orf260cra, a Novel Mitochondrial Gene, is Associated with the Homeotic Transformation of Stamens into Pistil-Like Structures (Pistillody) in Alloplasmic Wheat

Homeotic transformation of stamens into pistil-like structures (pistillody) can occur in cytoplasmic substitution (alloplasmic) lines of bread wheat (Triticum aestivum) that have the cytoplasm of the related species, Aegilops crassa. Previously we showed that pistillody results from altered patterns of expression of class B MADS-box genes mediated by mitochondrial gene(s) in the Ae. crassa cytoplasm. The wheat cultivar Chinese Spring does not show pistillody when Ae. crassa cytoplasm is introduced. The absence of an effect is due to a single dominant gene (designated Rfd1) located on the long arm of chromosome 7B. To identify the mitochondrial gene involved in pistillody induction, we performed a subtraction analysis using cDNAs derived from young spikes of a pistillody line and a normal line. We found that mitochondrial cDNA clone R04 was abundant in the young spikes of the pistillody line but was down-regulated in the normal line that carried nuclear Rfd1. Sequencing of the full-length cDNA corresponding to clone R04 showed that two genes were present, cox I (cytochrome c oxidase subunit I) and orf260(cra). orf260(cra) shows high sequence similarity to orf256, the T. timopheevii mitochondrial gene responsible for cytoplasmic male sterility (CMS). orf260(cra) was also present in the cytoplasms of Ae. juvenalis and Ae. vavilovii, which induce pistillody, but not in the cytoplasms of other species not associated with pistillody. Furthermore, Western blot analysis revealed that the ORF260cra protein was more abundant in the pistillody line than in the normal line. We suggest therefore that orf260(cra) is associated with pistillody induction.