SUPER WOMAN 2 (SPW2) maintains organ identity in spikelets by inhibiting the expression of floral homeotic genes OsMADS3, OsMADS58, OsMADS13, and DROOPING LEAF

Abstract

Flower organ identity in rice is mainly determined by the A-, B-, C- and E-class genes, with the majority encoding MADS-box transcription factors. However, few studies have investigated how the expression of these floral organ identity genes is regulated during flower development. In this study, we identified a gene named SUPER WOMAN 2 (SPW2), which is necessary for spikelet/floret development in rice by participating in the regulation of the expression of pistil identity genes such as OsMADS3, OsMADS13, OsMADS58 and DL. In the spw2 mutant, ectopic stigma/ovary-like tissues were observed in the non-pistil organs, including sterile lemma, lemma, palea, lodicule, and stamen, suggesting that the identities of these organs were severely affected by mutations in SPW2. SPW2 was shown to encode a plant-specific EMF1-like protein that is involved in H3K27me3 modification as an important component of the PRC2 complex. Expression analysis showed that the SPW2 mutation led to the ectopic expression of OsMADS3, OsMADS13, OsMADS58, and DL in non-pistil organs of the spikelet. The ChIP-qPCR results showed significant reductions in the levels of H3K27me3 modification on the chromatin of these genes. Thus, we demonstrated that SPW2 can mediate the process of H3K27me3 modification of pistil-related genes to regulate their expression in non-pistil organs of spikelets in rice. The results of this study expand our understanding of the molecular mechanism by which SPW2 regulates floral organ identity genes through epigenetic regulation.