Unequal genetic redundancy among the rice transcription factors OsMADS2 and OsMADS4 reveals distinct roles in floret organ development

Abstract

AbstractFunctional diversification of transcription factors and of their downstream targets contributes to the emergence of new organ morphologies. To better understand the molecular mechanisms underlying floral organ specification in rice, we investigated the function ofOsMADS2andOsMADS4, the ricePISTILLATA(PI) paralogs controlling lodicule and stamen formation. The morphological characterization ofosmads2null mutants generated by CRISPR/Cas9 reiterated OsMADS2’s nonredundant role in lodicule development. Furthermore, it uncovered unreported roles in controlling flowering time, floral meristem size, floral organ number, cell wall metabolism, and sterile lemma development. Although downregulation ofOsMADS4did not affect floral organ development, doubly perturbedosmads2d8/d8osmads4kdflorets exhibited more severe abnormalities than theosmads2single mutant. Doubly perturbed inflorescences also exhibited defective rachis extension and parthenocarpy. Remarkably, ubiquitous overexpression ofOsMADS4inosmads2rescued the different abnormalities of theosmads2mutant but resulted in defective anther development and reduced seed set, phenotypes also observed whenArabidopsis PIwas overexpressed in wild-type rice. These findings suggest that increased ubiquitousPIactivity is detrimental to anther development. To uncover genes whose (in)direct regulation may contribute to these phenotypes, we combined the genome-wide identification of OsMADS2 binding regions (ChIP-Seq) with transcriptome profiling (RNA-Seq). These analysis uncovered several OsMADS2 target genes that are implicated in lodicule and stamen development and in controlling floral organ number,e.g.,PECTIN METHYLESTERASE 24(PME24),GLYCOSIDE HYDROLASE 9B16(GH9B16),TAPETUM DEGENERATION RETARDATION(TDR),SPOROCYTELESS(SPL),FLORAL ORGAN NUMBER 1(FON1),YUCCA3 andYUCCA7. Altogether, our results provide insights on the underlying molecular mechanisms of ricePIparalogs in floral organ specification, thereby expanding our understanding of their function as well as their conservation and diversification.