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

Abiotic stress upregulated TaZFP34 represses the expression of type-B response regulator and SHY2 genes and enhances root to shoot ratio in wheat

Q-type C₂H₂ zinc finger proteins (ZFPs) are plant-specific DNA-binding proteins containing a conserved QALGGH motif. This study investigated the function of abiotic stress-inducible and predominantly root-expressed Triticum aestivum ZFPs (TaZFP22, TaZFP34 and TaZFP46) with a focus on TaZFP34. Expression of TaZFP34 in roots was upregulated by high salinity, dehydration, oxidative and cold stresses. Overexpression of TaZFP34 in wheat roots resulted in an increased root-to-shoot ratio, a phenomenon observed during plant adaptation to drying soil. Expression of a number of genes which are potentially involved in modulating root growth was significantly altered in the roots of TaZFP34 overexpressing lines. In particular, the transcript levels of TaRR12B, TaRR12D and TaSHY2 that are homologues of known negative regulators of root growth were significantly reduced. Expression of shoot growth-related genes, such as GA3-ox and expansins, was downregulated in the transgenic shoots. TaZFP34 bound to (C/G)AGT(G/A)-like elements in the promoters of TaZFP34 down-regulated TaRR12D and TaSHY2 and transrepressed the reporter gene expression driven by TaRR12D and TaSHY2 promoters. Expression of the above reporter genes was also repressed by TaZFP46 and TaZFP22. These data suggest that TaZFP34 is a transcriptional repressor and is involved in modulating the root-to-shoot ratio.

The rice YABBY4 gene regulates plant growth and development through modulating the gibberellin pathway

YABBY genes encode seed plant-specific transcription factors that play pivotal roles in diverse aspects of leaf, shoot, and flower development. Members of the YABBY gene family are primarily expressed in lateral organs in a polar manner and function to specify abaxial cell fate in dicotyledons, but this polar expression is not conserved in monocotyledons. The function of YABBY genes is therefore not well understood in monocotyledons. Here we show that overexpression of the rice (Oryza sativa L.) YABBY4 gene (OsYABBY4) leads to a semi-dwarf phenotype, abnormal development in the uppermost internode, an increased number of floral organs, and insensitivity to gibberellin (GA) treatment. We report on an important role for OsYABBY4 in negative control of the expression of a GA biosynthetic gene by binding to the promoter region of the gibberellin 20-oxidase 2 gene (GA20ox2), which is a direct target of SLR1 (the sole DELLA protein negatively controlling GA responses in rice). OsYABBY4 also suppresses the expression level of SLR1 and interacts with SLR1 protein. The interaction inhibits GA-dependent degradation of SLR1 and therefore leads to GA insensitivity. These data together suggest that OsYABBY4 serves as a DNA-binding intermediate protein for SLR1 and is associated with the GA signaling pathway regulating gene expression during plant growth and development.

Survey of Genes Involved in Biosynthesis, Transport, and Signaling of Phytohormones with Focus on Solanum lycopersicum

Phytohormones control the development and growth of plants, as well as their response to biotic and abiotic stress. The seven most well-studied phytohormone classes defined today are as follows: auxins, ethylene, cytokinin, abscisic acid, jasmonic acid, gibberellins, and brassinosteroids. The basic principle of hormone regulation is conserved in all plants, but recent results suggest adaptations of synthesis, transport, or signaling pathways to the architecture and growth environment of different plant species. Thus, we aimed to define the extent to which information from the model plant Arabidopsis thaliana is transferable to other plants such as Solanum lycopersicum. We extracted the co-orthologues of genes coding for major pathway enzymes in A. thaliana from the translated genomes of 12 species from the clade Viridiplantae. Based on predicted domain architecture and localization of the identified proteins from all 13 species, we inspected the conservation of phytohormone pathways. The comparison was complemented by expression analysis of (co-) orthologous genes in S. lycopersicum. Altogether, this information allowed the assignment of putative functional equivalents between A. thaliana and S. lycopersicum but also pointed to some variations between the pathways in eudicots, monocots, mosses, and green algae. These results provide first insights into the conservation of the various phytohormone pathways between the model system A. thaliana and crop plants such as tomato. We conclude that orthologue prediction in combination with analysis of functional domain architecture and intracellular localization and expression studies are sufficient tools to transfer information from model plants to other plant species. Our results support the notion that hormone synthesis, transport, and response for most part of the pathways are conserved, and species-specific variations can be found.

The DELLA-CONSTANS Transcription Factor Cascade Integrates Gibberellic Acid and Photoperiod Signaling to Regulate Flowering

Gibberellin (GA) and photoperiod pathways have recently been demonstrated to collaboratively modulate flowering under long days (LDs). However, the molecular mechanisms underlying this collaboration remain largely unclear. In this study, we found that GA-induced expression of FLOWERING LOCUS T (FT) under LDs was dependent on CONSTANS (CO), a critical transcription factor positively involved in photoperiod signaling. Mechanistic investigation revealed that DELLA proteins, a group of crucial repressors in GA signaling, physically interacted with CO. The DELLA-CO interactions repressed the transcriptional function of CO protein. Genetic analysis demonstrated that CO acts downstream of DELLA proteins to regulate flowering. Disruption of CO rescued the earlier flowering phenotype of the gai-t6 rga-t2 rgl1-1 rgl2-1 mutant (dellap), while a gain-of-function mutation in GA INSENSITIVE (GAI, a member of the DELLA gene) repressed the earlier flowering phenotype of CO-overexpressing plants. In addition, the accumulation of DELLA proteins and mRNAs was rhythmic, and REPRESSOR OF GA1-3 protein was noticeably decreased in the long-day afternoon, a time when CO protein is abundant. Collectively, these results demonstrate that the DELLA-CO cascade inhibits CO/FT-mediated flowering under LDs, which thus provide evidence to directly integrate GA and photoperiod signaling to synergistically modulate flowering under LDs.

The regulatory roles of ethylene and reactive oxygen species (ROS) in plant salt stress responses

Soil salinity is one of the most commonly encountered environmental stresses affecting plant growth and crop productivity. Accordingly, plants have evolved a variety of morphological, physiological and biochemical strategies that enable them to adapt to saline growth conditions. For example, it has long been known that salinity-stress increases both the production of the gaseous stress hormone ethylene and the in planta accumulation of reactive oxygen species (ROS). Recently, there has been significant progress in understanding how the fine-tuning of ethylene biosynthesis and signaling transduction can promote salinity tolerance, and how salinity-induced ROS accumulation also acts as a signal in the mediation of salinity tolerance. Furthermore, recent advances have indicated that ethylene signaling modulates salinity responses largely via regulation of ROS-generating and ROS-scavenging mechanisms. This review focuses on these recent advances in understanding the linked roles of ethylene and ROS in salt tolerance.

Molecular cloning, expression analysis and subcellular localization of four DELLA genes from hybrid poplar

Gibberellic acid (GA) signaling regulates diverse aspects of plant growth and developmental processes. The DELLA repressors of GA signaling are named for an N-terminal conserved DELLA domain. In this study, four genes encoding DELLA proteins, PeRGA1, PeRGA2, PeGAI1 and PeGAI2, were isolated and characterized in poplar. A gene structural analysis revealed that the DELLA genes were all intron-free. Multiple protein sequence alignments revealed that these proteins contained seven highly conserved domains: the DELLA domain, the TVHYNP domain, leucine heptad repeat I (LHR I), the VHIID domain, leucine heptad repeat II (LHR II), the PFYRE domain, and the SAM domain. Temporal expression patterns of these genes were profiled during the adventitious root development of poplar. The four DELLA genes were expressed in root, stem and leaf in a dynamic manner. The subcellular localization demonstrated that these DELLA genes were mainly localized to the nucleus. These results suggest that the four DELLA genes may play diverse regulatory roles in the adventitious root, stem and leaf development of poplar, and contribute to improving our understanding of conserved and divergent aspects of DELLA proteins that restrain GA signaling in various species.

