Functional Analysis of Three Arabidopsis Transcriptional Co- Repressors Linking Floral Regulators to Aba Hormone Signaling

Title of Dissertation: FUNCTIONAL ANALYSIS OF THREE ARABIDOPSIS TRANSCRIPTIONAL COREPRESSORS LINKING FLORAL REGULATORS TO ABA HORMONE SIGNALING Jayashree Sitaraman, Doctor of Philosophy, 2004 Dissertation Directed By: Associate Professor Zhongchi Liu Department Of Cell Biology and Molecular Genetics Hormonal signaling and developmental patterning have mostly been studied as separate and independent processes. However, the highly plastic development of plants suggests that plant development must incorporate environmental and physiological signals and adapt accordingly. The question of how development and hormone signaling influence each other needs to be answered yet. This work shows that the regulators of flower development such as LUG, LUH and SEU also function in ABA (Abscisic Acid) hormone signaling. Further, it is shown that LUG and LUH, two paralogs in Arabidopsis, are partially redundant in function, but each plays a more prominent role in a different process. This study provides evidence that the basic mechanisms of plant developmental regulation and hormonal signaling are interconnected and utilize similar regulatory components. LUG and SEU are two transcription co-repressors previously shown to regulate the spatial and temporal domains of expression of a key regulator of flower development AGAMOUS (AG) in Arabidopsis. LUG and SEU was shown to form a co-repression complex similar to the TUP1/Ssn6 co-repressor complex of yeast. In Arabidopsis, the one and only homolog of LUG, LUH, is 44% identical to LUG. However, its function is completely unknown. A putative luh null mutation was identified and luh mutants show reduced sensitivity to ABA and reduced expression of ABA-response genes. LUH is proposed to act to repress the transcription of ERA1, a negative regulator of ABA signaling. This is supported by increased ERA1 expression in luh-1 mutants. This study also suggests a possible role of LUG and SEU in ABA signaling. Although luh mutant by itself does not have a floral phenotype, luh has been found to interact with lug and seu genetically during flower development, based on the enhanced floral phenotype in the double mutants between luh and lug and between luh and seu. Yeast two hybrid data suggests that, like LUG, LUH also physically interacts with SEU to form a co-repressor complex. Thus, while LUG plays a major role in flower development, LUH plays a more important role in ABA signaling. FUNCTIONAL ANALYSIS OF THREE ARABIDOPSIS TRANSCRIPTIONAL COREPRESSORS LINKING FLORAL REGULATORS TO ABA HORMONE SIGNALING