UC Davis

Plants are generally referred to as sessile organisms, being bound to the location where they germinated and rooted. Therefore, plants have developed a remarkable ability to alter the growth of different organs in response to their environment, giving them the best opportunity to collect resources, such as light, water, or nutrients. These environmental response processes can be critical for plant success and consequently have been widely studied. One of the most striking examples of plant movements is sunflower heliotropism, a process in which sunflowers bend west throughout the day tracking the sun and then reorient back to face east at night. The bending occurs through differential growth between the sides of the stems, such that the east sides of the stems grow more during the day, and the west sides of the stems grow more at night. This process has been assumed to be a specialized form of phototropism. Although it has been studied for over a hundred years, the molecular mechanisms that regulate this differential growth pattern remain poorly understood. In this thesis, I examine the transcriptional profiles and post-translational modifications of sunflower stems during heliotropism to begin to understand underlying molecular mechanisms. In chapter one, I examine and compare the transcriptional profiles of sunflower stems undergoing heliotropism, phototropism, and autostraightening. Sunflower stems undergoing phototropism display the expected rapid upregulation of auxin-regulated and growth-related genes on the shaded sides of the stems. However, the expression profiles of these phototropic genes during heliotropism are quite different than during phototropism, suggesting the two processes are regulated by different pathways. Heliotropism can be initiated during the first day in the field, and the expression patterns of the phototropic genes during this initiation are distinct from both established heliotropism and phototropism. An interesting difference is that genes involved in shade avoidance are rapidly upregulated on the west sides of stems, albeit only during the first day in the field. Tracking movements in the field are also unaffected by alterations in perceived light quality, suggesting that multiple light pathways play a role in regulating sunflower heliotropism. In chapter two, I explore the role of auxin gradients and post-translational modifications during heliotropism. I find strong gating of both the transcriptional and growth responses to auxin application during heliotropism. There is very little transcriptional response at dawn compared to other times of day and a growth response during the day but a reduced response at night. Exogenous application of fusicoccin evokes a very similar growth response to auxin, suggesting the regulation of heliotropic growth is at the level of autoinhibited plasma membrane H+-ATPase (AHA) proteins or further downstream. Examination of the phosphoproteome of tracking stems revealed a higher abundance of AHA peptides phosphorylated on a residue of known to promote ATPase activity on the east sides during the day and the west sides at night. This correlates with the growth patterns of the stems and suggests that sunflower heliotropic growth may be regulated by the phosphorylation status of sunflower AHA proteins.