The regulation of starch synthesis by phytochrome A&B, thioredoxins and plastidial kinases in Arabidopsis thaliana leaves
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Starch is an important storage carbon in many plant leaves which is synthesized during the day and degraded at night to maintain cellular maintenance and eventually growth. The biosynthesis of starch is a highly regulated process in response to photoperiod, carbon availability, environmental stress and so forth. Whereas starch synthesis enzymes have been well identified up to now, their regulation is less understood. In this thesis, we adopted multiple techniques including TEM immunogold labelling, co-IP, GCMS-TOF metabolite profiling, starch phenotype screen from mutants, and investigated the regulatory role of photoreceptors, TRXs and other identified potential mediators of starch synthesis in Arabidopsis leaves. It turned out PHYA and PHYB exert a control on starch accumulation and also a considerable number of other primary metabolites in Arabidopsis leaves in response to light including light spectral content (R:FR ratio) and light intensity. TRX f was identified out of six plastidial TRXs in Arabidopsis to influence starch accumulation in a light intensity and photoperiod dependent manner. Further in vivo visualization of TRX f revealed its suborganellar distribution on thylakoids, stroma and starch granules possibly play a regulatory role for its biological functions in chloroplast. In a subsequent co-IP experiment, we identified 16 potential TRX f-interacting proteins associated with distinct processes, in particular electron transport flow and translation. In order to discover new regulators involved in starch synthesis, a starch synthesis phenotype screen was performed for a series of Arabidopsis mutants defective in potential insulin-responsive targets, plastidial kinases or potential starch granule-bound proteins. Loss of three plastidial kinases resulted in reduced starch accumulation and/or starch turnover. Altogether, this study shows starch synthesis regulation network consists of divergent components which coordinate starch metabolism and other physiological activities for optimal overall growth. This study expands the current understanding of starch synthesis regulation.