UCLA

Genome duplication in eukaryotes and particularly in plant species has led to gene families with both redundant and specialized functions. How closely related transcription factor families compete to alter gene expression and ultimately manifest a morphological phenotype is a key biological question. In Arabidopsis, the Class III Homedomain-Leucine Zipper (HD-ZIPIII) transcription factor family controls many developmental processes, such as embryo patterning and shoot stem cell formation, through complex genetic interactions. However the specific molecular mechanisms behind these genetic observations remain unresolved. I have discovered that HD-ZIPIIIs collaboratively and antagonistically regulate entire pathways at multiple, sometimes opposing steps, which can occur on a stage, tissue or cell type specific level. I found that HD-ZIPIIIs form preferential heterodimers in planta, co-occupy hundreds of genomic locations and regulate target gene expression both positively and negatively. Transcriptional repression by HD-ZIPIIIs is correlated with the co-occupancy and association with the co-repressor TOPLESS. Finally, I uncovered a novel HD-ZIPIII protein interaction partner that exhibits a similar genome-wide DNA binding profile and transcriptional response and interacts with both TPL and the HD-ZIPIIIs and thus may bridge a transcriptional complex required for HD-ZIPIII-mediated repression. These results demonstrate how a highly related family of TFs can collectively regulate important developmental signaling networks and will help unravel the inherent complexities of TF family interrelationships at both the molecular and network level.