The Polycomb Group Gene EMF2B is required for meristem determinacy and organ specification in rice flowers.

The Polycomb Repressive Complex 2 (PRC2) represses the transcriptional activity of target genes through trimethylation of Lysine 27 of Histone H3. The functions of the plant PRC2 have been chiefly described in Arabidopsis but specific PRC2 functions in other plant species, especially the cereals, are still largely unknown. Here we characterize mutants in the rice EMF2B gene, an ortholog of the Arabidopsis PRC2 gene EMBRYONIC FLOWER2 (EMF2). Loss of EMF2B in rice results in complete sterility, and mutant flowers have severe floral organ defects and indeterminacy that resemble loss of function mutants in rice E-function floral organ specification genes. Transcriptome analysis identified the E-function genes OsMADS1, OsMADS6 and OsMADS34 that are involved in floral development as differentially expressed in the emf2b mutant compared to wild type. OsMADS1 and OsMADS6 are known to be required for meristem determinacy in rice, and are down-regulated in the emf2b mutant, whereas OsMADS34 which interacts genetically with OsMADS1 was up-regulated. Chromatin immunoprecipitation for H3K27me3 followed by deep sequencing showed that all three genes are amongst the presumptive targets of PRC2 in the meristem. We propose that the PRC2 operates through a mechanism that involves regulation of E-function genes to play a major role in floral organ specification and floral meristem determinacy in rice, and possibly in other cereals. Overall design: RNA-seq: The transcriptome of WT and emf2b rice panicles were compared via RNA-seq.

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Conrad LJ, Khanday I, Johnson C, Guiderdoni E, An G, Vijayraghavan U, Sundaresan V