Abstract
Cytoplasmic male sterility (CMS) is a plant trait that involves interactions between nuclear- and mitochondrial genomes. In CMS, the nuclear restorer-of-fertility gene (Rf), a suppressor of male-sterility inducing mitochondria, is one of the best known genetic factors. Other unidentified genetic factors may exist but have not been well characterized. In sugar beet (Beta vulgaris L.), CMS is used for hybrid seed production, but few male-sterility inducing nuclear genotypes exist. Such genotypes could be introduced from a closely related plant such as leaf beet, but first the fertility restoring genotype of the related plant must be characterized. Here, we report the discovery of a Japanese leaf beet accession ‘Fukkoku-ouba’ that has both male-sterility inducing and fertility restoring genotypes. We crossed the leaf beet accession with a sugar beet CMS line, developed succeeding generations, and examined the segregation of two DNA markers that are linked to two sugar beet Rfs, Rf1 and Rf2. Only the Rf2 marker co-segregated with fertility restoration in every generation, implying that the Rf1 locus in leaf beet is occupied by a non-restoring allele. Fertility restoration was incomplete without a genetic factor closely linked to Rf1, leading to the assumption that the Rf1 locus encodes a modifier that cannot restore fertility by itself but perhaps strengthens another Rf. We sequenced the apparently non-restoring ‘Fukkoku-ouba’ rf1 gene-coding region and found that it closely resembles a restoring allele. The protein product demonstrated its potential to suppress CMS in transgenic suspension cells. In contrast, ‘Fukkoku-ouba’ rf1 transcript abundance was highly reduced compared to that of the restoring Rf1. Consistently, changes in protein complexes containing CMS-associated mitochondrial protein in anthers were very minor. Accordingly, we concluded that ‘Fukkoku-ouba’ rf1 is a hypomorph that acts as a non-restoring allele but has the potential to support another Rf, i.e. it is a modifier candidate.
Highlights
Mitochondria are endosymbiotic organelles that originated from α-proteobacteria [1]
Expression of cytoplasmic male sterility (CMS) is explained by a genetic model in which mitochondrial factor S, ‘sterile’, that causes male sterility interacts with nuclear factor restorer-of-fertility gene (Rf), ‘restorer of fertility’, whose dominant allele suppresses the action of S
A Japanese leaf beet accession ‘Fukkoku-ouba’ has a fertility restoring genotype for Owen-type CMS, which is used for hybrid seed production in sugar beet
Summary
Mitochondria are endosymbiotic organelles that originated from α-proteobacteria [1] Mitochondria possess their own genomes, their genetic information is insufficient to sustain mitochondrial function, for which a large number of nuclear genes are required [2]. Expression of CMS is explained by a genetic model in which mitochondrial factor S, ‘sterile’, that causes male sterility interacts with nuclear factor Rf, ‘restorer of fertility’, whose dominant allele suppresses the action of S. In this genetic model, male sterility is expressed only when the S plant has a non-restoring allele of Rf, i.e. the genotype [S]rfrf
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