Functional analysis of CESTA

Brassinosteroids (BRs) are plant-specific steroid hormones that play an essential role in plant growth and development. BRs regulate cell division and differentiation and help to control overall developmental programmes leading to morphogenesis. The analysis of mutants impaired in BR biosynthesis or BR perception/signalling highlights the importance of these phytohormones in development. BR mutants of Arabidopsis thaliana grown in the light are severe dwarfs with a characteristic leave morphology and also exhibit reduced fertility and apical dominance. Grown in the dark many of these mutants display light-grown phenotypes such as de-etiolation and lack of normal skotomorphogenesis, suggesting a potential interaction between BRs and light signalling pathways. Whereas BR biosynthesis has been characterised in much detail BR signal-transduction events, linking the hormone to its many biological effects, are not as well understood. The current model of BR signalling suggests that BRs are perceived by a cell surface receptor complex that includes the receptor kinase BRI1, which triggers a signalling cascade that ultimately involves the dephosphorylation and accumulation in the nucleus of two transcriptional modulators BES1 and BRZ1, which regulate BR target-gene expression. BR-homeostasis is regulated via multiple mechanisms. These include inactivation, such as hydroxylation and glucosylation, and through the precise control of BR biosynthesis. To date the factors that directly regulate BR biosynthesis are unknown, with the only exception of BZR1, that is thought to mediate both downstream BR-responses, as well as play a part in the feedback regulation of BR biosynthesis. This research proposal is based on the recent identification of a novel Arabidopsis mutant, cesta (ces), in which BR over-accumulation phenotypes correlate with an increased expression of genes involved in BR biosynthesis and altered levels of endogenous BRs. The ces mutant phenotype is caused by constitutive over-expression of a transcription factor. Loss of CES function results in reduced expression of BR biosynthesis genes. Using a complementary set of approaches the research proposed in this funding application aims to verify the current hypothesis that CES regulates BR biosynthesis. The research will provide the essential foundation for revealing the role of CES in BR metabolism in Arabidopsis and will contribute to our understanding of the regulation of steroid hormone homeostasis in plants.

Brassinosteroids (BRs) are plant-specific steroid hormones that play an essential role in plant growth and development. BRs regulate cell division and differentiation and help to control overall developmental programmes leading to morphogenesis. The analysis of mutants impaired in BR biosynthesis or BR perception/signalling highlights the importance of these phytohormones in development. BR mutants of Arabidopsis thaliana grown in the light are severe dwarfs with a characteristic leave morphology and also exhibit reduced fertility and apical dominance. Grown in the dark many of these mutants display light-grown phenotypes such as de-etiolation and lack of normal skotomorphogenesis, suggesting a potential interaction between BRs and light signalling pathways. Whereas BR biosynthesis has been characterised in much detail BR signal-transduction events, linking the hormone to its many biological effects, are not as well understood. The current model of BR signalling suggests that BRs are perceived by a cell surface receptor complex that includes the receptor kinase BRI1, which triggers a signalling cascade that ultimately involves the dephosphorylation and accumulation in the nucleus of two transcriptional modulators BES1 and BRZ1, which regulate BR target-gene expression. BR-homeostasis is regulated via multiple mechanisms. These include inactivation, such as hydroxylation and glucosylation, and through the precise control of BR biosynthesis. To date the factors that directly regulate BR biosynthesis are unknown, with the only exception of BZR1, that is thought to mediate both downstream BR-responses, as well as play a part in the feedback regulation of BR biosynthesis. This research proposal is based on the recent identification of a novel Arabidopsis mutant, cesta (ces), in which BR over-accumulation phenotypes correlate with an increased expression of genes involved in BR biosynthesis and altered levels of endogenous BRs. The ces mutant phenotype is caused by constitutive over-expression of a transcription factor. Loss of CES function results in reduced expression of BR biosynthesis genes. Using a complementary set of approaches the research proposed in this funding application aims to verify the current hypothesis that CES regulates BR biosynthesis. The research will provide the essential foundation for revealing the role of CES in BR metabolism in Arabidopsis and will contribute to our understanding of the regulation of steroid hormone homeostasis in plants.