Precise Mapping of Growth Hormone Effects by Cell-Specific Gene Activation Response

Plant yield largely depends on a complex interplay and feedback mechanisms of distinct hormonal pathways. Over the past decade great progress has been made in elucidating the global molecular mechanisms by which each hormone is produced and perceived. However, our knowledge of how interactions between hormonal pathways are spatially and temporally regulated remains rudimentary. For example, we have demonstrated that although the BR receptor BRI1 is widely expressed, the perception of BRs in epidermal cells is sufficient to control whole-organ growth. Supported by additional recent works, it is apparent that hormones are acting in selected cells of the plant body to regulate organ growth, and furthermore, that local cell-cell communication is an important mechanism. In this proposal our goals were to identify the global profile of translated genes in response to BR stimulation and depletion in specific tissues in Arabidopsis; determine the spatio-temporal dependency of BR response on auxin transport and signaling and construct an interactive public website that will provide an integrated analysis of the data set. Our technology incorporated cell-specific polysome isolation and sequencing using the Solexa technology. In the first aim, we generated and confirmed the specificity of novel transgenic lines expressing tagged ribosomal protein in various cell types in the Arabidopsis primary root. We next crossed these lines to lines with targeted expression of BRI1 in the bri1 background. All lines were treated with BRs for two time points. The RNA-seq of their corresponding immunopurified polysomal RNA is nearly completed and the bioinformatic analysis of the data set will be completed this year. Followed, we will construct an interactive public website (our third aim). In the second aim we started revealing how spatio-temporalBR activity impinges on auxin transport in the Arabidopsis primary root. We discovered the unexpected role of BRs in controlling the expression of specific auxin efflux carriers, post-transcriptionally (Hacham et al, 2012). We also showed that this regulation depends on the specific expression of BRI1 in the epidermis. This complex and long term effect of BRs on auxin transport led us to focus on high resolution analysis of the BR signaling per se. Taking together, our ongoing collaboration and synergistic expertise (hormone action and plant development (IL) and whole-genome scale data analysis (US)) enabled the establishment of a powerful system that will tell us how distinct cell types respond to local and systemic BR signal. BR research is of special agriculture importance since BR application and BR genetic modification have been shown to significantly increase crop yield and to play an important role in plant thermotolerance. Hence, our integrated dataset is valuable for improving crop traits without unwanted impairment of unrelated pathways, for example, establishing semi-dwarf stature to allow increased yield in high planting density, inducing erect leaves for better light capture and consequent biomass increase and plant resistance to abiotic stresses.