bHLH cascade regulate 24-nt phasiRNA biogenesis in maize anther

Anthers are the male reproductive floral organs, but loss of the 24-nt class of phased small-interfering RNAs (phasiRNAs) may confer temperature-sensitive male sterility in maize. 24-nt phasiRNAs from 176 loci are abundant coordinately in fertile maize anthers after the last periclinal divisions are complete, all four wall layers are formed, and meiosis has initiated in the pollen mother cells. In this study, we found male sterile maize ms23, ms32, and bhlh122 mutants with various degrees of tapetal defects lack 24-PHAS mRNA precursors and 24-nt phasiRNAs, which were not affected in another male sterile mutant, bhlh51. Transcription from most of 24-PHAS loci depends on bHLH122 while bHLH122 and Dcl5 transcription depends on both Ms23 and Ms32. Multiple lines of evidence suggest that 24-nt phasiRNAs biogenesis is primarily controlled at multiple layers by MS23 and MS32, with distinctive action on 24-PHAS transcription by bHLH122, creating a transcription factor cascade in maize tapetal cells. Transcriptome and small RNA profiling of maize bhlh male sterile anthers by RNA- and sRNA-seq, comparing to fertile anthers