Genetic characterization of a pentatricopeptide repeat protein gene, <i>orf687</i>, that restores fertility in the cytoplasmic male‐sterile Kosena radish

TLDR

Cytoplasmic male sterility in plants is a maternally inherited pollen‑deficiency linked to mitochondrial DNA abnormalities, and nuclear restorer loci (Rf) have been identified, including a previously mapped Rf locus for the Kosena radish. The study aimed to identify the Rf gene by introducing lambda and cosmid clones into CMS Brassica napus and assessing fertility restoration. A 687‑amino‑acid pentatricopeptide repeat protein, ORF687, was isolated from a restoring fragment and is proposed to lower mitochondrial ORF125 protein levels, thereby restoring fertility. A 4.7‑kb fragment containing ORF687 restored male fertility in CMS B.

Abstract

Cytoplasmic male sterility (CMS) in plants is a maternally inherited inability to produce functional pollen, and is often associated with mitochondrial DNA abnormalities. Specific nuclear loci that suppress CMS, termed as restorers of fertility (Rf), have been identified. Previously, we identified an Rf for the CMS Kosena radish and used genetic analysis to identify the locus and create a contig covering the critical interval. To identify the Rf gene, we introduced each of the lambda and cosmid clones into the CMS Brassica napus and scored for fertility restoration. Fertility restoration was observed when one of the lambda clones was introduced into the CMS B. napus. Furthermore, introduction of a 4.7-kb BamHI/HpaI fragment of the lambda clone is enough to restore male fertility. A cDNA strand isolated from a positive fragment contained a predicted protein (ORF687) of 687 amino acids comprising 16 repeats of the 35-amino acid pentatricopeptide repeat (PPR) motif. Kosena CMS radish plants were found to express an allele of this gene possessing four substituted amino acids in the second and third repeats of the PPR suggesting that the domains formed by these repeats in ORF687 are essential for fertility restoration. Protein levels of the Kosena CMS-associated mitochondrial protein ORF125 were considerably reduced in plants in which fertility was restored, although mRNA expression was normal. Regarding the possible role for PPR-containing proteins in the regulation of the mitochondrial gene, we propose that ORF687 functions either directly or indirectly to lower the levels of ORF125, resulting in the restoration of fertility in CMS plants.