| This thesis describes the functional characterization of four members of a small plant-specific HULK gene family in Arabidopsis thaliana. Gene expression studies indicated that four HULK genes display similar expression patterns but one of them is expressed at a low level. Phenotypic analysis of plants carrying mutations in one or several HULK family genes revealed that they affect flowering time, embryo development, pollen viability, fertility and plant size. Only mutations in the HUA2 gene cause a visible phenotype, whereas hulk1, hulk2 and hulk3 mutants appear similar to wild type. The loss-of-function hua2-7 allele combined with hulk1, hulk2 or hulk3 alleles causes early flowering. In addition, hua2-7/hulk1 double mutants show sterility in early flowers and embryo defects. The hulk2/hulk3 mutants are late flowering, whereas hulk1/hulk2 and hulk1/hulk3 double mutants resemble wild type plants. The early and late flowering phenotypes of several hulk mutants are correlated with FLC transcript levels. The expressions of wild type HULK genes in single and double mutants are altered, signifying the complexity of the molecular changes in these genetic backgrounds. The embryo defects and sterile flower phenotype were gradually enhanced when additional HULK family genes were compromised revealing an extensive functional redundancy among the HULK genes. The hua2-7/hulk1 mutant in combination with hulk2 enhances the embryo defect phenotype, and with hulk3 causes male sterility due to loss of pollen viability. I was unable to recover the mutant lacking all four HHULK genes. Through over-expression analysis, HULK genes exhibited a series of developmental phenotypes, including delay in floral transition, serrated leaves and sterile flowers. Taken together, the results presented in this thesis have established essential roles of HULK genes in plant development. |
