GSK3-like kinases integrate brassinosteroid signaling and stomatal development

Role of Basal ABA in Plant Growth and Development

Abscisic acid (ABA) regulates various aspects of plant physiology, including promoting seed dormancy and adaptive responses to abiotic and biotic stresses. In addition, ABA plays an im-portant role in growth and development under non-stressed conditions. This review summarizes phenotypes of ABA biosynthesis and signaling mutants to clarify the roles of basal ABA in growth and development. The promotive and inhibitive actions of ABA in growth are characterized by stunted and enhanced growth of ABA-deficient and insensitive mutants, respectively. Growth regulation by ABA is both promotive and inhibitive, depending on the context, such as concentrations, tissues, and environmental conditions. Basal ABA regulates local growth including hyponastic growth, skotomorphogenesis and lateral root growth. At the cellular level, basal ABA is essential for proper chloroplast biogenesis, central metabolism, and expression of cell-cycle genes. Basal ABA also regulates epidermis development in the shoot, by inhibiting stomatal development, and deposition of hydrophobic polymers like a cuticular wax layer covering the leaf surface. In the root, basal ABA is involved in xylem differentiation and suberization of the endodermis. Hormone crosstalk plays key roles in growth and developmental processes regulated by ABA. Phenotypes of ABA-deficient and insensitive mutants indicate prominent functions of basal ABA in plant growth and development.

Versatile Physiological Functions of Plant GSK3-Like Kinases

The plant glycogen synthase kinase 3 (GSK3)-like kinases are highly conserved protein serine/threonine kinases that are grouped into four subfamilies. Similar to their mammalian homologs, these kinases are constitutively active under normal growth conditions but become inactivated in response to diverse developmental and environmental signals. Since their initial discoveries in the early 1990s, many biochemical and genetic studies were performed to investigate their physiological functions in various plant species. These studies have demonstrated that the plant GSK3-like kinases are multifunctional kinases involved not only in a wide variety of plant growth and developmental processes but also in diverse plant stress responses. Here we summarize our current understanding of the versatile physiological functions of the plant GSK3-like kinases along with their confirmed and potential substrates.

GSK3/shaggy-like kinase 1 ubiquitously regulates cell growth from Arabidopsis to Moso bamboo (Phyllostachys edulis)

Moso bamboo (Phyllostachys edulis) is one of the fastest growing species with a maximum growth rate of 1 m/day. However, the regulator genes for this explosive growth phenomenon have not been functionally studied. Here, we found that Moso bamboo GSK3/shaggy-like kinase 1 (PeGSK1) acts as a negative regulator of cell growth. Over-expression of PeGSK1 in Arabidopsis showed significant growth arrest phenotypes, including dwarfism, small leaves, reduced cell length, and disturbed cell elongation of petiole. Furthermore, Overexpression of PeGSK1 fully inhibited the longer hypocotyl phenotype of Arabidopsis atgsk1 mutants. In addition, PeGSK1-overexpressing lines were resistant to exogenous BR treatment and PeGSK1 interacted with the brassinosteroid signal transduction key regulator BZR1. The BZR1-dependent cell growth genes were down-regulated in PeGSK1-overexpressing lines. These results indicated that PeGSK1 is functionally similar to AtGSK1 and inhibited cell growth via the brassinosteroid signaling pathway. Importantly, PeGSK1 also interacted with PeBZR1, and the expression pattern of PeGSK1 was negatively correlated with the internode elongation of bamboo, indicating that PeGSK1 is involved in the cell growth of bamboo. In summary, our results provide insight into the role of brassinosteroids in the rapid-growth of bamboo culms and identifying target genes for the genetic manipulation of plant height.