Control of Asymmetric Cell Divisions during Root Ground Tissue Maturation

Controlling the production of diverse cell/tissue types is essential for the development of multicellular organisms such as animals and plants. The Arabidopsis thaliana root, which contains distinct cells/tissues along longitudinal and radial axes, has served as an elegant model to investigate how genetic programs and environmental signals interact to produce different cell/tissue types. In the root, a series of asymmetric cell divisions (ACDs) give rise to three ground tissue layers at maturity (endodermis, middle cortex, and cortex). Because the middle cortex is formed by a periclinal (parallel to the axis) ACD of the endodermis around 7 to 14 days post-germination, middle cortex formation is used as a parameter to assess maturation of the root ground tissue. Molecular, genetic, and physiological studies have revealed that the control of the timing and extent of middle cortex formation during root maturation relies on the interaction of plant hormones and transcription factors. In particular, abscisic acid and gibberellin act synergistically to regulate the timing and extent of middle cortex formation, unlike their typical antagonism. The SHORT-ROOT, SCARECROW, SCARECROW-LIKE 3, and DELLA transcription factors, all of which belong to the plant-specific GRAS family, play key roles in the regulation of middle cortex formation. Recently, two additional transcription factors, SEUSS and GA- AND ABA-RESPONSIVE ZINC FINGER, have also been characterized during ground tissue maturation. In this review, we provide a detailed account of the regulatory networks that control the timing and extent of middle cortex formation during post-embryonic root development.

Novel players fine-tune plant trade-offs

Jasmonates (JAs) are essential signalling molecules that co-ordinate the plant response to biotic and abiotic challenges, as well as co-ordinating several developmental processes. Huge progress has been made over the last decade in understanding the components and mechanisms that govern JA perception and signalling. The bioactive form of the hormone, (+)-7-iso-jasmonyl-L-isoleucine (JA-Ile), is perceived by the COI1-JAZ co-receptor complex. JASMONATE ZIM DOMAIN (JAZ) proteins also act as direct repressors of transcriptional activators such as MYC2. In the emerging picture of JA-Ile perception and signalling, COI1 operates as an E3 ubiquitin ligase that upon binding of JA-Ile targets JAZ repressors for degradation by the 26S proteasome, thereby derepressing transcription factors such as MYC2, which in turn activate JA-Ile-dependent transcriptional reprogramming. It is noteworthy that MYCs and different spliced variants of the JAZ proteins are involved in a negative regulatory feedback loop, which suggests a model that rapidly turns the transcriptional JA-Ile responses on and off and thereby avoids a detrimental overactivation of the pathway. This chapter highlights the most recent advances in our understanding of JA-Ile signalling, focusing on the latest repertoire of new targets of JAZ proteins to control different sets of JA-Ile-mediated responses, novel mechanisms of negative regulation of JA-Ile signalling, and hormonal cross-talk at the molecular level that ultimately determines plant adaptability and survival.

Deep-sequencing transcriptome analysis of field-grown Medicago sativa L. crown buds acclimated to freezing stress

Medicago sativa L. (alfalfa) 'Zhaodong' is an important forage legume that can safely survive in northern China where winter temperatures reach as low as -30 °C. Survival of alfalfa following freezing stress depends on the amount and revival ability of crown buds. In order to investigate the molecular mechanisms of frost tolerance in alfalfa, we used transcriptome sequencing technology and bioinformatics strategies to analyze crown buds of field-grown alfalfa during winter. We statistically identified a total of 5605 differentially expressed genes (DEGs) involved in freezing stress including 1900 upregulated and 3705 downregulated DEGs. We validated 36 candidate DEGs using qPCR to confirm the accuracy of the RNA-seq data. Unlike other recent studies, this study employed alfalfa plants grown in the natural environment. Our results indicate that not only the CBF orthologs but also membrane proteins, hormone signal transduction pathways, and ubiquitin-mediated proteolysis pathways indicate the presence of a special freezing adaptation mechanism in alfalfa. The antioxidant defense system may rapidly confer freezing tolerance to alfalfa. Importantly, biosynthesis of secondary metabolites and phenylalanine metabolism, which is of potential importance in coordinating freezing tolerance with growth and development, were downregulated in subzero temperatures. The adaptive mechanism for frost tolerance is a complex multigenic process that is not well understood. This systematic analysis provided an in-depth view of stress tolerance mechanisms in alfalfa.

Strigolactone versus gibberellin signaling: reemerging concepts?

MAIN CONCLUSION

In this review, we compare knowledge about the recently discovered strigolactone signaling pathway and the well established gibberellin signaling pathway to identify gaps of knowledge and putative research directions in strigolactone biology. Communication between and inside cells is integral for the vitality of living organisms. Hormonal signaling cascades form a large part of this communication and an understanding of both their complexity and interactive nature is only beginning to emerge. In plants, the strigolactone (SL) signaling pathway is the most recent addition to the classically acting group of hormones and, although fundamental insights have been made, knowledge about the nature and impact of SL signaling is still cursory. This narrow understanding is in spite of the fact that SLs influence a specific spectrum of processes, which includes shoot branching and root system architecture in response, partly, to environmental stimuli. This makes these hormones ideal tools for understanding the coordination of plant growth processes, mechanisms of long-distance communication and developmental plasticity. Here, we summarize current knowledge about SL signaling and employ the well-characterized gibberellin (GA) signaling pathway as a scaffold to highlight emerging features as well as gaps in our knowledge in this context. GA signaling is particularly suitable for this comparison because both signaling cascades share key features of hormone perception and of immediate downstream events. Therefore, our comparative view demonstrates the possible level of complexity and regulatory interfaces of SL signaling.

Association genetics and transcriptome analysis reveal a gibberellin-responsive pathway involved in regulating photosynthesis

Gibberellins (GAs) regulate a wide range of important processes in plant growth and development, including photosynthesis. However, the mechanism by which GAs regulate photosynthesis remains to be understood. Here, we used multi-gene association to investigate the effect of genes in the GA-responsive pathway, as constructed by RNA sequencing, on photosynthesis, growth, and wood property traits, in a population of 435 Populus tomentosa By analyzing changes in the transcriptome following GA treatment, we identified many key photosynthetic genes, in agreement with the observed increase in measurements of photosynthesis. Regulatory motif enrichment analysis revealed that 37 differentially expressed genes related to photosynthesis shared two essential GA-related cis-regulatory elements, the GA response element and the pyrimidine box. Thus, we constructed a GA-responsive pathway consisting of 47 genes involved in regulating photosynthesis, including GID1, RGA, GID2, MYBGa, and 37 photosynthetic differentially expressed genes. Single nucleotide polymorphism (SNP)-based association analysis showed that 142 SNPs, representing 40 candidate genes in this pathway, were significantly associated with photosynthesis, growth, and wood property traits. Epistasis analysis uncovered interactions between 310 SNP-SNP pairs from 37 genes in this pathway, revealing possible genetic interactions. Moreover, a structural gene-gene matrix based on a time-course of transcript abundances provided a better understanding of the multi-gene pathway affecting photosynthesis. The results imply a functional role for these genes in mediating photosynthesis, growth, and wood properties, demonstrating the potential of combining transcriptome-based regulatory pathway construction and genetic association approaches to detect the complex genetic networks underlying quantitative traits.

