1. We have molecularly, subcellularly and phenotypically characterized the gene products of five maize DELLA alleles in transgenic Arabidopsis. In dissecting the molecular differences in D9-1, a critical residue for normal DELLA function has been uncovered, corresponding to E600 of the D9 protein. The gibberellin-insensitive D9-1 was found to produce dwarfing and, notably, earlier flowering in Arabidopsis. Conversely, overexpression of the D9-1 allele delayed flowering in transgenic maize, while overexpression of the d9 allele led to earlier flowering. These results corroborate findings that DELLA proteins are at the crux of many plant developmental pathways and suggest differing mechanisms of flowering time control by DELLAs in maize and Arabidopsis.

A rice gid1 suppressor mutant reveals that gibberellin is not always required for interaction between its receptor, GID1, and DELLA proteins

To investigate gibberellin (GA) signaling using the rice (Oryza sativa) GA receptor GIBBERELLIN-INSENSITIVE DWARF1 (GID1) mutant gid1-8, we isolated a suppressor mutant, Suppressor of gid1-1 (Sgd-1). Sgd-1 is an intragenic mutant containing the original gid1-8 mutation (L45F) and an additional amino acid substitution (P99S) in the loop region. GID1(P99S) interacts with the rice DELLA protein SLENDER RICE1 (SLR1), even in the absence of GA. Substitution of the 99th Pro with other amino acids revealed that substitution with Ala (P99A) caused the highest level of GA-independent interaction. Physicochemical analysis using surface plasmon resonance revealed that GID1(P99A) has smaller K(a) (association) and K(d) (dissociation) values for GA(4) than does wild-type GID1. This suggests that the GID1(P99A) lid is at least partially closed, resulting in both GA-independent and GA-hypersensitive interactions with SLR1. One of the three Arabidopsis thaliana GID1s, At GID1b, can also interact with DELLA proteins in the absence of GA, so we investigated whether GA-independent interaction of At GID1b depends on a mechanism similar to that of rice GID1(P99A). Substitution of the loop region or a few amino acids of At GID1b with those of At GID1a diminished its GA-independent interaction with GAI while maintaining the GA-dependent interaction. Soybean (Glycine max) and Brassica napus also have GID1s similar to At GID1b, indicating that these unique GID1s occur in various dicots and may have important functions in these plants.

The perception of gibberellins: clues from receptor structure

The discovery of GID1, a soluble receptor for gibberellins (GAs), has revealed new insights into how GA is perceived. X-ray analysis has demonstrated similarities in the tertiary structure of GID1 to hormone sensitive lipase (HSL), and the GA-binding pocket of GID1 corresponds to the active site of HSL. X-ray analysis has also revealed the structural basis of the GA-GID1 interaction, and evolutionary aspects of GID1 have been discovered by comparison to GID1 from non-flowering plants. Recent studies have also demonstrated the complexity of GA signaling in Arabidopsis, which is mediated by three GID1 and five DELLA proteins. Finally, mechanistic and structural similarities for hormone signaling are compared for GA, auxin and abscisic acid, three hormones where the receptor protein structure was recently described.

Characterization of the molecular mechanism underlying gibberellin perception complex formation in rice

The DELLA protein SLENDER RICE1 (SLR1) is a repressor of gibberellin (GA) signaling in rice (Oryza sativa), and most of the GA-associated responses are induced upon SLR1 degradation. It is assumed that interaction between GIBBERELLIN INSENSITIVE DWARF1 (GID1) and the N-terminal DELLA/TVHYNP motif of SLR1 triggers F-box protein GID2-mediated SLR1 degradation. We identified a semidominant dwarf mutant, Slr1-d4, which contains a mutation in the region encoding the C-terminal GRAS domain of SLR1 (SLR1(G576V)). The GA-dependent degradation of SLR1(G576V) was reduced in Slr1-d4, and compared with SLR1, SLR1(G576V) showed reduced interaction with GID1 and almost none with GID2 when tested in yeast cells. Surface plasmon resonance of GID1-SLR1 and GID1-SLR1(G576V) interactions revealed that the GRAS domain of SLR1 functions to stabilize the GID1-SLR1 interaction by reducing its dissociation rate and that the G576V substitution in SLR1 diminishes this stability. These results suggest that the stable interaction of GID1-SLR1 through the GRAS domain is essential for the recognition of SLR1 by GID2. We propose that when the DELLA/TVHYNP motif of SLR1 binds with GID1, it enables the GRAS domain of SLR1 to interact with GID1 and that the stable GID1-SLR1 complex is efficiently recognized by GID2.