Lotus SHAGGY-like kinase 1 is required to suppress nodulation in Lotus japonicus

Glycogen synthase kinase/SHAGGY-like kinases (SKs) are a highly conserved family of signaling proteins that participate in many developmental, cell-differentiation, and metabolic signaling pathways in plants and animals. Here, we investigate the involvement of SKs in legume nodulation, a process requiring the integration of multiple signaling pathways. We describe a group of SKs in the model legume Lotus japonicus (LSKs), two of which respond to inoculation with the symbiotic nitrogen-fixing bacterium Mesorhizobium loti. RNAi knock-down plants and an insertion mutant for one of these genes, LSK1, display increased nodulation. Ηairy-root lines overexpressing LSK1 form only marginally fewer mature nodules compared with controls. The expression levels of genes involved in the autoregulation of nodulation (AON) mechanism are affected in LSK1 knock-down plants at low nitrate levels, both at early and late stages of nodulation. At higher levels of nitrate, these same plants show the opposite expression pattern of AON-related genes and lose the hypernodulation phenotype. Our findings reveal an additional role for the versatile SK gene family in integrating the signaling pathways governing legume nodulation, and pave the way for further study of their functions in legumes.

Integrating Phosphoproteomics and Bioinformatics to Study Brassinosteroid-Regulated Phosphorylation Dynamics in Arabidopsis

BACKGROUND

Protein phosphorylation regulated by plant hormone is involved in the coordination of fundamental plant development. Brassinosteroids (BRs), a group of phytohormones, regulated phosphorylation dynamics remains to be delineated in plants. In this study, we performed a mass spectrometry (MS)-based phosphoproteomics to conduct a global and dynamic phosphoproteome profiling across five time points of BR treatment in the period between 5 min and 12 h. MS coupling with phosphopeptide enrichment techniques has become the powerful tool for profiling protein phosphorylation. However, MS-based methods tend to have data consistency and coverage issues. To address these issues, bioinformatics approaches were used to complement the non-detected proteins and recover the dynamics of phosphorylation events.

RESULTS

A total of 1104 unique phosphorylated peptides from 739 unique phosphoproteins were identified. The time-dependent gene ontology (GO) analysis shows the transition of biological processes from signaling transduction to morphogenesis and stress response. The protein-protein interaction analysis found that most of identified phosphoproteins have strongly connections with known BR signaling components. The analysis by using Motif-X was performed to identify 15 enriched motifs, 11 of which correspond to 6 known kinase families. To uncover the dynamic activities of kinases, the enriched motifs were combined with phosphorylation profiles and revealed that the substrates of casein kinase 2 and mitogen-activated protein kinase were significantly phosphorylated and dephosphorylated at initial time of BR treatment, respectively. The time-dependent kinase-substrate interaction networks were constructed and showed many substrates are the downstream of other signals, such as auxin and ABA signaling. While comparing BR responsive phosphoproteome and gene expression data, we found most of phosphorylation changes were not led by gene expression changes. Our results suggested many downstream proteins of BR signaling are induced by phosphorylation via various kinases, not through transcriptional regulation.

CONCLUSIONS

Through a large-scale dynamic profile of phosphoproteome coupled with bioinformatics, a complicated kinase-centered network related to BR-regulated growth was deciphered. The phosphoproteins and phosphosites identified in our study provide a useful dataset for revealing signaling networks of BR regulation, and also expanded our knowledge of protein phosphorylation modification in plants as well as further deal to solve the plant growth problems.

Consequences of induced brassinosteroid deficiency in Arabidopsis leaves

BACKGROUND

The identification of brassinosteroid (BR) deficient and BR insensitive mutants provided conclusive evidence that BR is a potent growth-promoting phytohormone. Arabidopsis mutants are characterized by a compact rosette structure, decreased plant height and reduced root system, delayed development, and reduced fertility. Cell expansion, cell division, and multiple developmental processes depend on BR. The molecular and physiological basis of BR action is diverse. The BR signalling pathway controls the activity of transcription factors, and numerous BR responsive genes have been identified. The analysis of dwarf mutants, however, may to some extent reveal phenotypic changes that are an effect of the altered morphology and physiology. This restriction holds particularly true for the analysis of established organs such as rosette leaves.

RESULTS

In this study, the mode of BR action was analysed in established leaves by means of two approaches. First, an inhibitor of BR biosynthesis (brassinazole) was applied to 21-day-old wild-type plants. Secondly, BR complementation of BR deficient plants, namely CPD (constitutive photomorphogenic dwarf)-antisense and cbb1 (cabbage1) mutant plants was stopped after 21 days. BR action in established leaves is associated with stimulated cell expansion, an increase in leaf index, starch accumulation, enhanced CO2 release by the tricarboxylic acid cycle, and increased biomass production. Cell number and protein content were barely affected.

