0546503 CAREER: Novel Signaling Components For Plant Steroid Regulated Gene Expression In Arabidopsis
Plant steroid hormones called brassinosteroids (BRs) control many diverse processes such as cell growth, senescence and stress responses. Application of BRs has been shown to increase crop yield and improve plant resistance to drought, high temperature and various diseases. Although significant progress has been made in identifying BR receptors and several other signaling components, little is known about how plant steroids regulate gene expression, thereby controlling different biological responses. Modern genetic and genomic approaches will be used to address this question. A BR-regulated transcription factor BES1 mediates many of the hormone responses and provides a powerful tool to study BR-regulated gene expression. To reveal the gene regulatory circuits, the functions of several BES1-induced transcription factors will be determined by over-expression and knock-out technologies. To identify other signaling components involved in BR-regulated gene expression, mutant plants with altered responses to plant steroids will be isolated, the underlying genes will be identified and characterized. The results from these studies will reveal the complex network through which BRs control growth and responses to both biotic and abiotic stresses. Broader impacts: Since BRs control many different biological processes, it is highly desirable to be able to manipulate individual responses. For example, it would be beneficial to promote BR-induced growth and stress responses and to reduce BR-induced senescence. The acquired knowledge from the proposed research can be used to achieve these goals. Several BR mutants will be introduced into an undergraduate Plant Physiology laboratory course to illustrate how steroids control plant growth. In addition, students from underrepresented groups will be recruited to work on the project through the Program for Woman in Science and Engineering (PWSE) at Iowa State University. The integrated research and teaching program will therefore provide ample training opportunities for high school, college, graduate and postgraduate students.
Plant steroid hormones called Brassinosteroids (BRs) are growth-promoting hormones that play important roles in plant growth, development and response to stresses. Application of exogenous BR or elevation of endogenous BR level has been shown to increase crop yield by 10-40% and have enhanced crop performance under stress conditions. The long-term goal of this research is to define the molecular mechanisms and genetic networks through which BRs regulate gene expression and control specific biological processes using Arabidopsis as model. The knowledge can be used to increase yield and disease or stress resistance of important crops. The research, therefore, can help meet the increasing demand for agricultural products like food, feed and biofuel due to constant population increase and climate changes. Intellectual Merit: It was previously established that BRs function through receptor BRI1 to regulate BES1 and BZR1 family transcription factors, which mediate the expression of thousands of genes required for BR responses (Cell 2005, 120:249; Science 2005 307: 1634). However, how BES1 and BZR1 regulate gene expression was largely unknown. With the support of the NSF grant, genetic and genomic approaches have been used to fill the gap. The mechanism and network for BR-regulated gene expression have been revealed (Fig 1). More than 10 articles in top scientific journals have been published based on research supported by the NSF grant. Specifically: (1) Mechanism of BR-regulated gene expression has been discovered: It was found that two histone modifying enzymes EFL6 and REF6, previously known to be involved in flowering time control, interact with BES1 to regulate BR target gene expression and provide a molecular link between plant growth and reproduction (Fig 2, PNAS 2008 105:7618). In addition, BES1 recruits a transcription elongation factor IWS1 to control BR-responsive gene expression (PNAS 2010 107:3918), establishing a novel mechanism for the regulation of gene expression (Fig 3). IWS1 target genes have been identified by a genomic technique called ChIP-seq (Chromatin Immunoprecipitation and high throughput sequencing), which revealed that IWS1 coordinates BR response with several other hormone and stress response pathways. (2) Network for BR-regulated gene expression have been established: BES1 direct target genes have been identified by ChIP experiments and a BR Gene Regulatory Network has been established by computational modeling (Fig 4, Plant J. 2011 65: 634). Recent functional studies have demonstrated that BES1 targeted transcription factors (BTFs) act with BES1 in a combinatorial fashion to activate or repress different sets of genes for various BR responses (Fig 5). Interestingly, some BTFs are directly regulated by BIN2 kinase, a well-established negative regulator in the BR signaling pathway and BIN2 phosphorylation has opposite effects on different BTFs. (3) A novel signaling pathway for plant growth have been discovered (Fig 6): Several BR regulated genes, named HERCULES Receptor-like Kinases (HERKs) are required for optimal plant growth at vegetative stage. Gene expression studies demonstrated that these RLKs control the expression of a unique set of genes including those implicated in plant growth. Our results, therefore, identify a previously unknown pathway that functions cooperatively with, but largely independent of the BR pathway to regulate plant growth. We have recently found that HERKs are required in plant defense response against bacterial pathogen. (4) Collaborative research has been established to study BR functions in crop plants such as maize, rice, and switchgrass, which will help transfer the knowledge obtained in model system to crop plants. Broader Impacts: The research has been successfully integrated into education and outreach activities. First, the data generated have been deposited to public database; antibodies and mutant lines have been distributed to more than 20 labs all over the world. Second, four graduate students, eight undergraduate students, two high school teachers and one research assistant scientist have been working on the research projects with the NSF support. Three of the undergraduate students have contributed significantly to the research and have coauthored three of our research papers and two of the undergraduate students went to graduate schools studying plant biology. Third, the BR mutants have been introduced to several high schools biology classes in Iowa and two high school teachers have been working on the NSF supported project in the laboratory. The NSF support thus has directly contributed to K12 education. Finally, the BR mutants have been introduced to an introductory biology laboratory course at Iowa State University, in which about 1100 students every year carry out a hypothesis driven research to explore how plant steroid hormone regulates plant growth. In summary, the research has generated a large amount of data and provided significant new insight into BR function in plants. The knowledge will be used to improve crop yield, stress tolerance and disease resistance. The NSF grant has also provided outstanding training and educational opportunities to the new generation of scientists, workforces and future leaders in various fields.
