Post‐translational mechanisms are associated with fertility restoration of cytoplasmic male sterility in sugar beet (<i>Beta vulgaris</i>)

Abstract

Summary Genetic conflict between cytoplasmically inherited elements and nuclear genes arising from their different transmission patterns can be seen in cytoplasmic male sterility ( CMS ), the mitochondrion‐encoded inability to shed functional pollen. CMS is associated with a mitochondrial open reading frame ( ORF ) that is absent from non‐sterility inducing mitochondria ( S‐orf ). Nuclear genes that suppress CMS are called restorer‐of‐fertility ( Rf ) genes. Post‐transcriptional and translational repression of S‐orf mediates the molecular action of Rf that encodes a class of RNA ‐binding proteins with pentatricopeptide repeat ( PPR ) motifs. Besides the PPR ‐type of Rf s, there are also non‐ PPR Rf s, but the molecular interactions between non‐ PPR Rf and S‐orf have not been described. In this study, we investigated the interaction of bv ORF 20 , a non‐ PPR Rf from sugar beet ( Beta vulgaris ), with preSatp6 , the S‐orf from sugar beet. Anthers expressing bv ORF 20 contained a protein that interacted with pre SATP 6 protein. Analysis of anthers and transgenic calli expressing a FLAG ‐tagged bv ORF 20 suggested the binding of pre SATP 6 to bv ORF 20. To see the effect of bv ORF 20 on pre SATP 6, which exists as a 250 ‐ kD a protein complex in CMS plants, signal bands of pre SATP 6 in bv ORF 20 ‐expressing and non‐expressing anthers were compared by immunoblotting combined with Blue Native polyacrylamide gel electrophoresis. The signal intensity of the 250 ‐ kD a band decreased significantly, and 200‐ and 150 ‐ kD a bands appeared in bv ORF 20‐ expressing anthers. Transgenic callus expressing bv ORF 20 also generated the 200‐ and 150 ‐ kD a bands. The 200 ‐ kD a complex is likely to include both pre SATP 6 and bv ORF 20. Post‐translational interaction between pre SATP 6 and bv ORF 20 appears to alter the higher order structure of pre SATP 6 that may lead to fertility restoration in sugar beet.