MicroRNAs in Control of Plant Development

In the long evolutionary history, plant has evolved elaborate regulatory network to control functional gene expression for surviving and thriving, such as transcription factor-regulated transcriptional programming. However, plenty of evidences from the past decade studies demonstrate that the 21-24 nucleotides small RNA molecules, majorly microRNAs (miRNAs) play dominant roles in post-transcriptional gene regulation through base pairing with their complementary mRNA targets, especially prefer to target transcription factors in plants. Here, we review current progresses on miRNA-controlled plant development, from miRNA biogenesis dysregulation-caused pleiotropic developmental defects to specific developmental processes, such as SAM regulation, leaf and root system regulation, and plant floral transition. We also summarize some miRNAs that are experimentally proved to greatly affect crop plant productivity and quality. In addition, recent reports show that a single miRNA usually displays multiple regulatory roles, such as organ development, phase transition, and stresses responses. Thus, we infer that miRNA may act as a node molecule to coordinate the balance between plant development and environmental clues, which may shed the light on finding key regulator or regulatory pathway for uncovering the mysterious molecular network.

Developmental mechanisms underlying variable, invariant and plastic phenotypes

BACKGROUND

Discussions of phenotypic robustness often consider scenarios where invariant phenotypes are optimal and assume that developmental mechanisms have evolved to buffer the phenotypes of specific traits against stochastic and environmental perturbations. However, plastic plant phenotypes that vary between environments or variable phenotypes that vary stochastically within an environment may also be advantageous in some scenarios.

SCOPE

Here the conditions under which invariant, plastic and variable phenotypes of specific traits may confer a selective advantage in plants are examined. Drawing on work from microbes and multicellular organisms, the mechanisms that may give rise to each type of phenotype are discussed.

CONCLUSION

In contrast to the view of robustness as being the ability of a genotype to produce a single, invariant phenotype, changes in a phenotype in response to the environment, or phenotypic variability within an environment, may also be delivered consistently (i.e. robustly). Thus, for some plant traits, mechanisms have probably evolved to produce plasticity or variability in a reliable manner.

Trails to the gibberellin receptor, GIBBERELLIN INSENSITIVE DWARF1

The researches on the identification of gibberellin receptor are reviewed from the early attempts in 1960s to the identification of GIBBERELLIN INSENSITIVE DWARF1 (GID1) as the receptor in 2005. Unpublished data of the gibberellin-binding protein in the seedlings of adzuki bean (Vigna angularis) are also included, suggesting that the active principle of the gibberellin-binding protein was a GID1 homolog.

Critical analysis of protein signaling networks involved in the regulation of plant secondary metabolism: focus on anthocyanins

Anthocyanin biosynthesis in Arabidopsis is a convenient and relatively simple model for investigating the basic principles of secondary metabolism regulation. In recent years, many publications have described links between anthocyanin biosynthesis and general defense reactions in plants as well as photomorphogenesis and hormonal signaling. These relationships are complex, and they cannot be understood intuitively. Upon observing the lacuna in the Arabidopsis interactome (an interaction map of the factors involved in the regulation of Arabidopsis secondary metabolism is not available), we attempted to connect various cellular processes that affect anthocyanin biosynthesis. In this review, we revealed the main signaling protein modules that regulate anthocyanin biosynthesis. To our knowledge, this is the first reconstruction of a network of proteins involved in plant secondary metabolism.

Seed vigour and crop establishment: extending performance beyond adaptation

Seeds are central to crop production, human nutrition, and food security. A key component of the performance of crop seeds is the complex trait of seed vigour. Crop yield and resource use efficiency depend on successful plant establishment in the field, and it is the vigour of seeds that defines their ability to germinate and establish seedlings rapidly, uniformly, and robustly across diverse environmental conditions. Improving vigour to enhance the critical and yield-defining stage of crop establishment remains a primary objective of the agricultural industry and the seed/breeding companies that support it. Our knowledge of the regulation of seed germination has developed greatly in recent times, yet understanding of the basis of variation in vigour and therefore seed performance during the establishment of crops remains limited. Here we consider seed vigour at an ecophysiological, molecular, and biomechanical level. We discuss how some seed characteristics that serve as adaptive responses to the natural environment are not suitable for agriculture. Past domestication has provided incremental improvements, but further actively directed change is required to produce seeds with the characteristics required both now and in the future. We discuss ways in which basic plant science could be applied to enhance seed performance in crop production.

Gibberellin Deficiency Confers Both Lodging and Drought Tolerance in Small Cereals

Tef [Eragrostis tef (Zucc.) Trotter] and finger millet [Eleusine coracana Gaertn] are staple cereal crops in Africa and Asia with several desirable agronomic and nutritional properties. Tef is becoming a life-style crop as it is gluten-free while finger millet has a low glycemic index which makes it an ideal food for diabetic patients. However, both tef and finger millet have extremely low grain yields mainly due to moisture scarcity and susceptibility of the plants to lodging. In this study, the effects of gibberellic acid (GA) inhibitors particularly paclobutrazol (PBZ) on diverse physiological and yield-related parameters were investigated and compared to GA mutants in rice (Oryza sativa L.). The application of PBZ to tef and finger millet significantly reduced the plant height and increased lodging tolerance. Remarkably, PBZ also enhanced the tolerance of both tef and finger millet to moisture deficit. Under moisture scarcity, tef plants treated with PBZ did not exhibit drought-related symptoms and their stomatal conductance was unaltered, leading to higher shoot biomass and grain yield. Semi-dwarf rice mutants altered in GA biosynthesis, were also shown to have improved tolerance to dehydration. The combination of traits (drought tolerance, lodging tolerance and increased yield) that we found in plants with altered GA pathway is of importance to breeders who would otherwise rely on extensive crossing to introgress each trait individually. The key role played by PBZ in the tolerance to both lodging and drought calls for further studies using mutants in the GA biosynthesis pathway in order to obtain candidate lines which can be incorporated into crop-breeding programs to create lodging tolerant and climate-smart crops.

Structural and Functional Analysis of the GRAS Gene Family in Grapevine Indicates a Role of GRAS Proteins in the Control of Development and Stress Responses

GRAS transcription factors are involved in many processes of plant growth and development (e.g., axillary shoot meristem formation, root radial patterning, nodule morphogenesis, arbuscular development) as well as in plant disease resistance and abiotic stress responses. However, little information is available concerning this gene family in grapevine (Vitis vinifera L.), an economically important woody crop. We performed a model curation of GRAS genes identified in the latest genome annotation leading to the identification of 52 genes. Gene models were improved and three new genes were identified that could be grapevine- or woody-plant specific. Phylogenetic analysis showed that GRAS genes could be classified into 13 groups that mapped on the 19 V. vinifera chromosomes. Five new subfamilies, previously not characterized in other species, were identified. Multiple sequence alignment showed typical GRAS domain in the proteins and new motifs were also described. As observed in other species, both segmental and tandem duplications contributed significantly to the expansion and evolution of the GRAS gene family in grapevine. Expression patterns across a variety of tissues and upon abiotic and biotic conditions revealed possible divergent functions of GRAS genes in grapevine development and stress responses. By comparing the information available for tomato and grapevine GRAS genes, we identified candidate genes that might constitute conserved transcriptional regulators of both climacteric and non-climacteric fruit ripening. Altogether this study provides valuable information and robust candidate genes for future functional analysis aiming at improving the quality of fleshy fruits.

