Synergistic roles of LAX1 and FZP in the development of rice sterile lemma

Rice florets are subtended by two sterile lemmas, whose origin and biological functions have not been studied extensively. Here we demonstrate that two putative transcription factors, LAX PANICLE1 (LAX1) and FRIZZY PANICLE (FZP), synergistically control the development of sterile lemmas. Both LAX1 and FZP are previously known for their roles in panicle and floret development. Disruption of either LAX1 or FZP greatly reduces the number of floret development. We generated new lax1 mutants (lax1-c) using CRISPR/Cas9 gene editing technology. In addition to the expected lax panicle phenotypes, we noticed that a significant number of spikelets of lax1-c developed elongated sterile lemmas. Moreover, our characterization of lax1-RNAi plants also revealed sterile lemma phenotypes similar to lax1-c mutants. We isolated a weak allele of fzp (fzp-14) in a genetic screen for lax1–1 enhancers. The fzp-14 lax1–1 double mutants completely eliminated flower development. Interestingly, the isolated fzp-14 produced spikelets with elongated sterile lemmas. Furthermore, fzp-14 was haploid-insufficient in the lax1–1 background whereas fzp-14 heterozygous plants were indistinguishable from wild type plants. The lax1–1 fzp-14^+/− also developed elongated sterile lemma as observed in lax1-c, lax1-RNAi, and fzp-14, suggesting that LAX1 and FZP synergistically control sterile lemma development.