Mitochondrial orf107 transcription, editing, and nucleolytic cleavage conferred by the gene Rf3 are expressed in sorghum pollen

Abstract

 Restoration of male fertility in the A3, IS1112C source of cytoplasmic male sterility (cms) in sorghum is exacted in a gametophytic manner. One required nuclear gene, Rf3, regulates a nucleolytic transcript processing activity, cleaving sequences internal to the chimeric mitochondrial open reading frame orf107. We examined mitochondrial transcription, RNA editing, and action of Rf3 in developing pollen from a male-sterile line, the progenitor, a male-fertile line, and the fertile F1 to determine if these expression processes were manifested at the haploid pollen stage. Steady-state levels of orf107 transcripts and nucleolytic processing conferred by Rf3 were similar to observations from leaves, indicating comparable expression in pollen. RNA editing frequency at two of three sites in orf107 was differentially suppressed compared to leaves, but editing was higher in male-sterile plants than in fertile plants, consistent with the possibility that nucleolytic cleavage is enhanced by editing. The differential suppression of editing frequency at two sites in orf107 contrasts with near-complete editing of a third site in orf107, shared with atp9, indicating that factors influencing editing frequency of the chimeric transcript are temporally regulated and sequence-specific. Since action of the nuclear gene Rf3 is manifested at the diploid and haploid stages, pollen-specific expression of this fertility restoration gene is not required in the A3 gametophytic cms system.