CMS system inNicotiana: flower development, patterns of mitochondrial DNA and mitochondrial gene expression

Summary

A detailed description of in planta floral ontogeny based on scanning electron microscopy, developmental histology and morphology is presented for three different alloplasmic gene-cytoplasmic male-sterile (CMS)Nicotiana tabacum isonuclear lines with cytoplasms ofN. bigelovii, N. debneyi and N. suaveolens and compared to the corresponding nuclear donorN. tabacum genotype. This allowed the precise determination of the developmental stages affected in the mutant forms as well as a thorough phenotypic characterization of them. The organization of the mitochondrial genome and expression of mitochondrial DNA (mtDNA) was investigated in the three different alloplasmic CMS tobacco analogs and compared to the corresponding malefertile parentalNicotiana species. Southern hybridizations of total cellular DNA and mtDNA from the different sets of lines with probes specific for mitochondrial genes coding for cytochrome oxidase subunits I, II and III, apocytochrome b, ATPase subunits α and 9 as well as 18S-5S ribosomal RNA indicated that: (a) mtDNA organization is different between mitochondrial genomes of fertile and sterile lines but identical in two different fertile tobacco lines; however genetic similarity among different mitochondrial genome types can be revealed by restriction fragment patterns; (b) although several differences were detected between the male-sterile and male-fertile plants, most of these were related to the origin of the mitochondria (cytoplasm donorNicotiana species); (c) identical mtDNA rearrangements — distinct to the cytoplasm donor — occur in cytoplasmic malesterile tobacco analogs bearing cytoplasm fromN. bigelovii in two differentN. tabacum nuclear backgrounds, indicating that mitochondrial genome structure inNicotiana is altered by substitution of the nuclear back-ground, since (d) inter- and intraspecific mitochondrial genome diversity among differentNicotiana species and the corresponding alloplasmic CMS tobacco analogs can be determined by hybridization with mtDNA specific probes. Analysis of in organello translation products in the three CMS-systems described confirmed the presence of variant proteins synthesized by mitochondria from CMS and male-fertileNicotiana isonuclear lines. In addition, differences due to the origin of both the nucleus and the cytoplasm, which involve changes in the presence or size of particular polypeptides, are apparent. A common feature of all three systems — including two different nuclear backgrounds — is the enhanced synthesis of a 31-kDa polypeptide in the strictly isonuclear CMS lines compared to the male-fertile tobacco. In addition, organellar gene expression was studied by Northern blot analysis of transcripts homologous to mitochondrial gene probes, revealing variant mtRNA species associated with some CMS lines.