ClBBM and ClPLT2 function redundantly during both male and female gametophytes development in watermelon

As a prerequisite for sexual reproduction, gametophyte development is an interesting process involving cell proliferation, differentiation and specialization. Gametogenesis has been extensively explored in model plants, but the regulatory mechanism of gametophyte development largely remains unknown in Cucurbitaceae species. In present study, we have shown that watermelon ClBBM and ClPLT2, two AP2/ERF transcription factors, participated in both male and female gametophyte development. Clbbm and Clplt2 single mutants resembled wild-type phenotypes in both vegetative and reproductive development. But Clbbm/Clplt2 double mutant showed partial pollen abortion and bore less seeds comparing to WT. Our results indicated that the abnormal pollen grains were caused by premature tapetum degeneration, and reduced seed-set was due to faulty embryo sac development. ClBBM and ClPLT2 were expressed in FG4 embryo sacs, and their transcripts were also detectable in the tapetum and microspore of stage 9 anthers, which was consistent with developmental stages of defective phenotypes observed in double mutant. The expression of genes essential for tapetum development, ClATM1, ClAMS, ClMS1 and ClMS188, was decreased in Clbbm/Clplt2 double mutants. Moreover, the transcriptome analysis indicated that ClBBM and ClPLT2 participated in tapetum and pollen wall development by regulate cell cycle, transmembrane transport, glucan and cellulose metabolic process. Collectively, ClBBM and ClPLT2 were functionally redundant in regulating gametophyte development in watermelon, and their functions differ from their homologous genes in model plant Arabidopsis.