| Plants are sessile organisms that optimize available resources via adjustments in growth and development. Light responses, termed photomorphogenesis, are of primary importance due to the photoautotrophic nature of plants. Light perception often triggers a change in plant cell expansion, which leads to a change in organ size or position, and results in optimization of the ambient light environment. Elucidating the molecular mechanism between light perception and changes in organ size remains a daunting challenge. Hormones are well known mediators of cell and organ expansion, and recent evidence implicates light regulation of hormone levels as a potential mechanism of adjusting growth and development. This dissertation investigates the nature of light regulated steroid hormone inactivation as a potential molecular mechanism of photomorphogenesis. Active brassinosteroids (BRs), such as brassinolide (BL) and castasterone (CS), are steroid phytohormones. An Arabidopsis cytochrome P450 monooxygenase (CYP734A1, formerly CYP72B1), encoded by the BAS1 gene, inactivates BRs and modulates photomorphogenesis. Findings presented include identification of SOB7/CYP72C1, a BR-inactivating cytochrome P450 homologous to BAS1. Investigation of BAS1 and SOB7 biochemistry indicated separate mechanism of BR inactivation. Physiological analysis of single- and double-null mutants indicated redundant functions of the BAS1 and SOB7 genes in mediating photomorphogenesis. Thus BAS1 and SOB7 act redundantly to mediate light responses via inactivation of BRs, but the encoded enzymes have unique biochemistries with respect to how BRs are inactivated. These results broaden our understanding of photomorphogenesis as a response to light that results in decreased or increased organ expansion, both of which are influenced by regulating the BR-inactivation pathway, and hence the level of active BRs. Changes in BR level are likely to affect BR signaling and result in changes in growth and development that optimize the available light. As the mechanism of light modulation of BR inactivation is elucidated, advances in other hormone and environmental response pathways may reveal additional points of cross-talk between these pathways. As our understanding of such a complex network of interconnected pathways is advanced it may become clear that greater connectivity gives rise to greater adaptability. As sessile and photoautotrophic organisms, adaptability to available resources is surely a fundamental driving force to the success of land plants. |
