| When threatened by severe environmental conditions, plants enhance their chances of survival by activating stress response pathways. The plant hormone abscisic acid (ABA) is a key player in the regulation of stress response as well as other developmental processes such as seed maturation. ABA signaling has been demonstrated to act in part via the transcription factor ABI5 ( ABA Insensitive 5). To better understand how ABI5 activity is regulated by ABA signaling, a screen for ABI5 interacting proteins was performed, which identified a plant-specific protein family [ABI Five Binding Protein (AFP) Family] of unknown function. This dissertation work addresses several questions about this family. Are the AFPs involved in mediating ABA/stress response? If so, how do their interactions with ABI5 affect this process? Finally, do the AFPs function redundantly? The genes encoding the AFPs have different expression patterns in seeds and seedlings and older tissues exposed to stress. Loss-of-function lines have varying sensitivities to ABA, sugar, and salt stresses as well as effects on ABI5-dependent and -independent gene expression. The AFPs also appear to interact to varying extents vii with other members of the ABI5/ABF/AREB family of transcription factors. Taken together, these findings suggest that different AFPs act throughout development and respond to assorted stress cues in order to regulate seed maturation and stress tolerance.; In addition to demonstrating that functional redundancy may exist in the AFP family, as AFP1 and AFP2 both appear to be negative regulators of ABA response by affecting ABI5 protein accumulation, this work provides evidence that AFP2 may regulate the activity of ABI5 versus absolute protein levels. This raises an important question. Why would a negative regulator of ABI5 activity be active during a time when ABI5 is needed for stress response? I propose that, within a population of seeds, subtle differences in AFP2 and ABI5 activities result in differential responses to stress. Therefore, some seeds germinate, when AFP2 is active, and some remain in an arrested state, when ABI5 is active. In the presence of stress, this somewhat stochastic process ensures the best chances of survival for the population. |
