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

Restoration of male fertility in the A3, IS1112C source of cytoplasmic male sterility (cms) in sorghum is exacted in a gametophytic manner. One requir ed nuclear gene, Rf3, regulates a nucleolytic transcript processing activit y, cleaving sequences internal to the chimeric mitochondrial open reading f rame orf107. We examined mitochondrial transcription, RNA editing, and acti on 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 proc esses were manifested at the haploid pollen stage. Steady-state levels of o rf107 transcripts and nucleolytic processing conferred by Rf3 were similar to observations from leaves, indicating comparable expression in pollen. RN A editing frequency at two of three sites in orf107 was differentially supp ressed compared to leaves, but editing was higher in male-sterile plants th an in fertile plants, consistent with the possibility that nucleolytic clea vage is enhanced by editing. The differential suppression of editing freque ncy at two sites in orfl07 contrasts with near-complete editing of a third site in orf107, shared with atp9, indicating that factors influencing editi ng frequency of the chimeric transcript are temporally regulated and sequen ce-specific. Since action of the nuclear gene Rf3 is manifested at the dipl oid and haploid stages, pollen-specific expression of this fertility restor ation gene is not required in the A3 gametophytic ems system.