CONCLUSION

Previous analysis of BR promoted growth focused on genomic effects. However, the link between growth and changes in gene expression patterns barely provided clues to the physiological and metabolic basis of growth. Our study analysed comprehensive metabolic data sets of leaves with altered BR levels. The data suggest that BR promoted growth may depend on the increased provision and use of carbohydrates and energy. BR may stimulate both anabolic and catabolic pathways.

Signaling to stomatal initiation and cell division

Stomata are two-celled valves that control epidermal pores whose opening and spacing optimizes shoot-atmosphere gas exchange. Arabidopsis stomatal formation involves at least one asymmetric division and one symmetric division. Stomatal formation and patterning are regulated by the frequency and placement of asymmetric divisions. This model system has already led to significant advances in developmental biology, such as the regulation of cell fate, division, differentiation, and patterning. Over the last 30 years, stomatal development has been found to be controlled by numerous intrinsic genetic and environmental factors. This mini review focuses on the signaling involved in stomatal initiation and in divisions in the cell lineage.

Transcription factor HAT1 is phosphorylated by BIN2 kinase and mediates brassinosteroid repressed gene expression in Arabidopsis

Plant steroid hormones, brassinosteroids (BRs), play essential roles in modulating cell elongation, vascular differentiation, senescence and stress responses. BRs signal through plasma membrane-localized receptor and other components to modulate the BES1/BZR1 (BRI1-EMS SUPPRESSOR 1/BRASSINAZOLE RESISTANT 1) family of transcription factors that modulate thousands of target genes. Arabodopsis thaliana homeodomain-leucine zipper protein 1 (HAT1), which encodes a homeodomain-leucine zipper (HD-Zip) class II transcription factor, was identified through chromatin immunoprecipitation (ChIP) experiments as a direct target gene of BES1. Loss-of-function and gain-of-function mutants of HAT1 display altered BR responses. HAT1 and its close homolog HAT3 act redundantly, as the double mutant hat1 hat3 displayed a reduced BR response that is stronger than the single mutants alone. Moreover, hat1 hat3 enhanced the phenotype of a weak allele of the BR receptor mutant bri1 and suppressed the phenotype of constitutive BR response mutant bes1-D. These results suggest that HAT1 and HAT3 function to activate BR-mediated growth. Expression levels of several BR-repressed genes are increased in hat1 hat3 and reduced in HAT1OX, suggesting that HAT1 functions to repress the expression of a subset of BR target genes. HAT1 and BES1 bind to a conserved homeodomain binding (HB) site and BR response element (BRRE) respectively, in the promoters of some BR-repressed genes. BES1 and HAT1 interact with each other and cooperate to inhibit BR-repressed gene expression. Furthermore, HAT1 can be phosphorylated and stabilized by GSK3 (GLYCOGEN SYNTHASE KINASE 3)-like kinase BIN2 (BRASSINOSTEROID-INSENSITIVE 2), a well established negative regulator of the BR pathway. Our results thus revealed a previously unknown mechanism by which BR signaling modulates BR-repressed gene expression and coordinates plant growth.

Crosstalk between secondary messengers, hormones and MAPK modules during abiotic stress signalling in plants

The crosstalk between second messengers, hormones and mitogen-activated protein kinases (MAPKs) in plant signalling systems facilitates adaptation and survival in the face of diverse environmental stresses. This review focuses on the transduction of second messenger and hormone signals by MAPK modules in plant abiotic stress responses. We discuss how this crosstalk regulates gene expression (e.g. by controlling transcription factor activity) and other cellular and physiological responses to enable adaptation and/or resistance to abiotic stresses.

Recent insights into brassinosteroid signaling in plants: its dual control of plant immunity and stomatal development

Brassinosteroid (BR) signaling, plant innate immunity, and stomatal developments are three pathways that are initiated by receptor-like kinases. This commentary focuses on the latest findings in the role of BR signaling in plant immunity and stomatal development that provide some insight into the molecular mechanism of the BR signal pathway interacting with other receptor signaling pathways.