Brassinosteroids

plants. In Arabidopsis, brassinosteroids are perceived by receptor kinases that transduce the signal from the cell surface to the nucleus by an intracellular cascade of phosphorylation mediated protein-protein interactions, involving kinases, phosphatases, 14-3-3 proteins, and nuclear transcription factors. In addition, brassinosteroid signalling is regulated by the plant endocytic machinery because an increased endosomal localization of the brassinosteroid receptor enhances the signalling.
The main objective of the group is to combine genetic, molecular and cell biology tools to study the mechanism of brassinosteroid signalling regulation in plants. One aspect of the research is to understand the subcellular compartmentalization and trafficking of brassinosteroid receptor complexes and their relevance to brassinosteroids physiological responses. We use chemical genomics and proteomics to investigate the subcellular localization, mobility, transport routes and binding interactions of different brassinosteroid signalling components. In addition, we want to position important downstream brassinosteroid signalling regulators such as the Arabidopsis GSK3-like kinases in subcellular compartments important for brassinosteroid receptor activity.
The potential application of brassinosteroids in agriculture is based not only on their ability to increase crop yields, but also on the fact that they increase resistance to different stress conditions, such as high salinity, drought, and fungal and viral infections. Thus, unravelling the regulatory mechanisms of brassinosteroid signalling on the level of signalling components, brassinosteroid target genes, and endomembrane trafficking regulators, or identifying chemicals that modulate any of those components can be used to develop selective strategies for high-yielding plants.

Growing evidence suggests that plants have adapted endocytosis for signal transduction and use mechanisms similar to those of animals to regulate receptor internalization. Despite progress in the description of some endocytic routes of plant plasma membrane proteins, we are far from a complete understanding of the endocytic trafficking of receptor complexes and to which extent their signalling activity requires and is modulated by these routes. The project aims at filling this gap by gaining basic knowledge of the relationship between the endocytic transport and signalling activity of brassinosteroid hormones. In addition, the discovery of novel components of plant endocytic machinery will provide evolutionary insight into why certain gene families expanded in plants, while others range from being absent to having been maintained at an ancestral minimum.

Genetic studies of brassinosteroid signalling and of its interplay with endocytosis are hampered by gene redundancy, the very dynamic nature of endomembrane trafficking and the high degree of lethality of the genes encoding endomembrane components. Chemical genetics is a powerful approach that overcomes these limitations by the use of small molecules that perturb the protein function in a specific, fast, and conditional manner. The function of redundant proteins can be simultaneously perturbed by a general antagonist, thereby revealing novel phenotypes. Additionally, embryonic lethality due to interference with essential signalling components can be avoided by modulation of the dosage of the chemical compounds or by treatment at a later developmental stage. The main objective of this research is to identify a large set of small chemicals affecting endomembrane trafficking of different plasma membrane proteins by effectively blocking and accumulating these cargos in different endomembrane compartments.

Only recently it became apparent that signalling components in plants, as in mammalian systems, are involved in more than one signalling pathway, creating a need for research to understand the mechanisms of specificity and integration of these pathways within an individual cell. Our aim is to molecularly characterize the interaction between brassinosteroid and stomatal signalling pathways at the level of the brassinosteroid-regulated GSK3-like kinases. We explore the hypothesis that shared signalling components are recruited into different protein complexes to prevent cross signalling and that compartmentalization of the signal in the cell contributes to specificity.
The aim of the project is to combine genetics, proteomics and molecular cell biology to study the mechanisms of brassinosteroid signalling interaction at the level of BIN2 to eventually answer the question how specificity is achieved in brassinosteroid signalling.