A chimeric and truncated mitochondrial atpA gene is transcribed in alloplasmic cytoplasmic male-sterile tobacco with Nicotiana bigelovii mitochondria

Protoplast fusions were performed between two sexually produced alloplasmic male-sterile tobacco cultivars, with cytoplasms from Nicotiana bigelovii [Nta (big)S] and N. undulata[Nta(und)S], both of which exhibit homeotic-like phenotypes affecting the petal and stamen whorls. Among the fusion products obtained, both novel male-sterile and pollen-producing cybrid plants were identified. Of the pollen-producing cybrid plants, all of which were indehiscent, some had flowers with stamens that appeared normal when compared to male-fertile tobacco plants. Other hybrid plants were incompletely restored as they exhibited petaloid structures on the anther-bearing pollen-producing stamens. In this study, gel-blot analyses with mitochondrial geneprobes were conducted comparing the mitochondrial DNA of cybrids and male-sterile parents. It was found that the flower morphology typical of the Nta(big)S parental plants, as well as of the novel male-sterile cybrids, coincided with the presence of a chimeric atpA gene copy where an open reading frame of unknown origin was found to be linked in-frame to the 3'-end of a truncated atpA gene. RNA gel-blot hybridizations revealed the presence of atpA transcripts in the malesterile parent Nta(big)S and novel male-sterile cybrids, but which were absent in cybrids capable of pollen production.

How do alterations in plant mitochondrial genomes disrupt pollen development?

Cytoplasmic male sterility arises when mitochondrial activities are disrupted that are essential for pollen development. Rearrangements in the mitochondrial genome that create novel open reading frames are strongly correlated with CMS phenotypes in a number of systems. The morphological aberrations which indicate CMS-associated degeneration are frequently restricted to the male sporogenous tissue and a limited number of vegetative tissues. In several cases, this tissue specificity may result from interactions between the mitochondrial genome and nuclear genes that regulate mitochondrial gene expression. A molecular mechanism by which CMS might be caused has not been conclusively demonstrated for any system. Several hypotheses for general mechanisms by which mitochondrial dysfunction might disrupt pollen development are discussed, based on similarities between the novel CMS-associated genes from a number of systems.

Intraspecific heterogeneity of the Vicia faba mitochondrial genome: evidence for multiregional rearrangements in the mitochondrial chromosome associated with coxII-orf192 chimeric gene formation

Previous RFLP-analysis of mtDNA isolated from different lines and cultivars of Vicia faba with respect to variability of the coxII gene revealed two types of mitochondrial genome: one with a normal coxII gene and the other with both normal coxII and chimeric coxII-orf192 genes. In this study we analyzed other regions of these two types of mitochondrial genome and found significant differences in the arrangement of regions around the coxII, coxIII, cob, rrn26 and atpA genes. More detailed analysis of the rrn26 and atpA gene regions showed that these genes are associated with recombinationally active repeats. Restriction maps of the rrn26 and atpA gene regions in different recombinative variants are presented.

Nucleotide sequence of the mitochondrial 5S rRNA gene from lupine (Lupinus luteus)

A lupine mitochondrial clone containing 5S rRNA gene is characterized. The gene is located on the same strand as 18S rRNA and separated from it by 190 nucleotides. The intergenic region in different plants shows high degree of homology. In the case of lupine and soybean 43 nucleotides upstream of 5S rRNA gene exhibits 100% of homology. Comparisons of lupine 5S rRNA gene sequence with other plant mitochondrial 5S rRNA genes displays high degree of homology (from 89.8% to 95.8%).

Characterization of Vicia faba mitochondrial genomes with normal coxII and coxII-orf 192 chimeric genes

Southern hybridization analysis of mitochondrial genomes from different lines and cultivars of Vicia faba, with respect to variability of the coxII gene sequence, revealed two predominant types of mitochondrial genomes. The type I mitochondrial genome contained the coxII gene sequence in a 6.5-kb BamHI fragment. Type II had two copies of the coxII sequence: the first in a 6.5-kb and the second in a 2.6-kb BamHI fragment. The second copy was represented by a coxII-orf192 chimeric gene. We found several pure lines with type I and type II mitochondrial genomes. Each type of genome was stably inherited. No chimeric gene was found in mitochondria of the male-sterile line cms447. Nucleotide sequences of Vicia faba mitochondrial DNA (mtDNA) containing normal and chimeric coxII genes are presented. The sequence of the normal coxII gene was compared to the coxII gene from mitochondria of Pisum sativum. The similarity of nucleotide sequences and of predicted amino acid sequences between these two genes was more than 98%. A very high similarity between transcription initiation and termination signals was also observed. The sequence of the chimeric gene was characterized at the 5' end by the almost complete sequence of the normal coxII gene, up to the fifth nucleotide before the termination codon. The 3' end of the chimeric gene was represented by the 3' part of an orf previously called orf128+. The full size of this orf was 576 nucleotides, and the full size of the predicted polypeptide was 192 amino acid residues. Therefore, this orf can be finally called orf192. Northern hybridization analysis showed that orf192 was actively transcribed into a 1.4-kb transcript. The chimeric gene was also transcribed into a minor transcript of about 3 kb. Comparative analysis of the normal coxII gene and orf192 supported the suggestion that the chimeric gene resulted from nonhomologous recombination.

Mitochondrial genome organization and expression associated with cytoplasmic male sterility in sunflower (Helianthus annuus)

A comparative investigation of the organization and expression of the mitochondrial genome in fertile and cytoplasmic male sterile (CMS) sunflower (Helianthus annuus) has been undertaken. A region of mitochondrial genome variation between the two phenotypes has been located in the 3' flanking region of the gene encoding the alpha subunit of the F1 ATPase (atpA). Physical mapping and sequence analysis have been used to show that a rearrangement involving an inversion and an insertion has occurred immediately downstream of the atpA coding region in the mitochondrial DNA from sterile sunflower. This rearrangement has resulted in the creation of a new open reading frame (ORFc) which is co-transcribed with atpA in sterile sunflower. In organello labelling of mitochondrial translation products from the two types of sunflower shows that a 15 kDa protein is synthesized by the mitochondria from sterile sunflower but not by those from fertile plants. The ORFc sequence could encode this 15 kDa protein which may be causally related to the CMS phenotype.