MECHANISMS OF MERISTEM MAINTENANCE BY MAIZE TRANSCRIPTIONAL COREPRESSORS

The formation of the plant body proceeds in a sequential, post-embryonic manner through the action of meristems, organized structures containing plant stem cells. Tightly coordinated meristematic regulation is required for proper development and reproductive success, eventually determining yield in crop species. In maize, the RAMOSA ENHANCER LOCUS2 (REL2) family of transcriptional corepressor proteins includes four members, REL2, RELK1 (REL2-LIKE1), RELK2, and RELK3. In an unbiased genetic screen for rel2 enhancers, we identified double mutants with fewer and shorter internodes, and enlarged female inflorescence meristems (IMs) carrying mutations in RELK1. We show that RELK1 partially buffers the developmental defects of rel2 mutants and actively compensates for the loss of REL2-RELK function. Expression and genetic analysis show that REL2 and RELK1 cooperatively regulate female IM development by controlling genes involved in redox balance, hormone catabolism, and differentiation, ultimately tipping the meristem toward a molecular environment favorable to expanded expression of the ZmWUSCHEL1 gene, a key transcription factor regulating stem cell populations. We further investigated the functional relationship among the various family members and showed that RELK genes have partially redundant yet diverse functions in maintenance of various meristem types. In particular, REL2, RELK1, RELK2, and RELK3 function redundantly during embryogenesis and in the formation and maintenance of the shoot apical meristem; ear formation requires REL2 and RELK3 while REL2, RELK1 and RELK2 are required for maintenance of ear IM. By exploiting slight increases in ear inflorescence meristem size caused by the loss of one functional copy of REL2, we also promoted extra rows of kernels across a diverse set of F1 hybrid ears. Our findings reveal that the REL2 family of transcriptional corepressors maintains maize development from embryonic initiation to reproductive growth and can potentially be harnessed forincreasing seed yield in a major crop species.