Endogenous diterpenes derived from ent-kaurene, a common gibberellin precursor, regulate protonema differentiation of the moss Physcomitrella patens

Gibberellins (GAs) are a group of diterpene-type plant hormones biosynthesized from ent-kaurene via ent-kaurenoic acid. GAs are ubiquitously present in seed plants. The GA signal is perceived and transduced by the GID1 GA receptor/DELLA repressor pathway. The lycopod Selaginella moellendorffii biosynthesizes GA and has functional GID1-DELLA signaling components. In contrast, no GAs or functionally orthologous GID1-DELLA components have been found in the moss Physcomitrella patens. However, P. patens produces ent-kaurene, a common precursor for GAs, and possesses a functional ent-kaurene synthase, PpCPS/KS. To assess the biological role of ent-kaurene in P. patens, we generated a PpCPS/KS disruption mutant that does not accumulate ent-kaurene. Phenotypic analysis demonstrates that the mutant has a defect in the protonemal differentiation of the chloronemata to caulonemata. Gas chromatography-mass spectrometry analysis shows that P. patens produces ent-kaurenoic acid, an ent-kaurene metabolite in the GA biosynthesis pathway. The phenotypic defect of the disruptant was recovered by the application of ent-kaurene or ent-kaurenoic acid, suggesting that ent-kaurenoic acid, or a downstream metabolite, is involved in protonemal differentiation. Treatment with uniconazole, an inhibitor of ent-kaurene oxidase in GA biosynthesis, mimics the protonemal phenotypes of the PpCPS/KS mutant, which were also restored by ent-kaurenoic acid treatment. Interestingly, the GA(9) methyl ester, a fern antheridiogen, rescued the protonemal defect of the disruption mutant, while GA(3) and GA(4), both of which are active GAs in angiosperms, did not. Our results suggest that the moss P. patens utilizes a diterpene metabolite from ent-kaurene as an endogenous developmental regulator and provide insights into the evolution of GA functions in land plants.

ABA receptors: the START of a new paradigm in phytohormone signalling

The phytohormone abscisic acid (ABA) plays a central role in plant development and in plant adaptation to both biotic and abiotic stressors. In recent years, knowledge of ABA metabolism and signal transduction has advanced rapidly to provide detailed glimpses of the hormone's activities at the molecular level. Despite this progress, many gaps in understanding have remained, particularly at the early stages of ABA perception by the plant cell. The search for an ABA receptor protein has produced multiple candidates, including GCR2, GTG1, and GTG2, and CHLH. In addition to these candidates, in 2009 several research groups converged on a novel family of Arabidopsis proteins that bind ABA, and thereby interact directly with a class of protein phosphatases that are well known as critical players in ABA signal transduction. The PYR/PYL/RCAR receptor family is homologous to the Bet v 1-fold and START domain proteins. It consists of 14 members, nearly all of which appear capable of participating in an ABA receptor-signal complex that responds to the hormone by activating the transcription of ABA-responsive genes. Evidence is provided here that PYR/PYL/RCAR receptors can also drive the phosphorylation of the slow anion channel SLAC1 to provide a fast and timely response to the ABA signal. Crystallographic studies have vividly shown the mechanics of ABA binding to PYR/PYL/RCAR receptors, presenting a model that bears some resemblance to the binding of gibberellins to GID1 receptors. Since this ABA receptor family is highly conserved in crop species, its discovery is likely to usher a new wave of progress in the elucidation and manipulation of plant stress responses in agricultural settings.

