Polycomb proteins RING1A/B promote H2A monoubiquitination to regulate female gametophyte development in Arabidopsis.

A functional female gametophyte is the basis of successful sexual reproduction in flowering plants. During female gametophyte development, the megaspore mother cell (MMC), differentiated from a single subepidermal somatic cell in the nucellus, undergoes meiosis to produce four megaspores; only the one at the chalazal end, referred to as functional megaspore (FM), undergoes three rounds of mitosis and develops into a mature embryo sac. Here, we reported that RING1A and RING1B (RING1A/B), two functionally redundant Polycomb proteins in Arabidopsis, are critical for female gametophyte development. The mutations of RING1A/B resulted in defects in MMC and FM's specification and subsequent mitosis of FM, thereby leading to aborted ovules. Gene expression analysis revealed several genes essential for female gametophyte development, including Argonaute (AGO) family genes and critical transcription factors, were ectopically expressed in ring1a ring1b. Furthermore, RING1A/B bound some of these genes to promote H2A monoubiquitination (H2Aub) deposition. Together, RING1A/B promote H2Aub modification at genes essential for female gametophyte development, suppressing their expression to ensure the progression of female gametophyte development.