Role of non-hypophototropic hypocotyl₃ (NPH₃) in regulation of phototropism in Arabidopsis
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Light is arguably one of the most important environmental factor that not only provides the plant with its energy needs but also cues to modulate its growth and development. Plants being immobile have developed various adaptive responses to interpret and utilize light directionality, quantity and quality. One such adaptive response is phototropism where the plant organs bend towards a directional light source.Using Arabidopsis thaliana as a model plant system, two photoreceptors: phot1 and phot2 has been identified that mediates phototropism. NPH3 is a phot1 interacting protein that is absolutely required for phototropism. NPH3 contains BTB and coiled-coil sequence conserved domains and no NPH3-like proteins have been identified so far in other non-plant organisms. Given the unique properties of this critical protein mediating phototropism, yet little is known about how phot1 signals through NPH3. In this dissertation research work, I have demonstrated that NPH3 exists as a phosphorylated protein and that BL stimulates its dephosphorylation in phot1 dependent manner. Moreover, data presented in this dissertation has demonstrated that NPH3 likely functions as a substrate specific adapter for CUL3-based E3 ubiquitin ligase. Moreover, a putative substrate of NPH3-CUL3 E3 ligase has been identified which is likey monubiquitinated in planta in a blue light dependent manner. Interestingly phot1 is degraded in high light intensities dependent on NPH3. Altogether, these results indicate that NPH3 has a novel dual function in modulating phot1-dependent phototropism based on light intensities. At low light intensities, NPH3 likely promotes phototropism whereas at higher light intensities, NPH3 likely targets phot1 for degradation to modulate global phot1 responsiveness. The phosphorylation state of NPH3 likely determines the functional status of such an E3 ligase and that differential regulation of this E3 is required for normal phototropic responsiveness.--From public.pdf
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