WP1. BR mediated and non-BR mediated processes

Work package leader

De Vries lab

Participants

De Vries Lab, Kemmerling Lab, Hardtke Lab

Subprojects

The BRI1-SERK complexes

The objective of this subproject is to establish whether the main brassinosteroid response can be subdivided into partial responses attributed to a particular combination of main receptors and co-receptors.

The BAK1/SERK3 receptor in innate immunity

The objective of this subproject is to investigate the role of BAK1/SERK3 (BRI1-associated receptor kinase 1) and other members of the SERK family in plant immunity. bak1 mutants were shown to be less sensitive to brassinolide (BL) treatment but also more susceptible to necrotizing pathogens. The altered immune responses of the bak1 mutants to microbial infection appear to be independent of BR signalling. Integration of microarray expression data, yeast-two-hybrid, and co-immunoprecipitation data revealed potential receptor molecules for which BAK1 might also functions as a co-receptor. The molecular and functional analyses of the BAK1 interacting proteins will reveal their role in development and/or plant immunity and will allow determining how the specificity of BAK1 in different physiological pathways is established and maintained.

The BRX protein as a link between BR and auxin signalling

By analysing natural genetic variation of root growth in Arabidopsis thaliana, we have identified a gene of unknown function, named BREVIS RADIX (BRX), which is required for proper root growth. Physiological and expression profiling experiments have implicated BRX in growth-related hormone pathways, suggesting that BRX establishes a link between brassinosteroid biosynthesis and auxin action in the root. The objective of this subproject is to place BRX within the network of known brassinosteroid and auxin signalling factors, and to determine the biochemical and cell biological functions of the BRX protein.

WP2. Downstream developmental processes

Work package leader

Russinova lab

Participants

Russinova lab, Can Delgado Lab, Szekeres Lab

Subprojects

The role of brassinosteroid perception in plant growth

Loss-of-function mutations in BR receptor complex result in dwarf plants resembling the BR biosynthetic mutants. In contrast, overexpression of BR receptors makes plants bigger. These observations suggest that changes in BR perception directly affect organ size. Plants grow because their cells progress through developmental phases of proliferation and expansion to reach maturity. The growth of an organ is determined by the rate and duration of the cell division together with postmeristematic expansion. It is generally accepted that the growth-promoting effects of BRs result from stimulation of cell expansion. Several independent experiments strongly suggest that BRs directly influence the expression of the cell cycle genes. In this part of the project we want to determine the cellular mechanisms by which BRs regulate organ growth and size and answer which elements of the cell cycle machinery are involved.

BRs and vascular development

BRs play a crucial role in regulating the formation and organisation of vascular tissues. However, the specific action of these hormones in the (pro)cambial cells as well as the cross-talk mechanisms that exist between them during vascular cell specification are not yet understood. The identification of BR-receptors BRL1 and BRL3 (BRASSINOSTEROID RECEPTOR LIKE 1 and 3 respectively) that function in vascular development in Arabidopsis open new window for these studies We want to understand the role of BR in vascular cell differentiation in Arabidopsis. The proposed research project is based on the hypothesis that a specific signal transduction mechanism for brassinosteroid receptors BRL1 and BRL3 present in the provascular tissues acts during in vascular cell differentiation.

Correlation between BR level and observed physiological responses

We propose that local BR responses depend on changes in BR level and/or BR susceptibility. In this context, we would like to clarify how these conditions are controlled by environmental factors (e.g. light, temperature, other phytohormones or wounding) and differential organ-specific or developmental regulation. Our previous results showed a correlation between the accumulation of bioactive BRs and the activity of BR-biosynthetic genes. On the other hand, we also saw that physiological conditions can strongly influence the extent of the BR response. Previously it has been suggested that elongation of etiolated seedlings could be accompanied by upregulated BR synthesis in the dark. We found that in Arabidopsis key BR-biosynthetic genes are down-, rather than up-regulated in the dark, while the amount of endogenous bioactive BRs does not change appreciably upon extended dark growth. As our preliminary results suggest an increase of BR sensitivity in the dark, we wish to find out how the processes of BR synthesis and perception are coordinated in light-grown and etiolated seedlings.

WP3. BRs for crop improvement

Work package leader

Christophe Reuzeau, CropDesign N.V

Participants

Russinova lab, Bishop Lab, CropDesign N.V

Subprojects

Screening for bioactive compounds to modify BR signalling

We aim to discover small synthetic molecules that will reversibly modulate the trafficking of BR signalling components chemical genetics approach and to further identify their potential targets. Such compounds can be used as bioactive molecular probes for functional analysis of the endocytosis of BR receptor complex and for further understanding of its significance for signal transduction in plants.

BRs in tomato

To enable the rapid exploitation of our understanding of BR biosynthesis and signalling from model species to crops it is important to know the level of conservation in such processes. Tomato is an ideal species that can be utilised for analysis of conservation of BR synthesis and signalling and is also a model system for fruit production. In tomato two key novel findings have been made in BR synthesis and signalling, namely the discovery of a putative fruit specific BL-synthase (CYP85A3) and that the peptide hormone systemin binds to the BR receptor BRI1. To further our understanding of BRs role in tomato it will be important to discern where and when BRs are made and perceived. It will also be important to focus this research on fruit development.

Identifying genes that can modify brassinosteroid responses, plant architecture and seed yield in rice

New approaches are necessary to modify and improve plant growth and plant yield .We will review all known genes involved in brassinosteroid pathway, identify all data published concerning those genes and mutants and transgenics, particularly focus on architectural and seed yield traits. Based on this analysis we will elect and test the function of a number of new important genes in rice. Using the CropDesign high throughput TraitMill phenotyping platform we will precisely examine the effects of selected transgenes on the development of rice and the establishment of the final phenotype. We will focus on architectural traits such as height and branching, leaf structure, but also panicle structure and development and effect on final yield.