Comparative Transcriptome Analysis of Genes Involved in GA-GID1-DELLA Regulatory Module in Symbiotic and Asymbiotic Seed Germination of Anoectochilus roxburghii (Wall.) Lindl. (Orchidaceae)

Anoectochilus roxburghii (Wall.) Lindl. (Orchidaceae) is an endangered medicinal plant in China, also called “King Medicine”. Due to lacking of sufficient nutrients in dust-like seeds, orchid species depend on mycorrhizal fungi for seed germination in the wild. As part of a conservation plan for the species, research on seed germination is necessary. However, the molecular mechanism of seed germination and underlying orchid-fungus interactions during symbiotic germination are poorly understood. In this study, Illumina HiSeq 4000 transcriptome sequencing was performed to generate a substantial sequence dataset of germinating A. roxburghii seed. A mean of 44,214,845 clean reads were obtained from each sample. 173,781 unigenes with a mean length of 653 nt were obtained. A total of 51,514 (29.64%) sequences were annotated, among these, 49 unigenes encoding proteins involved in GA-GID1-DELLA regulatory module, including 31 unigenes involved in GA metabolism pathway, 5 unigenes encoding GID1, 11 unigenes for DELLA and 2 unigenes for GID2. A total of 11,881 genes showed significant differential expression in the symbiotic germinating seed sample compared with the asymbiotic germinating seed sample, of which six were involved in the GA-GID1-DELLA regulatory module, and suggested that they might be induced or suppressed by fungi. These results will help us understand better the molecular mechanism of orchid seed germination and orchid-fungus symbiosis.

Plant Survival in a Changing Environment: The Role of Nitric Oxide in Plant Responses to Abiotic Stress

Nitric oxide in plants may originate endogenously or come from surrounding atmosphere and soil. Interestingly, this gaseous free radical is far from having a constant level and varies greatly among tissues depending on a given plant's ontogeny and environmental fluctuations. Proper plant growth, vegetative development, and reproduction require the integration of plant hormonal activity with the antioxidant network, as well as the maintenance of concentration of reactive oxygen and nitrogen species within a narrow range. Plants are frequently faced with abiotic stress conditions such as low nutrient availability, salinity, drought, high ultraviolet (UV) radiation and extreme temperatures, which can influence developmental processes and lead to growth restriction making adaptive responses the plant's priority. The ability of plants to respond and survive under environmental-stress conditions involves sensing and signaling events where nitric oxide becomes a critical component mediating hormonal actions, interacting with reactive oxygen species, and modulating gene expression and protein activity. This review focuses on the current knowledge of the role of nitric oxide in adaptive plant responses to some specific abiotic stress conditions, particularly low mineral nutrient supply, drought, salinity and high UV-B radiation.

Does gibberellin biosynthesis play a critical role in the growth of Lolium perenne? Evidence from a transcriptional analysis of gibberellin and carbohydrate metabolic genes after defoliation

Global meat and milk production depends to a large extent on grazed pastures, with Lolium perenne being the major forage grass in temperate regions. Defoliation and subsequent regrowth of leaf blades is a major and essential event with respect to L. perenne growth and productivity. Following defoliation, carbohydrates (mainly fructans and sucrose) have to be mobilized from heterotrophic tissues to provide energy and carbon for regrowth of photosynthetic tissues. This mobilization of reserve carbohydrates requires a substantial change in the expression of genes coding for enzymes involved in carbohydrate metabolism. Here we tested the hypothesis that gibberellins (GA) are at the core of the processes regulating the expression of these genes. Thus, we examined the transcript profiles of genes involved in carbohydrate and GA metabolic pathways across a time course regrowth experiment. Our results show that following defoliation, the immediate reduction of carbohydrate concentrations in growing tissues is associated with a concomitant increase in the expression of genes encoding carbohydrate mobilizing invertases, and was also associated with a strong decrease in the expression of fructan synthesizing fructosyltransferase genes. We also show that the decrease in fructan levels is preceded by increased expression of the GA activating gene GA 3-oxidase and decreased expression of the GA inactivating gene GA 2 -oxidase in sheaths. GA 3-oxidase expression was negatively, while GA 2 -oxidase positively linked to sucrose concentrations. This study provides indicative evidence that gibberellins might play a role in L. perenne regrowth following defoliation and we hypothesize that there is a link between gibberellin regulation and sugar metabolism in L. perenne.

Insights into molecular and metabolic events associated with fruit response to post-harvest fungal pathogens

Due to post-harvest losses more than 30% of harvested fruits will not reach the consumers' plate. Fungal pathogens play a key role in those losses, as they cause most of the fruit rots and the customer complaints. Many of the fungal pathogens are already present in the unripe fruit but remain quiescent during fruit growth until a particular phase of fruit ripening and senescence. The pathogens sense the developmental change and switch into the devastating necrotrophic life style that causes fruit rotting. Colonization of unripe fruit by the fungus initiates defensive responses that limit fungal growth and development. However, during fruit ripening several physiological processes occur that correlate with increased fruit susceptibility. In contrast to plant defenses in unripe fruit, the defense posture of ripe fruit entails a different subset of defense responses that will end with fruit rotting and losses. This review will focus on several aspects of molecular and metabolic events associated with fleshy fruit responses induced by post-harvest fungal pathogens during fruit ripening.

Degradation of the ABA co-receptor ABI1 by PUB12/13 U-box E3 ligases

Clade A protein phosphatase 2Cs (PP2Cs) are abscisic acid (ABA) co-receptors that block ABA signalling by inhibiting the downstream protein kinases. ABA signalling is activated after PP2Cs are inhibited by ABA-bound PYR/PYL/RCAR ABA receptors (PYLs) in Arabidopsis. However, whether these PP2Cs are regulated by other factors remains unknown. Here, we report that ABI1 (ABA-INSENSITIVE 1) can interact with the U-box E3 ligases PUB12 and PUB13, but is ubiquitinated only when it interacts with ABA receptors in an in vitro assay. A mutant form of ABI1-1 that is unable to interact with PYLs is more stable than the wild-type protein. Both ABI1 degradation and all tested ABA responses are reduced in pub12 pub13 mutants compared with the wild type. Introducing the abi1-3 loss-of-function mutation into pub12 pub13 mutant recovers the ABA-insensitive phenotypes of the pub12 pub13 mutant. We thus uncover an important regulatory mechanism for regulating ABI1 levels by PUB12 and PUB13.

Hydrogen peroxide priming modulates abiotic oxidative stress tolerance: insights from ROS detoxification and scavenging

Plants are constantly challenged by various abiotic stresses that negatively affect growth and productivity worldwide. During the course of their evolution, plants have developed sophisticated mechanisms to recognize external signals allowing them to respond appropriately to environmental conditions, although the degree of adjustability or tolerance to specific stresses differs from species to species. Overproduction of reactive oxygen species (ROS; hydrogen peroxide, H2O2; superoxide, [Formula: see text]; hydroxyl radical, OH(⋅) and singlet oxygen, (1)O2) is enhanced under abiotic and/or biotic stresses, which can cause oxidative damage to plant macromolecules and cell structures, leading to inhibition of plant growth and development, or to death. Among the various ROS, freely diffusible and relatively long-lived H2O2 acts as a central player in stress signal transduction pathways. These pathways can then activate multiple acclamatory responses that reinforce resistance to various abiotic and biotic stressors. To utilize H2O2 as a signaling molecule, non-toxic levels must be maintained in a delicate balancing act between H2O2 production and scavenging. Several recent studies have demonstrated that the H2O2-priming can enhance abiotic stress tolerance by modulating ROS detoxification and by regulating multiple stress-responsive pathways and gene expression. Despite the importance of the H2O2-priming, little is known about how this process improves the tolerance of plants to stress. Understanding the mechanisms of H2O2-priming-induced abiotic stress tolerance will be valuable for identifying biotechnological strategies to improve abiotic stress tolerance in crop plants. This review is an overview of our current knowledge of the possible mechanisms associated with H2O2-induced abiotic oxidative stress tolerance in plants, with special reference to antioxidant metabolism.

