A unique two-gene gametophytic male sterility system in Sorghum involving a possible role of RNA editing in fertility restoration

The sorghum line IS1112C carries a male sterility-inducing cytoplasm when i ntroduced into nuclear backgrounds that do not include fertility restoratio n genes. An mtDNA chimeric configuration resulting from recombination/dupli cation with atp9 resulted in the formation of orf107, a chimeric open readi ng frame. Transcription of orf107 is driven by three promoters, and abundan t whole-length transcripts are detected in male-sterile lines. Fertility re storation is exacted through a unique two-gene gametophytic system requirin g complementary action of genes designated Rf3 and Rf4. In male-sterile lin es carrying Rf3, or lines restored to fertility, an enhanced nucleolytic tr anscript processing activity is targeted within orf107, cleaving 75% of who le-length transcripts. Rf3 thus confers or regulates the nucleolytic proces sing activity. A correlation between the frequency of RNA editing at two si tes in orf107 and transcript processing suggests that processing may be dep endent on templates edited at these sites. In addition, editing of atp6 tra nscripts is specifically reduced in anthers/pollen of male-sterile lines. P artially restored F(1)s and segregating F(2)s exhibit atp6 editing frequenc ies consistent with the possibility that Rf4 may confer the restitution of normal editing frequency. Thus RNA editing may be involved in features of f ertility restoration in this unusual system.