OsMADS1 as a transcriptional regulator of rice floral organ fate affects auxin and cytokinin signaling pathways

The SEPALLATA genes encode a functionally diverse group of MADS domain transcription factors including redundant floral organ fate determinants in Arabidopsis and non-redundant factors like OsMADS1 in rice. Earlier functional studies show that OsMADS1 regulates the identity of all floret organs and controls floret meristem determinacy. To investigate OsMADS1 as a regulator of floret development we have identified its likely target genes using microarray analysis. Many of the affected genes in OsMADS1 knockdown panicles belong to auxin and cytokinin signaling pathways. Transcripts for 11 of the 25 rice auxin response factors (ARFs) including OsETTIN1 and 2; for a third of all predicted rice Aux/IAA proteins that are repressors of ARFs and nearly half of all GH3 family of proteins involved in hormone homeostasis are affected. Further, genes encoding cytokinin biosynthetic enzymes (e.g. LOG) and response regulators (OsRR1 and OsRR9) are up-regulated. The consequences of mis-expression of an OsMADS1 target, OsMGH3, an auxin responsive member of GH3 family were examined in transgenic plants. Its ectopic over expression creates extreme dwarfs with no apical dominance, while its targeted over expression during panicle branching reduces panicle length—a phenotype also seen on OsMADS1 overexpression. The enlarged carpel phenotype created on its partial knockdown may arise from loss of floral determinacy and could provide a plausible mechanism for the determinacy defects of OsMADS1 mutants. Overall our data reinforce the role of hormone homeostasis and its transcription regulation during floral organ development