A missense mutation in the VHYNP motif of a DELLA protein causes a semi-dwarf mutant phenotype in Brassica napus

Although dwarf genes have been widely used to improve lodging resistance and enhance harvest index in cereal crops, lodging is still a serious problem in rapeseed (Brassica napus) production. A semi-dwarf B. napus mutant, ds-1, was identified through EMS mutagenesis of a microspore-cultured DH line. The mutant had a significant reduction in height due to a lower first branch position and shorter internodes when compared with wild-type cultivars. This dwarfism was inherited as a single semi-dominant gene, ds-1. DS-1 locus was mapped to chromosome A6, and co-segregated with a microsatellite marker BnEMS1125 derived from the gene BnRGA. BnRGA encodes a DELLA protein that functions as a GA signaling repressor. The expression of a mutant BnRGA allele from ds-1, Bnrga-ds, caused dwarf phenotypes in Arabidopsis. Comparative sequencing of RGA open-reading frames (ORFs) of ds-1 and wild-type cultivars revealed a single proline (P)-to-leucine (L) substitution that may lead to a gain-of-function mutation in GA signaling. The expression of the Arabidopsis homolog, Atrga-ds, bearing this site-directed mutation also rendered dwarf phenotypes in Arabidopsis, which demonstrated that the P-to-L mutation in the VHYNP motif of Bnrga-ds is responsible for the dwarfism. A yeast two-hybrid assay confirmed that this mutation inhibited the interaction between Bnrga-ds/Atrga-ds and the GA receptor, AtGID1A, in the presence of GA(3), suggesting that the conserved proline residue in the VHYNP motif of DELLA protein directly participates in DELLA-GID1 interaction. Identification and characterization of the dwarf gene ds-1 will facilitate its utilization in improving lodging resistance in Brassica breeding.

Rice early flowering1, a CKI, phosphorylates DELLA protein SLR1 to negatively regulate gibberellin signalling

The plant hormone gibberellin (GA) is crucial for multiple aspects of plant growth and development. To study the relevant regulatory mechanisms, we isolated a rice mutant earlier flowering1, el1, which is deficient in a casein kinase I that has critical roles in both plants and animals. el1 had an enhanced GA response, consistent with the suppression of EL1 expression by exogenous GA₃. Biochemical characterization showed that EL1 specifically phosphorylates the rice DELLA protein SLR1, proving a direct evidence for SLR1 phosphorylation. Overexpression of SLR1 in wild-type plants caused a severe dwarf phenotype, which was significantly suppressed by EL1 deficiency, indicating the negative effect of SLR1 on GA signalling requires the EL1 function. Further studies showed that the phosphorylation of SLR1 is important for maintaining its activity and stability, and mutation of the candidate phosphorylation site of SLR1 results in the altered GA signalling. This study shows EL1 a novel and key regulator of the GA response and provided important clues on casein kinase I activities in GA signalling and plant development.

N-terminal domains of DELLA proteins are intrinsically unstructured in the absence of interaction with GID1/gibberellic acid receptors

The plant growth-repressing DELLA proteins (DELLAs) are known to represent a convergence point in integration of multiple developmental and environmental signals in planta, one of which is hormone gibberellic acid (GA). Binding of the liganded GA receptor (GID1/GA) to the N-terminal domain of DELLAs is required for GA-induced degradation of DELLAs via the ubiquitin-proteasome pathway, thus derepressing plant growth. However, the conformational changes of DELLAs upon binding to GID1/GA, which are the key to understanding the precise mechanism of GID1/GA-mediated degradation of DELLAs, remain unclear. Using biophysical, biochemical, and bioinformatics approaches, we demonstrated for the first time that the unbound N-terminal domains of DELLAs are intrinsically unstructured proteins under physiological conditions. Within the intrinsically disordered N-terminal domain of DELLAs, we have identified several molecular recognition features, sequences known to undergo disorder-to-order transitions upon binding to interacting proteins in intrinsically unstructured proteins. In accordance with the molecular recognition feature analyses, we have observed the binding-induced folding of N-terminal domains of DELLAs upon interaction with AtGID1/GA. Our results also indicate that DELLA proteins can be divided into two subgroups in terms of their molecular compactness and their interactions with monoclonal antibodies.

Why so repressed? Turning off transcription during plant growth and development

To ensure correct patterns of gene expression, eukaryotes use a variety of strategies to repress transcription. The transcriptional regulators mediating this repression can be broadly categorized as either passive or active repressors. While passive repressors rely on mechanisms such as steric hindrance of transcriptional activators to repress gene expression, active repressors display inherent repressive abilities commonly conferred by discrete repression domains. Recent studies have indicated that both categories of regulators function in a variety of plant processes, including hormone signal transduction, developmental pathways, and response to biotic and abiotic stresses.

