ABSTRACT: Plant inflorescence meristems and floral meristems possess specific boundary domains that result in floral organ separation, and in proper numbers of floral organs. HANABA TARANU (HAN) encodes a boundary-expressing GATA type zinc finger transcription factor that regulates shoot apical meristem organization, cell division and flower development in Arabidopsis, but the underlying mechanism remains unclear. Through time-course whole genome oligonucleotide microarray analyses following transient overexpression of HAN, we find that HAN represses hundreds of genes, especially genes involved in hormone responses and floral organ regulation. Transient overexpression of HAN also causes the repression of HAN itself and three other HAN family genes: HANL2 (HAN-LIKE2), GNC (GATA, NITRATE-INDUCIBLE, CARBON-METABOLISM-INVOLVED) and GNL (GNC LIKE), forming a negative regulatory feedback loop. Double- and triple-mutant strains of han with hanl2, gnc and gnl show synergistic effects on sepal fusion, petal number, and silique length, and embryo development, as well as carpelloid stamens. Transcripts of HANL2, GNC and GNL have similar accumulation patterns, specifically in petals, stamens, carpels and inflorescence meristems, which are partially overlapping with the expression pattern of HAN, suggesting that HAN and HAN family genes share redundant functions during flower development. We further show by yeast two hybrid assays that HAN can homodimerize as well as heterodimerize with other HAN family proteins. Chromatin-immunoprecipitation analyses indicate that HAN directly binds to its own promoter and the promoter of GNC in vivo. These findings, together with the fact that constitutive overexpression of HAN has an even stronger phenotype than a loss of function mutation, support the hypothesis that HAN may function as a key repressor that regulates floral development via intricate regulatory networks involving genes in the GATA3 family, hormone actions and floral organ specification.
