Photoperiod-sensitive cytoplasmic male sterility in wheat: nuclear-mitochondrial incompatibility results in differential processing of the mitochondrial orf25 gene.

An alloplasmic wheat line with the cytoplasm of Aegilops crassa expresses photoperiod-sensitive cytoplasmic male sterility (PCMS). Southern- and Northern-hybridization analyses showed that this line contains alterations in both the gene structure and transcription patterns of the mitochondrial gene orf25. In this study, the nucleotide sequence around the orf25 gene of Ae. crassa (CR-orf25) and common wheat (AE-orf25) was determined, and we found that the upstream region of CR-orf25 had been replaced by that of rps7 of common wheat (AE-rps7) through recombination. A novel open reading frame (orf48) is present upstream of CR-orf25. In these three genes, transcription was initiated from the consensus promoter motif of plant mitochondrial genes located in the upstream regions. Processing enzymes in Ae. crassa and common wheat cleave the respective precursor mRNAs, namely CR-orf25 and AE-rps7, at sites similar to that of the premature mitochondrial 26S rRNA. In contrast, the precursor mRNA is not effectively processed at the target sequence of CR-orf25 in the alloplasmic wheat line. Because major transcripts of the euplasmic CR-orf25 and AE-rps7 genes would result in a truncated orf48 product, one possibility is that the orf48 protein might disturb mitochondrial function at a specific stage and hence affect the expression of the PCMS trait.