Uncovering DELLA-Independent Gibberellin Responses by Characterizing New Tomato procera Mutants

Gibberellin (GA) regulates plant development primarily by triggering the degradation/deactivation of the DELLA proteins. However, it remains unclear whether all GA responses are regulated by DELLAs. Tomato (Solanum lycopersicum) has a single DELLA gene named PROCERA (PRO), and its recessive pro allele exhibits constitutive GA activity but retains responsiveness to external GA. In the loss-of-function mutant pro(ΔGRAS), all examined GA developmental responses were considerably enhanced relative to pro and a defect in seed desiccation tolerance was uncovered. As pro, but not pro(ΔGRAS), elongation was promoted by GA treatment, pro may retain residual DELLA activity. In agreement with homeostatic feedback regulation of the GA biosynthetic pathway, we found that GA20oxidase1 expression was suppressed in pro(ΔGRAS) and was not affected by exogenous GA3. In contrast, expression of GA2oxidase4 was not affected by the elevated GA signaling in pro(ΔGRAS) but was strongly induced by exogenous GA3. Since a similar response was found in Arabidopsis thaliana plants with impaired activity of all five DELLA genes, we suggest that homeostatic GA responses are regulated by both DELLA-dependent and -independent pathways. Transcriptome analysis of GA-treated pro(ΔGRAS) leaves suggests that 5% of all GA-regulated genes in tomato are DELLA independent.

A Gibberellin-Mediated DELLA-NAC Signaling Cascade Regulates Cellulose Synthesis in Rice

Cellulose, which can be converted into numerous industrial products, has important impacts on the global economy. It has long been known that cellulose synthesis in plants is tightly regulated by various phytohormones. However, the underlying mechanism of cellulose synthesis regulation remains elusive. Here, we show that in rice (Oryza sativa), gibberellin (GA) signals promote cellulose synthesis by relieving the interaction between SLENDER RICE1 (SLR1), a DELLA repressor of GA signaling, and NACs, the top-layer transcription factors for secondary wall formation. Mutations in GA-related genes and physiological treatments altered the transcription of CELLULOSE SYNTHASE genes (CESAs) and the cellulose level. Multiple experiments demonstrated that transcription factors NAC29/31 and MYB61 are CESA regulators in rice; NAC29/31 directly regulates MYB61, which in turn activates CESA expression. This hierarchical regulation pathway is blocked by SLR1-NAC29/31 interactions. Based on the results of anatomical analysis and GA content examination in developing rice internodes, this signaling cascade was found to be modulated by varied endogenous GA levels and to be required for internode development. Genetic and gene expression analyses were further performed in Arabidopsis thaliana GA-related mutants. Altogether, our findings reveal a conserved mechanism by which GA regulates secondary wall cellulose synthesis in land plants and provide a strategy for manipulating cellulose production and plant growth.

Digital Single-Cell Analysis of Plant Organ Development Using 3DCellAtlas

Diverse molecular networks underlying plant growth and development are rapidly being uncovered. Integrating these data into the spatial and temporal context of dynamic organ growth remains a technical challenge. We developed 3DCellAtlas, an integrative computational pipeline that semiautomatically identifies cell types and quantifies both 3D cellular anisotropy and reporter abundance at single-cell resolution across whole plant organs. Cell identification is no less than 97.8% accurate and does not require transgenic lineage markers or reference atlases. Cell positions within organs are defined using an internal indexing system generating cellular level organ atlases where data from multiple samples can be integrated. Using this approach, we quantified the organ-wide cell-type-specific 3D cellular anisotropy driving Arabidopsis thaliana hypocotyl elongation. The impact ethylene has on hypocotyl 3D cell anisotropy identified the preferential growth of endodermis in response to this hormone. The spatiotemporal dynamics of the endogenous DELLA protein RGA, expansin gene EXPA3, and cell expansion was quantified within distinct cell types of Arabidopsis roots. A significant regulatory relationship between RGA, EXPA3, and growth was present in the epidermis and endodermis. The use of single-cell analyses of plant development enables the dynamics of diverse regulatory networks to be integrated with 3D organ growth.

Knockdown of a JmjC domain-containing gene JMJ524 confers altered gibberellin responses by transcriptional regulation of GRAS protein lacking the DELLA domain genes in tomato

Plants integrate responses to independent hormonal and environmental signals to survive adversity. In particular, the phytohormone gibberellin (GA) regulates a variety of developmental processes and stress responses. In this study, the Jumonji-C (JmjC) domain-containing gene JMJ524 was characterized in tomato. JMJ524 responded to circadian rhythms and was upregulated by GA treatment. Knockdown of JMJ524 by RNAi caused a GA-insensitive dwarf phenotype with shrunken leaves and shortened internodes. However, in these transgenic plants, higher levels of endogenous GAs were detected. A genome-wide gene expression analysis by RNA-seq indicated that the expression levels of two DELLA-like genes, SlGLD1 ('GRAS protein Lacking the DELLA domain') and SlGLD2, were increased in JMJ524-RNAi transgenic plants. Nevertheless, only the overexpression of SlGLD1 in tomato resulted in a GA-insensitive dwarf phenotype, suggesting that SlGLD1 acts as a repressor of GA signalling. This study proposes that JMJ524 is required for stem elongation by altering GA responses, at least partially by regulating SlGLD1.

Combinatorial activities of SHORT VEGETATIVE PHASE and FLOWERING LOCUS C define distinct modes of flowering regulation in Arabidopsis

Background

The initiation of flowering is an important developmental transition as it marks the beginning of the reproductive phase in plants. The MADS-box transcription factors (TFs) FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE (SVP) form a complex to repress the expression of genes that initiate flowering in Arabidopsis. Both TFs play a central role in the regulatory network by conferring seasonal patterns of flowering. However, their interdependence and biological relevance when acting as a complex have not been extensively studied.

Results

We characterized the effects of both TFs individually and as a complex on flowering initiation using transcriptome profiling and DNA-binding occupancy. We find four major clusters regulating transcriptional responses, and that DNA binding scenarios are highly affected by the presence of the cognate partner. Remarkably, we identify genes whose regulation depends exclusively on simultaneous action of both proteins, thus distinguishing between the specificity of the SVP:FLC complex and that of each TF acting individually. The downstream targets of the SVP:FLC complex include a higher proportion of genes regulating floral induction, whereas those bound by either TF independently are biased towards floral development. Many genes involved in gibberellin-related processes are bound by the SVP:FLC complex, suggesting that direct regulation of gibberellin metabolism by FLC and SVP contributes to their effects on flowering.

Conclusions

The regulatory codes controlled by SVP and FLC were deciphered at the genome-wide level revealing substantial flexibility based on dependent and independent DNA binding that may contribute to variation and robustness in the regulation of flowering.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-015-0597-1) contains supplementary material, which is available to authorized users.

Identification of phloem-mobile mRNA

Signaling between cells, tissues and organs is essential for multicellular organisms to coordinate and adapt their development and growth to internal and environmental changes. Plants have evolved a plant-specific symplasmic pathway, called plasmodesmata, for efficient intercellular communication, in addition to the receptor-ligand-based apoplasmic pathway. Long-distance signaling between distant organs is enabled via the phloem tube system, where plasmodesmata contribute to phloem loading and unloading for photosynthate allocation. In addition to signaling by small molecules such as metabolites and phytohormones, the transport of proteins, small RNAs and mRNAs is also considered an important mechanism to achieve long-distance signaling in plants. Recent studies on phloem-mobile proteins and small RNAs have revealed their role in crucial physiological processes including flowering, systemic silencing and nutrient allocation. However, the biological role of mRNAs found in the phloem tube is not yet clear, though their mobility over long-distances has been well evidenced. To gain this knowledge, it is important to collect further information on mRNA profiles in the phloem translocation stream. In this review, I summarize the current approaches to identifying the mRNA population in the phloem translocation system, and discuss the possible role of short- and long-distance mRNA transport.

