MOLECULAR-BASIS OF NUCLEAR AND CYTOPLASMIC MALE-STERILITY IN HIGHER-PLANTS

Two major types of male sterility, Mendelian sterility (MST) and cytop lasmic male sterility (CMS) can be distinguished according to their ge netic control. Molecular advances concerning MST arose initially from the isolation and the characterization of genes expressed specifically in anther and pollen. These genes were manipulated to induce pollen f ailure. MST was induced by tapetal-specific expression of different ge nes such as an RNase gene, a modified glucanase gene, the Agrobacteriu m rhizogenes rolC gene or an unedited atp9 mitochondrial gene in tobac co transformants, and by antisense inhibition of flavonoid biosynthesi s in anthers of petunia transformants. Other recent data on MST molecu lar genetics were obtained from the analysis of three categories of mu tants. In Arabidopsis thaliana, chemical and transposon mutagenesis le d to MST lines of which the corresponding target genes were isolated. MST was also induced in Nicotiana sylvestris after protoplast culture and plant regeneration. Molecular advances concerning CMS came mainly from studies on structure and expression of higher plant mitochondrial genomes. In different CMS systems, the male sterile trait appeared to be associated with mtDNA rearrangements, resulting in the expression of chimeric genes supposed to interfere with normal pollen development . Protoplast fusion inducing mitochondrial intergenomic recombination was used to transfer CMS from one species to another and to identify C MS-correlated mtDNA regions. CMS plants were also regenerated in the a bsence of fusion, by culture of protoplasts of a fertile line of N. sy lvestris. Protoplast fusion as well as fertility restoration by specif ic dominant nuclear genes indicated that the CMS trait results from in compatibility between the nucleus and the mitochondria.