Mitochondrial atp9 genes from petaloid male-sterile and male-fertile carrots differ in their status of heteroplasmy, recombination involvement, post-transcriptional processing as well as accumulation of RNA and protein product

KEY MESSAGE: Petaloid cytoplasmic male-sterile carrots exhibit overexpression of the mitochondrial atp9 genes which is associated with specific features in organization and expression of these sequences. ABSTRACT: In carrots, the S(p)-cytoplasm causes transformation of stamens into petal-like organs, while plants carrying normal N-cytoplasm exhibit normal flower morphology. Our work was aimed at characterization of distinct features both cytoplasms display with respect to organization and expression of the mitochondrial atp9 genes. We show that two carrot atp9 genes, previously reported as cytoplasm-specific, in fact occur in heteroplasmic condition. In the S(p)-cytoplasm the atp9-1 version dominates over atp9-3, while in N-cytoplasmic plants this proportion is reversed. Herein, we also indicate the presence and recombination activity of a 130-/172-bp sequence repeat which likely shaped the present organization of carrot atp9 loci. Furthermore, cDNA sequence examination revealed that the atp9 open reading frames (ORFs) were C to U edited in 4 nucleotide positions. One of the editing events turns a glutamine triplet into the stop codon, thereby equalizing ORFs of atp9-1 and atp9-3. A certain fraction of partially edited molecules was identified—they all represented the atp9-3 sequence. In either S(p)- or N-cytoplasmic plants multiple 5′ transcript termini were observed. Of these, the ones mapping more distantly from the atp9 ORF were more pronounced in case of petaloid accessions. It was also shown that despite comparable copy number of the genomic atp9 sequences, the level of the respective mRNAs was approximately 3 times higher in case of petaloid carrots. The latter fact corresponded to the elevated content of the ATP9 protein in plants carrying S(p)-cytoplasm. The semi-fertile phenotype of such plants is associated with a drop in ATP9 accumulation. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00122-014-2331-x) contains supplementary material, which is available to authorized users.