Developmentally distinct activities of the exocyst enable rapid cell elongation and determine meristem size during primary root growth in Arabidopsis

BACKGROUND

Exocytosis is integral to root growth: trafficking components of systems that control growth (e.g., PIN auxin transport proteins) to the plasma membrane, and secreting materials that expand the cell wall to the apoplast. Spatiotemporal regulation of exocytosis in eukaryotes often involves the exocyst, an octameric complex that tethers selected secretory vesicles to specific sites on the plasma membrane and facilitates their exocytosis. We evaluated Arabidopsis lines with mutations in four exocyst components (SEC5, SEC8, EXO70A1 and EXO84B) to explore exocyst function in primary root growth.

RESULTS

The mutants have root growth rates that are 82% to 11% of wild-type. Even in lines with the most severe defects, the organization of the quiescent center and tissue layers at the root tips appears similar to wild-type, although meristematic, transition, and elongation zones are shorter. Reduced cell production rates in the mutants are due to the shorter meristems, but not to lengthened cell cycles. Additionally, mutants demonstrate reduced anisotropic cell expansion in the elongation zone, but not the meristematic zone, resulting in shorter mature cells that are similar in shape to wild-type. As expected, hypersensitivity to brefeldin A links the mutant root growth defect to altered vesicular trafficking. Several experimental approaches (e.g., dose-response measurements, localization of signaling components) failed to identify aberrant auxin or brassinosteroid signaling as a primary driver for reduced root growth in exocyst mutants.

CONCLUSIONS

The exocyst participates in two spatially distinct developmental processes, apparently by mechanisms not directly linked to auxin or brassinosteroid signaling pathways, to help establish root meristem size, and to facilitate rapid cell expansion in the elongation zone.

Through the doors of perception to function in arbuscular mycorrhizal symbioses

The formation of an arbuscular mycorrhizal (AM) symbiosis is initiated by the bidirectional exchange of diffusible molecules. While strigolactone hormones, secreted from plant roots,stimulate hyphal branching and fungal metabolism, fungal short-chain chitin oligomers as well assulfated and nonsulfated lipochitooligosaccharides (s/nsMyc-LCOs) elicit pre-symbiosis responses in the host. Fungal LCO signals are structurally related to rhizobial Nod-factor LCOs. Genome-wide expression studies demonstrated that defined sets of genes were induced by Nod-, sMyc- and nsMyc-LCOs, indicating LCO-specific perception in the pre-symbiosis phase. During hyphopodium formation and the subsequent root colonization, cross-talk between plant roots and AM fungi also involves phytohormones. Notably, gibberellins control arbuscule formation via DELLA proteins, which themselves serve as positive regulators of arbuscule formation. The establishment of arbuscules is accompanied by a substantial transcriptional and post-transcriptional reprogramming of host roots, ultimately defining the unique protein composition of arbuscule-containing cells. Based on cellular expression profiles, key check points of AM development as well as candidate genes encoding transcriptional regulators and regulatory microRNAs were identified. Detailed functional analyses of promoters specified short motifs sufficient for cell-autonomous gene regulation in cells harboring arbuscules, and suggested simultaneous, multi-level regulation of the mycorrhizal phosphate uptake pathway by integrating AM symbiosis and phosphate starvation response signaling.

Cotton WRKY1 mediates the plant defense-to-development transition during infection of cotton by Verticillium dahliae by activating JASMONATE ZIM-DOMAIN1 expression

Plants have evolved an elaborate signaling network to ensure an appropriate level of immune response to meet the differing demands of developmental processes. Previous research has demonstrated that DELLA proteins physically interact with JASMONATE ZIM-DOMAIN1 (JAZ1) and dynamically regulate the interaction of the gibberellin (GA) and jasmonate (JA) signaling pathways. However, whether and how the JAZ1-DELLA regulatory node is regulated at the transcriptional level in plants under normal growth conditions or during pathogen infection is not known. Here, we demonstrate multiple functions of cotton (Gossypium barbadense) GbWRKY1 in the plant defense response and during development. Although GbWRKY1 expression is induced rapidly by methyl jasmonate and infection by Verticillium dahliae, our results show that GbWRKY1 is a negative regulator of the JA-mediated defense response and plant resistance to the pathogens Botrytis cinerea and V. dahliae. Under normal growth conditions, GbWRKY1-overexpressing lines displayed GA-associated phenotypes, including organ elongation and early flowering, coupled with the down-regulation of the putative targets of DELLA. We show that the GA-related phenotypes of GbWRKY1-overexpressing plants depend on the constitutive expression of Gossypium hirsutum GhJAZ1. We also show that GhJAZ1 can be transactivated by GbWRKY1 through TGAC core sequences, and the adjacent sequences of this binding site are essential for binding specificity and affinity to GbWRKY1, as revealed by dual-luciferase reporter assays and electrophoretic mobility shift assays. In summary, our data suggest that GbWRKY1 is a critical regulator mediating the plant defense-to-development transition during V. dahliae infection by activating JAZ1 expression.

Control of cotton fibre elongation by a homeodomain transcription factor GhHOX3

Cotton fibres are unusually long, single-celled epidermal seed trichomes and a model for plant cell growth, but little is known about the regulation of fibre cell elongation. Here we report that a homeodomain-leucine zipper (HD-ZIP) transcription factor, GhHOX3, controls cotton fibre elongation. GhHOX3 genes are localized to the 12th homoeologous chromosome set of allotetraploid cotton cultivars, associated with quantitative trait loci (QTLs) for fibre length. Silencing of GhHOX3 greatly reduces (>80%) fibre length, whereas its overexpression leads to longer fibre. Combined transcriptomic and biochemical analyses identify target genes of GhHOX3 that also contain the L1-box cis-element, including two cell wall loosening protein genes GhRDL1 and GhEXPA1. GhHOX3 interacts with GhHD1, another homeodomain protein, resulting in enhanced transcriptional activity, and with cotton DELLA, GhSLR1, repressor of the growth hormone gibberellin (GA). GhSLR1 interferes with the GhHOX3–GhHD1 interaction and represses target gene transcription. Our results uncover a novel mechanism whereby a homeodomain protein transduces GA signal to promote fibre cell elongation.

Cotton fibre is the most important renewable material for textiles, with a huge economic output. Here the authors show that a homeodomain-leucine zipper transcription factor, GhHOX3, transduces a gibberellin signal that in turn promotes fibre cell elongation.

To grow or defend? Low red : far-red ratios reduce jasmonate sensitivity in Arabidopsis seedlings by promoting DELLA degradation and increasing JAZ10 stability

How plants balance resource allocation between growth and defense under conditions of competitive stress is a key question in plant biology. Low red : far-red (R : FR) ratios, which signal a high risk of competition in plant canopies, repress jasmonate-induced defense responses. The mechanism of this repression is not well understood. We addressed this problem in Arabidopsis by investigating the role of DELLA and JASMONATE ZIM domain (JAZ) proteins. We showed that a quintuple della mutant and a phyB mutant were insensitive to jasmonate for several physiological readouts. Inactivation of the photoreceptor phyB by low R : FR ratios rapidly reduced DELLA protein abundance, and the inhibitory effect of FR on jasmonate signaling was missing in the gai-1 mutant, which encodes a stable version of the GAI DELLA protein. We also demonstrated that low R : FR ratios and the phyB mutation stabilized the protein JAZ10. Furthermore, we demonstrated that JAZ10 was required for the inhibitory effect of low R : FR on jasmonate responses, and that the jaz10 mutation restored jasmonate sensitivity to the phyB mutant. We conclude that, under conditions of competition for light, plants redirect resource allocation from defense to rapid elongation by promoting DELLA degradation and enhancing JAZ10 stability.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Redox regulation of plant development

SIGNIFICANCE

We provide a conceptual framework for the interactions between the cellular redox signaling hub and the phytohormone signaling network that controls plant growth and development to maximize plant productivity under stress-free situations, while limiting growth and altering development on exposure to stress.

