Variation in mitochondrial translation products in fertile and cytoplasmic male-sterile sugar beets

Abstract

Intact and functional mitochondria were isolated from sugar beet plants (Beta vulgaris L.) containing normal fertile (F) or cytoplasmic male-sterile (S1-S4) cytoplasms. Incorporation of (35)S-methionine by mitochondria isolated from both roots and leaves showed approximately 20 major and ten minor translation products. Comparison of the polypeptide synthesis patterns produced by leaf mitochondria from fertile plants of three different species within the genus Beta revealed several taxonomically related differences. Contrary to this, the patterns of polypeptides synthesized by mitochondria from roots and leaves of sugar beet plants containing the F and S1-S4 cytoplasms were very similar; in the S1 and S2 cytoplasms no qualitative, and only a few quantitative, differences from the F cytoplasm were observed. Thus, in these cases, cytoplasmic male sterility in sugar beet is not correlated with the constitutive expression of variant polypeptides. In the S3 cytoplasm, however, an additional 6 kDa polypeptide was synthesized and in the S4 cytoplasm an additional 10 kDa polypeptide was observed when compared with the F cytoplasm. The expression of cytoplasmic male sterility in sugar beet may be associated with these variant polypeptides. The mitochondrial polypeptides synthesized were identical in plants with different nuclear backgrounds but with identical S1 cytoplasms. Mitochondria from plants with variants of the S4 cytoplasm in the same nuclear genotype also showed identical patterns of polypeptide synthesis, including the synthesis of the 10 kDa S4-specific polypeptide. Pulse-chase experiments did not affect the synthesis of this polypeptide.