The regulation of CESTA action

In 1979 a `Nature` publication by Carter Cook and co-workers revealed the surprising fact that plants produce steroid hormones, with high structural similarity to mammalian sex steroids. Today it is well established that, brassinosteroids (BRs) are used as signaling molecules to regulate cell division and differentiation and control overall developmental programmes. Rapid progress has since been made in understanding how BRs are synthesized and how their signals are transduced. However, the events controlling BR cellular homeostasis and thereby regulating their effects have remained ill defined. Recently we have identified the novel transcription factor CESTA (CES), which acts as a positive regulator of BR biosynthetic gene expression. CES is a nuclear protein that heterodimerizes with BEE1, a bHLH transcription factor previously implicated in BR signaling, and binds to G-box motifs present in the promoters of its targets. Interestingly BR signaling induces nuclear compartmentalization of CES and its homologues. Moreover CES is an in vitro substrate of the GSK3 shaggy-like kinase BIN2, a negative regulator of BR signaling, suggesting a model in which CES is controlled by BIN2 action, to allow for an adjustment of BR biosynthesis in response to variations in BR homeostasis. The research proposed in this application for funding aims to investigate mechanisms, which regulate CES nuclear localization and activity in planta. Using a complementary set of approaches it is intended to analyze possible mechanistic roles of phosphorylation in CES nuclear distribution, transcriptional activity and/or protein stability, and to analyze a potential interplay of phosphorylation and other post-translational modifications in the regulation of CES action.

Plants are sessile organisms. This requires them to constantly adapt their growth and development to changes in their environment. For example germination is inhibited in dry conditions or in winter, shading by neighbouring plants results in increased elongation growth towards the light, flowering and fruit development are affected by day length.Hormones are decisive in regulating the adaptive potential of plants and evidence suggests that in this process the class of brassinosteroids is of particular importance. Brassinosteroids are steroid hormones that are essential for cell division, differentiation and elongation. The BRs act as master regulators of developmental programs including germination, shoot and root growth, flowering, fruit development and senescence. Moreover BRs are thought to confer abiotic and biotic stress resistance.This project intended to elucidate molecular and biochemical events, which enable brassinosteroids to regulate growth. More specifically we were interested in characterizing the mode of action of the transcription factor CESTA, which acts as a positive regulator of brassinosteroid activity. Interestingly an activation of brassinosteroid signalling changes CESTA localization from a diffuse localization to a speckled pattern in the nucleus. An aim of the project was to characterize events, which control CESTA subnuclear dynamics. We could show that changes in the protein structure of CESTA by phosphorylation and SUMOylation are responsible for conferring CESTA localization and that these modifications in addition also control CESTA protein levels and activity.Our study was the first to reveal that interplay of phosphorylation and SUMOylation events takes place in the posttranslational modification of plant proteins and provided first evidence that this interplay controls steroid hormone activity.