RECENT ADVANCES

Enhanced cellular oxidation plays a key role in the regulation of plant growth and stress responses. Oxidative signals or cycles of oxidation and reduction are crucial for the alleviation of dormancy and quiescence, activating the cell cycle and triggering genetic and epigenetic control that underpin growth and differentiation responses to changing environmental conditions.

CRITICAL ISSUES

The redox signaling hub interfaces directly with the phytohormone network in the synergistic control of growth and its modulation in response to environmental stress, but a few components have been identified. Accumulating evidence points to a complex interplay of phytohormone and redox controls that operate at multiple levels. For simplicity, we focus here on redox-dependent processes that control root growth and development and bud burst.

FUTURE DIRECTIONS

The multiple roles of reactive oxygen species in the control of plant growth and development have been identified, but increasing emphasis should now be placed on the functions of redox-regulated proteins, along with the central roles of reductants such as NAD(P)H, thioredoxins, glutathione, glutaredoxins, peroxiredoxins, ascorbate, and reduced ferredoxin in the regulation of the genetic and epigenetic factors that modulate the growth and vigor of crop plants, particularly within an agricultural context.

Interplay between sugar and hormone signaling pathways modulate floral signal transduction

NOMENCLATURE The following nomenclature will be used in this article: Names of genes are written in italicized upper-case letters, e.g., ABI4.Names of proteins are written in non-italicized upper-case letters, e.g., ABI4.Names of mutants are written in italicized lower-case letters, e.g., abi4. The juvenile-to-adult and vegetative-to-reproductive phase transitions are major determinants of plant reproductive success and adaptation to the local environment. Understanding the intricate molecular genetic and physiological machinery by which environment regulates juvenility and floral signal transduction has significant scientific and economic implications. Sugars are recognized as important regulatory molecules that regulate cellular activity at multiple levels, from transcription and translation to protein stability and activity. Molecular genetic and physiological approaches have demonstrated different aspects of carbohydrate involvement and its interactions with other signal transduction pathways in regulation of the juvenile-to-adult and vegetative-to-reproductive phase transitions. Sugars regulate juvenility and floral signal transduction through their function as energy sources, osmotic regulators and signaling molecules. Interestingly, sugar signaling has been shown to involve extensive connections with phytohormone signaling. This includes interactions with phytohormones that are also important for the orchestration of developmental phase transitions, including gibberellins, abscisic acid, ethylene, and brassinosteroids. This article highlights the potential roles of sugar-hormone interactions in regulation of floral signal transduction, with particular emphasis on Arabidopsis thaliana mutant phenotypes, and suggests possible directions for future research.

The effect of mepiquat chloride on elongation of cotton (Gossypium hirsutum L.) internode is associated with low concentration of gibberellic acid

The growth regulator mepiquat chloride (MC) is globally used in cotton (Gossypium hirsutum L.) canopy manipulation to avoid excess growth and yield loss. However, little information is available as to whether the modification of plant architecture by MC is related to alterations in gibberellic acid (GA) metabolism and signaling. Here, the role of GA metabolism and signaling was investigated in cotton seedlings treated with MC. The MC significantly decreased endogenous GA3 and GA4 levels in the elongating internode, which inhibited cell elongation by downregulating GhEXP and GhXTH2, and then reducing plant height. Biosynthetic and metabolic genes of GA were markedly suppressed within 2-10d of MC treatment, which also downregulated the expression of DELLA-like genes. A remarkable feedback regulation was observed at the early stage of MC treatment when GA biosynthetic and metabolic genes expression was evidently upregulated. Mepiquat chloride action was controlled by temporal translocation and spatial accumulation which regulated GA biosynthesis and signal expression for maintaining GA homeostasis. The results suggested that MC application could reduce endogenous GA levels in cotton through controlled GA biosynthetic and metabolic genes expression, which might inhibit cell elongation, thereby shortening the internode and reducing plant height.

Arabidopsis DELLA protein degradation is controlled by a type-one protein phosphatase, TOPP4

Gibberellins (GAs) are a class of important phytohormones regulating a variety of physiological processes during normal plant growth and development. One of the major events during GA-mediated growth is the degradation of DELLA proteins, key negative regulators of GA signaling pathway. The stability of DELLA proteins is thought to be controlled by protein phosphorylation and dephosphorylation. Up to date, no phosphatase involved in this process has been identified. We have identified a dwarfed dominant-negative Arabidopsis mutant, named topp4-1. Reduced expression of TOPP4 using an artificial microRNA strategy also resulted in a dwarfed phenotype. Genetic and biochemical analyses indicated that TOPP4 regulates GA signal transduction mainly via promoting DELLA protein degradation. The severely dwarfed topp4-1 phenotypes were partially rescued by the DELLA deficient mutants rga-t2 and gai-t6, suggesting that the DELLA proteins RGA and GAI are required for the biological function of TOPP4. Both RGA and GAI were greatly accumulated in topp4-1 but significantly decreased in 35S-TOPP4 transgenic plants compared to wild-type plants. Further analyses demonstrated that TOPP4 is able to directly bind and dephosphorylate RGA and GAI, confirming that the TOPP4-controlled phosphorylation status of DELLAs is associated with their stability. These studies provide direct evidence for a crucial role of protein dephosphorylation mediated by TOPP4 in the GA signaling pathway.

Gibberellins (GAs) are essential regulators of plant growth and development. They are tightly related to crop productivity in the first “green revolution.” GA triggers its responses by targeting DELLA proteins, the important repressors, for degradation. This process is believed to be regulated by protein phosphorylation and dephosphorylation, but there are not any reports describing the identification of phosphatases regulating this critical event. By screening an ethyl methane sulfonate (EMS)-mutagenized Arabidopsis thaliana population, we identified a protein phosphatase TOPP4, a member of protein phosphatase 1 (PP1), that acts as a positive regulator in the GA signaling pathway. TOPP4 promotes the GA-induced degradation of DELLA proteins by directly dephosphorylating these proteins. This study provides an important insight for the switch role of protein phosphorylation and dephosphorylation in GA signal transduction and sheds light on PP1 protein phosphatases in regulating plant growth and development.

A GAMYB homologue CsGAMYB1 regulates sex expression of cucumber via an ethylene-independent pathway

Cucumber (Cucumis sativus L.) is a typical monoecious vegetable with individual male and female flowers, and has been used as a model plant for sex determination. It is well known that sex differentiation of cucumber can be regulated by phytohormones, such as gibberellic acid (GA) and ethylene. The molecular mechanism of female sex expression modulated by ethylene has been widely understood, but how GA controls male sex expression remains elusive. In hermaphroditic Arabidopsis and rice, GA can regulate stamen and anther development via the transcriptional regulation of GAMYB. Here we characterized a GAMYB homologue CsGAMYB1 in cucumber. We found that CsGAMYB1 is predominantly expressed in male flower buds, where its expression is upregulated by GA3 treatment. CsGAMYB1 protein is localized in the nucleus. CsGAMYB1 can partially rescue stamen development and fertility phenotypes of an Arabidopsis myb33 myb65 double mutant. However, constitutive overexpression of CsGAMYB1 in wild-type Arabidopsis resulted in male sterility, which mimics the effect of GA overdose in flower development. Knockdown of CsGAMYB1 in cucumber decreases the ratio of nodes with male and female flowers, and ethylene is not involved in this process. Our data suggest that CsGAMYB1 regulates sex expression of cucumber via an ethylene-independent pathway.