Visualization of molecular interactions using bimolecular fluorescence complementation analysis: characteristics of protein fragment complementation

Investigations of the molecular processes that sustain life must include studies of these processes in their normal cellular environment. The bimolecular fluorescence complementation (BiFC) assay provides an approach for the visualization of protein interactions and modifications in living cells. This assay is based on the facilitated association of complementary fragments of a fluorescent protein that are fused to interaction partners. Complex formation by the interaction partners tethers the fluorescent protein fragments in proximity to each other, which can facilitate their association. The BiFC assay enables sensitive visualization of protein complexes with high spatial resolution. The temporal resolution of BiFC analysis is limited by the time required for fluorophore formation, as well as the stabilization of complexes by association of the fluorescent protein fragments. Many modifications and enhancements to the BiFC assay have been developed. The multicolor BiFC assay enables simultaneous visualization of multiple protein complexes in the same cell, and can be used to investigate competition among mutually exclusive interaction partners for complex formation in cells. The ubiquitin-mediated fluorescence complementation (UbFC) assay enables visualization of covalent ubiquitin family peptide conjugation to substrate proteins in cells. The BiFC assay can also be used to visualize protein binding to specific chromatin domains, as well as other molecular scaffolds in cells. BiFC analysis therefore provides a powerful approach for the visualization of a variety of processes that affect molecular proximity in living cells. The visualization of macromolecular interactions and modifications is of great importance owing to the central roles of proteins, nucleic acids and other macromolecular complexes in the regulation of cellular functions. This tutorial review describes the BiFC assay, and discusses the advantages and disadvantages of this experimental approach. The review will be of interest to scientists interested in the investigation of macromolecular interactions or modifications who need exquisite sensitivity for the detection of their complexes or conjugates of interest.

GRAS-domain transcription factors that regulate plant development

The molecular mechanisms of plant growth and development have been analyzed in detail during the past years. The critical role of a plant-specific family of GRAS domain proteins in these processes has become apparent. In this review we highlight the importance of DELLA proteins in gibberellic acid (GA) and light signaling, the regulation of root patterning by SCR-SHR interactions and the requirement of two GRAS proteins from legumes, NSP1 and NSP2, for root nodule symbiosis. We discuss common and distinct molecular mechanisms underlying GRAS protein function and emphasise new discoveries regarding their function as transcription factors and the role of protein movement in refining their mode of action.

From dwarves to giants? Plant height manipulation for biomass yield

The increasing demand for lignocellulosic biomass for the production of biofuels provides value to vegetative plant tissue and leads to a paradigm shift for optimizing plant architecture in bioenergy crops. Plant height (PHT) is among the most important biomass yield components and is the focus of this review, with emphasis on the energy grasses maize (Zea mays) and sorghum (Sorghum bicolor). We discuss the scientific advances in the identification of PHT quantitative trait loci (QTLs) and the understanding of pathways and genes controlling PHT, especially gibberellins and brassinosteroids. We consider pleiotropic effects of QTLs or genes affecting PHT on other agronomically important traits and, finally, we discuss strategies for applying this knowledge to the improvement of dual-purpose or dedicated bioenergy crops.

Gibberellin signaling in the endodermis controls Arabidopsis root meristem size

Plant growth is driven by cell proliferation and elongation. The hormone gibberellin (GA) regulates Arabidopsis root growth by controlling cell elongation, but it is currently unknown whether GA also controls root cell proliferation. Here we show that GA biosynthetic mutants are unable to increase their cell production rate and meristem size after germination. GA signals the degradation of the DELLA growth repressor proteins GAI and RGA, promoting root cell production. Targeting the expression of gai (a non-GA-degradable mutant form of GAI) in the root meristem disrupts cell proliferation. Moreover, expressing gai in dividing endodermal cells was sufficient to block root meristem enlargement. We report a novel function for GA regulating cell proliferation where this signal acts by removing DELLA in a subset of, rather than all, meristem cells. We suggest that the GA-regulated rate of expansion of dividing endodermal cells dictates the equivalent rate in other root tissues. Cells must double in size prior to dividing but cannot do so independently, because they are physically restrained by adjacent tissues with which they share cell walls. Our study highlights the importance of probing regulatory mechanisms linking molecular- and cellular-scale processes with tissue and organ growth responses.

