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BRAssinosteroid Venture Increasing StudentS' International MObility

Final Report Summary - BRAVISSIMO (BRAssinosteroid Venture Increasing StudentS' International MObility)

The BRAVISSIMO project

BRAVISSIMO is an inter- and multidisciplinary training network that uses biochemical, genetic, molecular and cellular approaches, imaging technology, as well as high-throughput techniques to study the function of the plant hormone brassinosteroids (BRs) in Arabidopsis thaliana and translates this knowledge into model crop plants. The project integrates different expertise and provides a full training of young scientists to ensure that they will acquire a clear understanding and appreciation of how different techniques can be effectively used in the context of an integrated research program.

Motivation:

A challenge for the European agriculture is to satisfy the growing demands for food in a sustainable manner. Understanding the basic mechanisms of plant growth will ultimately increase the ability to improve crop yields, while decreasing the need for fertilizers and pesticides. Plant growth is regulated by developmental programs that can be modified by environmental cues acting through endogenous signaling molecules, such as plant hormones. Brassinosteroids (BRs) are plant growth-promoting hormones that determine important agronomic traits, including biomass, crop yield, and stress and pathogen adaptation. Although, besides the well-elucidated BR biosynthetic pathway, multiple BR signaling components have been identified, key issues remain unsolved. It is still unclear how BRs control growth, how the BR levels change throughout development and in response to environment and how different hormonal pathways interact within the cells. Furthermore, whether different signaling pathways mediate other non-BR effects and how the redundant BR signaling components fine tune the pleiotropic action of these hormones are still unknown. The BRAVISSIMO network provides a training program that establish the comprehensive education needed for a competitive and leading European science in the interdisciplinary field of BR signaling.

Objectives and Overall strategy:

The purpose of BRAVISSIMO is to train well-qualified young researchers in the field of hormone and pathogen-originated plant signaling research. Together with the industry, BRAVISSIMO applies novel technologies to BR signaling research and thereby improves the understanding of hormonal plant growth control, innate immunity, signal transduction, and hormonal crosstalk in plants. The BRAVISSIMO network consists of scientific and technological platforms that enable its partners to: (1) purify affinity-tagged versions of the BR receptors to identify protein complexes of which the individual components might not be identified by other approaches; (2) validate the in vivo association of novel protein components and candidate kinase targets by means of advanced microscopy techniques; (3) characterize molecularly, genetically and physiologically different BR downstream cellular processes, including growth and vascular differentiation, and clarify how they are affected by environmental factors; (4) use chemical and genetic modifiers of the BR signaling pathway to improve agriculturally important crop traits in tomato and in rice; (5) use the high-throughput TraitMill phenotyping platform developed in CropDesign N.V. to examine the effects of such modifiers in rice and establish the final phenotypes;

Description of the main results:

BRAVISSIMO ESRs:

During the BRAVISSIMO project, nine ESRs have been recruited. They were introduced to the concepts of personal career development and action plans and participated in the network activities, including two training courses, one workshop, one international conference and three network meetings. In addition, ESRs attended several external conferences and received training outside the network. The research progress achieved in all work packages was in line with the initial plan and can be summarized as follows.

WP1. BR-mediated and non-BR-mediated processes

The main objective of WP1 was to distinguish at the receptor level which processes use the same set of co-receptors for BR-dependent and non-BR-dependent responses. Within this work package, tools (constructs and transgenic lines) have been generated to dissect processes at the receptor level that depend on BRs and non-BRs. Which cellular compartment in which the sites of specific interactions between the SERK receptors and downstream transcriptional regulators occur has already been determined. Next, it has been analyzed how genetic interactions are translated into the dynamic composition of multiprotein complexes. The roles of BAK1 and other members of the SERK family in plant immunity have also been studied. BAK1-interacting proteins have been identified and their function in disease resistance is being characterized. By analyzing natural genetic variation of root growth in Arabidopsis thaliana, a gene of unknown function, named BREVIS RADIX (BRX), which is required for proper root growth has been isolated. By physiological and expression profiling experiments, BRX has been implicated in growth-related hormone pathways, suggesting that BRX establishes a link between BR biosynthesis and auxin action in the root. An objective was to place BRX within the network of known brassinosteroid and auxin signaling factors and to determine the biochemical and cell biological functions of the BRX protein. Major progress has been made in deciphering the physiology and cell biology of BRX. Evidence has been provided that the BRX protein is associated to the plasma membrane, but translocates to the nucleus upon auxin treatment to modulate the cellular growth depending either on the cellular auxin concentration or the auxin flux.

WP2 Downstream developmental processes

The main objective of WP2 was to characterize different cellular processes controlled by BRs, including plant growth, vascular development and to understand how BR levels change throughout development and in response to environment. During the project, a platforms for kinematic analysis of leaf and root growth has been established and used to characterize growth defects in BR-biosynthesis and signaling mutants. BR-regulated genes in the root vasculature have been identified by a transcriptomic approach and further validated. BR-responsive vascular-specific transcription factors have been selected to establish a genetic network on the regulation of root vascular development by BRs. In order to allow monitoring of physiological changes in the levels of bioactive BRs, stable transgenic Arabidopsis lines were generated that harbor promoter-reporter constructs controlled by the insertion of BR response elements.

WP3 BRs for crop improvement

The main objective of WP3 was to identify novel components of the BR signaling pathway and to modify existing ones to improve agriculturally important crop traits. The gene discovery is based on literature, results from WP1 and WP2, and the use of chemical genomics. In order to identify bioactive compounds that modify BR signaling, a phenotype-based chemical genomic screen for hypocotyl elongation in Arabidopsis seedlings was performed and their effect on BR responses was validated. To enable the rapid exploitation of our understanding of BR biosynthesis and signaling from model species to crops, it is important to know the conservation level in such processes. Tomato has been selected as an ideal species for the conservation analysis of BR synthesis and signaling and also as a model system for fruit production. Some tools have been created to better understand the physiological role of the putative fruit-specific brassinolide synthase in tomato development. Known genes involved in the BR pathway are being reviewed based on published data of genes, mutants and transgenic plants, with particular focus on architectural traits, such as height and branching, leaf structure, but also panicle structure and development and effects on final yield. Based on this analysis, genes have been selected for functional tests in rice