A GAMYB homologue CsGAMYB1 regulates sex expression of cucumber via an ethylene-independent pathway

Cucumber (Cucumis sativus L.) is a typical monoecious vegetable with individual male and female flowers, and has been used as a model plant for sex determination. It is well known that sex differentiation of cucumber can be regulated by phytohormones, such as gibberellic acid (GA) and ethylene. The molecular mechanism of female sex expression modulated by ethylene has been widely understood, but how GA controls male sex expression remains elusive. In hermaphroditic Arabidopsis and rice, GA can regulate stamen and anther development via the transcriptional regulation of GAMYB. Here we characterized a GAMYB homologue CsGAMYB1 in cucumber. We found that CsGAMYB1 is predominantly expressed in male flower buds, where its expression is upregulated by GA3 treatment. CsGAMYB1 protein is localized in the nucleus. CsGAMYB1 can partially rescue stamen development and fertility phenotypes of an Arabidopsis myb33 myb65 double mutant. However, constitutive overexpression of CsGAMYB1 in wild-type Arabidopsis resulted in male sterility, which mimics the effect of GA overdose in flower development. Knockdown of CsGAMYB1 in cucumber decreases the ratio of nodes with male and female flowers, and ethylene is not involved in this process. Our data suggest that CsGAMYB1 regulates sex expression of cucumber via an ethylene-independent pathway.

A cucumber DELLA homolog CsGAIP may inhibit staminate development through transcriptional repression of B class floral homeotic genes

In hermaphroditic Arabidopsis, the phytohormone gibberellin (GA) stimulates stamen development by opposing the DELLA repression of B and C classes of floral homeotic genes. GA can promote male flower formation in cucumber (Cucumis sativus L.), a typical monoecious vegetable with unisexual flowers, and the molecular mechanism remains unknown. Here we characterized a DELLA homolog CsGAIP in cucumber, and we found that CsGAIP is highly expressed in stem and male flower buds. In situ hybridization showed that CsGAIP is greatly enriched in the stamen primordia, especially during the hermaphrodite stage of flower development. Further, CsGAIP protein is located in nucleus. CsGAIP can partially rescue the plant height, stamen development and fertility phenotypes of Arabidopsis rga-24/gai-t6 mutant, and ectopic expression of CsGAIP in wide-type Arabidopsis results in reduced number of stamens and decreased transcription of B class floral homeotic genes APETALA3 (AP3) and PISTILLATA (PI). Our data suggest that monoecious CsGAIP may inhibit staminate development through transcriptional repression of B class floral homeotic genes in Arabidopsis.

Arabidopsis DELLA and JAZ proteins bind the WD-repeat/bHLH/MYB complex to modulate gibberellin and jasmonate signaling synergy

Integration of diverse environmental and endogenous signals to coordinately regulate growth, development, and defense is essential for plants to survive in their natural habitat. The hormonal signals gibberellin (GA) and jasmonate (JA) antagonistically and synergistically regulate diverse aspects of plant growth, development, and defense. GA and JA synergistically induce initiation of trichomes, which assist seed dispersal and act as barriers to protect plants against insect attack, pathogen infection, excessive water loss, and UV irradiation. However, the molecular mechanism underlying such synergism between GA and JA signaling remains unclear. In this study, we revealed a mechanism for GA and JA signaling synergy and identified a signaling complex of the GA pathway in regulation of trichome initiation. Molecular, biochemical, and genetic evidence showed that the WD-repeat/bHLH/MYB complex acts as a direct target of DELLAs in the GA pathway and that both DELLAs and JAZs interacted with the WD-repeat/bHLH/MYB complex to mediate synergism between GA and JA signaling in regulating trichome development. GA and JA induce degradation of DELLAs and JASMONATE ZIM-domain proteins to coordinately activate the WD-repeat/bHLH/MYB complex and synergistically and mutually dependently induce trichome initiation. This study provides deep insights into the molecular mechanisms for integration of different hormonal signals to synergistically regulate plant development.

Characterization of gibberellin-signalling elements during plum fruit ontogeny defines the essentiality of gibberellin in fruit development

Fruit growth is a coordinated, complex interaction of cell division, differentiation and expansion. Gibberellin (GA) involvement in the reproductive events is an important aspect of GA effects. Perennial fruit-trees such as plum (Prunus salicina L.) have distinct features that are economically important and provide opportunities to dissect specific GA mechanisms. Currently, very little is known on the molecular mechanism(s) mediating GA effects on fruit development. Determination of bioactive GA content during plum fruit ontogeny revealed that GA1 and GA4 are critical for fruit growth and development. Further, characterization of several genes involved in GA-signalling showed that their transcriptional regulation are generally GA-dependent, confirming their involvement in GA-signalling. Based on these results, a model is presented elucidating how the potential association between GA and other hormones may contribute to fruit development. PslGID1 proteins structure, Y2H and BiFC assays indicated that plum GA-receptors can form a complex with AtDELLA-repressors in a GA-dependent manner. Moreover, phenotypical-, molecular- and GA-analyses of various Arabidopsis backgrounds ectopically expressing PslGID1 sequences provide evidence on their role as active GA-signalling components that mediate GA-responsiveness. Our findings support the critical contribution of GA alone or in association with other hormones in mediating plum fruit growth and development.

DELLA activity is required for successful pollen development in the Columbia ecotype of Arabidopsis

Excessive gibberellin (GA) signalling, mediated through the DELLA proteins, has a negative impact on plant fertility. Loss of DELLA activity in the monocot rice (Oryza sativa) causes complete male sterility, but not in the dicot model Arabidopsis (Arabidopsis thaliana) ecotype Landsberg erecta (Ler), in which DELLA function has been studied most extensively, leading to the assumption that DELLA activity is not essential for Arabidopsis pollen development. A novel DELLA fertility phenotype was identified in the Columbia (Col-0) ecotype that necessitates re-evaluation of the general conclusions drawn from Ler. Fertility phenotypes were compared between the Col-0 and Ler ecotypes under conditions of chemical and genetic GA overdose, including mutants in both ecotypes lacking the DELLA paralogues REPRESSOR OF ga1-3 (RGA) and GA INSENSITIVE (GAI). Ler displays a less severe fertility phenotype than Col-0 under GA treatment. Col-0 rga gai mutants, in contrast with the equivalent Ler phenotype, were entirely male sterile, caused by post-meiotic defects in pollen development, which were rescued by the reintroduction of DELLA into either the tapetum or developing pollen. We conclude that DELLA activity is essential for Arabidopsis pollen development. Differences between the fertility responses of Col-0 and Ler might be caused by differences in downstream signalling pathways or altered DELLA expression.

The Arabidopsis gibberellin methyl transferase 1 suppresses gibberellin activity, reduces whole-plant transpiration and promotes drought tolerance in transgenic tomato

Previous studies have shown that reduced gibberellin (GA) level or signal promotes plant tolerance to environmental stresses, including drought, but the underlying mechanism is not yet clear. Here we studied the effects of reduced levels of active GAs on tomato (Solanum lycopersicum) plant tolerance to drought as well as the mechanism responsible for these effects. To reduce the levels of active GAs, we generated transgenic tomato overexpressing the Arabidopsis thaliana GA METHYL TRANSFERASE 1 (AtGAMT1) gene. AtGAMT1 encodes an enzyme that catalyses the methylation of active GAs to generate inactive GA methyl esters. Tomato plants overexpressing AtGAMT1 exhibited typical GA-deficiency phenotypes and increased tolerance to drought stress. GA application to the transgenic plants restored normal growth and sensitivity to drought. The transgenic plants maintained high leaf water status under drought conditions, because of reduced whole-plant transpiration. The reduced transpiration can be attributed to reduced stomatal conductance. GAMT1 overexpression inhibited the expansion of leaf-epidermal cells, leading to the formation of smaller stomata with reduced stomatal pores. It is possible that under drought conditions, plants with reduced GA activity and therefore, reduced transpiration, will suffer less from leaf desiccation, thereby maintaining higher capabilities and recovery rates.