Analyses of GA20ox- and GID1-over-expressing aspen suggest that gibberellins play two distinct roles in wood formation

Gibberellins (GAs) are involved in many aspects of plant development, including shoot growth, flowering and wood formation. Increased levels of bioactive GAs are known to induce xylogenesis and xylem fiber elongation in aspen. However, there is currently little information on the response pathway(s) that mediate GA effects on wood formation. Here we characterize an important element of the GA pathway in hybrid aspen: the GA receptor, GID1. Four orthologs of GID1 were identified in Populus tremula x P. tremuloides (PttGID1.1-1.4). These were functional when expressed in Arabidopsis thaliana, and appear to present a degree of sub-functionalization in hybrid aspen. PttGID1.1 and PttGID1.3 were over-expressed in independent lines of hybrid aspen using either the 35S promoter or a xylem-specific promoter (LMX5). The 35S:PttGID1 over-expressors shared several phenotypic traits previously described in 35S:AtGA20ox1 over-expressors, including rapid growth, increased elongation, and increased xylogenesis. However, their xylem fibers were not elongated, unlike those of 35S:AtGA20ox1 plants. Similar differences in the xylem fiber phenotype were observed when PttGID1.1, PttGID1.3 or AtGA20ox1 were expressed under the control of the LMX5 promoter, suggesting either that PttGID1.1 and PttGID1.3 play no role in fiber elongation or that GA homeostasis is strongly controlled when GA signaling is altered. Our data suggest that GAs are required in two distinct wood-formation processes that have tissue-specific signaling pathways: xylogenesis, as mediated by GA signaling in the cambium, and fiber elongation in the developing xylem.

The ubiquitin-26S proteasome system at the nexus of plant biology

Plants, like other eukaryotes, rely on proteolysis to control the abundance of key regulatory proteins and enzymes. Strikingly, genome-wide studies have revealed that the ubiquitin-26S proteasome system (UPS) in particular is an exceedingly large and complex route for protein removal, occupying nearly 6% of the Arabidopsis thaliana proteome. But why is the UPS so pervasive in plants? Data accumulated over the past few years now show that it targets numerous intracellular regulators that have central roles in hormone signalling, the regulation of chromatin structure and transcription, tailoring morphogenesis, responses to environmental challenges, self recognition and battling pathogens.

Gibberellin modulates anther development in rice via the transcriptional regulation of GAMYB

Gibberellins (GAs) play important roles in regulating reproductive development, especially anther development. Our previous studies revealed that the MYB transcriptional factor GAMYB, an important component of GA signaling in cereal aleurone cells, is also important for anther development. Here, we examined the physiological functions of GA during anther development through phenotypic analyses of rice (Oryza sativa) GA-deficient, GA-insensitive, and gamyb mutants. The mutants exhibited common defects in programmed cell death (PCD) of tapetal cells and formation of exine and Ubisch bodies. Microarray analysis using anther RNAs of these mutants revealed that rice GAMYB is involved in almost all instances of GA-regulated gene expression in anthers. Among the GA-regulated genes, we focused on two lipid metabolic genes, a cytochrome P450 hydroxylase CYP703A3 and beta-ketoacyl reductase, both of which might be involved in providing a substrate for exine and Ubisch body. GAMYB specifically interacted with GAMYB binding motifs in the promoter regions in vitro, and mutation of these motifs in promoter-beta-glucuronidase (GUS) transformants caused reduced GUS expression in anthers. Furthermore, a knockout mutant for CYP703A3 showed gamyb-like defects in exine and Ubisch body formation. Together, these results suggest that GA regulates exine formation and the PCD of tapetal cells and that direct activation of CYP703A3 by GAMYB is key to exine formation.

The angiosperm gibberellin-GID1-DELLA growth regulatory mechanism: how an "inhibitor of an inhibitor" enables flexible response to fluctuating environments

The phytohormone gibberellin (GA) has long been known to regulate the growth, development, and life cycle progression of flowering plants. However, the molecular GA-GID1-DELLA mechanism that enables plants to respond to GA has only recently been discovered. In addition, studies published in the last few years have highlighted previously unsuspected roles for the GA-GID1-DELLA mechanism in regulating growth response to environmental variables. Here, we review these advances within a general plant biology context and speculate on the answers to some remaining questions. We also discuss the hypothesis that the GA-GID1-DELLA mechanism enables flowering plants to maintain transient growth arrest, giving them the flexibility to survive periods of adversity.