Nucleus-Cytoplasm Interactions : Molecular Analysis and Their Implication in Cron Improvement

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Correct functioning of a plant cell requires coordination between three genetic compartments : the nucleus, the plastids and the mitochondria. We are very kignorant of how the activities of the nuclear and mitochondrial genetic systems are coodinated. Cytoplasmic male sterility (CMS) provides a good model to investigate nuclear-mitochondrial interactions.Several mitochondrial genes were identified as exhibiting differences in transcript patterns between normal and CMS sugarbeet. Of these genes only the coxl transcript pattern is affected by nuclear background. Two dominant nuclear genes (Rf1 and Rf2) have been reported to be necessary to restore fertility in sugarbeet. To determine if the coxl transcript differences observed between CMS and restored plants are associated with the action of Rf genes, we have analyzed the coxl transcripts in individuals of a BC1 backcross (rf1rf1rf2rf2*Rf1rf1Rf2rf2) population. Four progeny genotypes are exprected, Rf1rf1Rf2rf2 (fertile), Rf1rf1rf2rf2 (partial sterile), rf1rf1Rf2rf2 (partial sterile), and rf1rf1rf2rf2 (sterile), and these should occur in about equal numbers. Northern analysis of mitochondrial transcripts of 13 male-fertile and 16 male-sterile progeny was performed using cox1 probe. The results showed that the coxl transcript differences were due to a single nuclear locus and did not cosegregate with Rf genes. The sugarbeet coxl locus thus offers an attractive system for studying the mechanisms through which nuclear genes regulate mitochondrial gene expression. This project also included the issue concerning the gene transfer from the mitochondrial or chloroplast genome to the nucleus genome.