The nuclear gene Rf3 affects the expression of the mitochondrial chimeric sequence R implicated in S-type male sterility in maize.

The mitochondrial genomes of maize plants exhibiting S-type cytoplasmic male sterility (cms-S) contain a repeated DNA region designated R. This region was found to be rearranged in the mitochondria of all cms-S cytoplasmically revertant fertile plants in all nuclear backgrounds analyzed. A 1.6-kb mRNA transcribed from the R region in mitochondria of sterile plants was absent from all cytoplasmic revertants examined. The nuclear gene Rf3, which suppresses the cms-S phenotype, was found to have a specific effect on the expression of the R sequence; the abundance of the major R transcripts, including the cms-S-specific 1.6-kb mRNA, is decreased in mitochondria of restored plants. Nucleotide sequence analysis of R has revealed similarities to the R1 plasmid found in some South American maize races with RU cytoplasm, to the M1 plasmid found in one source of Zea luxurians teosinte, to the atp9 mitochondrial gene and its 3' flanking sequence, and also to a region 3' to the orf221 gene. The derived amino acid sequence of the R region predicts two open reading frames (ORFs). These ORFs contain the similarities to R1, M1, atp9 and orf221. The present report reveals the chimeric nature of the R region, describes the complex effect of Rf3 on the expression of the R sequence and implicates R in the sterile phenotype of cms-S maize.

Involvement of S2 episomal sequences in the generation of NCS4 deletion mutation in maize mitochondria.

Cytoplasmic male sterility (CMS) and the abnormal-growth, nochromosomal stripe (NCS) phenotypes are cytoplasmically determined traits in plants. Mitochondrial DNA rearrangements involving short repeats appear to be responsible for the production of CMS reversions to fertility and NCS mutations. NCS4, a new mutant of maize CMS-S, exhibits both abnormal growth and male fertility. This mutant is unique because both mutations occurred within the same plant. Free S1 and S2 episomes normally found in CMS-S mitochondria have been lost from NCS4 plants. An S2 sequence has recombined aberrantly with a ribosomal protein coding region, rps3/rpl16. One end of the S2 sequence and the 5' end of the rps3/rpl16 transcription unit are absent from the NCS4 recombinant genome. Loss of mitochondrial ribosomal protein function is lethal; therefore, NCS4 plants are heteroplasmic for the rps3 deletion. Loss of S sequences from CMS-S mitochondria is not lethal and plants regain pollen function. Thus, although NCS4 plants have very abnormal plant phenotypes, they are male-fertile.

Nuclear genotype affects mitochondrial genome organization of CMS-S maize.

A WF9 strain of maize with the RD subtype of the S male-sterile cytoplasm (CMS-S) was converted to the inbred M825 nuclear background by recurrent backcrossing. The organization of the mitochondrial genomes of the F1 and succeeding backcross progenies was analyzed and compared with the progenitor RD-WF9 using probes derived from the S1 and S2 mitochondrial episomes, and probes containing the genes for cytochrome c oxidase subunit I (coxI), cytochrome c oxidase subunit II (coxII) and apocytochrome b (cob). Changes in mitochondrial DNA (mtDNA) organization were observed for S1-, S2-, and coxI-homologous sequences that involve loss of homologous restriction enzyme fragments present in the RD-WF9 progenitor. With the coxI probe, the loss of certain fragments was accompanied by the appearance of a fragment not detectable in the progenitor. The changes observed indicate the effect of the nuclear genome on the differential replication of specific mitochondrial subgenomic entities.

A partially deleted mitochondrial cytochrome oxidase gene in the NCS6 abnormal growth mutant of maize.

The nonchromosomal stripe (NCS) mutations of maize are a set of maternally inherited mutations that result in reduced plant growth and lowered reproductive capacity. NCS6 mutants are characterized by stunted growth, yellow stripes on leaves, and sectors of aborted kernels on ears. The heteroplasmic NCS6 striped plants carry both partially deleted and normal versions of the mitochondrial cytochrome oxidase subunit 2 (cox2) gene and have low levels of cox2 transcripts when compared with nonstriped control plants. Other tested mitochondrial genes and their transcripts appear to be unaltered in NCS6 plants. The level of COXII protein is also specifically reduced in the striped plants relative to that of control plants. In the NCS6 mutant mitochondrial genome, a portion of the first exon of the cox2 gene is deleted along with the 5' flanking region. The formation of the defective cox2 gene appears to have involved a recombination event between 31-bp repeats.

An abnormal growth mutant in maize has a defective mitochondrial cytochrome oxidase gene.

We describe a new maternally inherited maize mutation, nonchromosomal stripe 5 (NCS5), that adversely affects plant growth and yield. Mutant plants are characterized by reduced height, defective yellow striping on leaves, and aborted kernels on ears. NCS5 striped plants carry both normal and partially deleted versions of the mitochondrial cytochrome oxidase subunit 2 gene and exhibit greatly reduced levels of cox2 transcripts when compared with nonstriped control plants. Other mitochondrial genes and their mRNAs are not affected. Thus, the defective plant phenotype is correlated with a reduction in the number of functional cytochrome oxidase subunit 2 genes. The NCS5 mutant mitochondrial genome appears to have arisen by amplification of a rare homologous recombination product.

A comparison of cytoplasmic revertants to fertility from different CMS-S maize sources.

The mitochondrial genome organizations of a number of independent culture-derived fertile CMS-S revertants with the nuclear genotype W182BN were compared to spontaneous field revertants with the genotypes WF9, M825/Oh07 and 38-11. Regions of the genome around sequences homologous to the terminal repeats of the linear S1 and S2 episomes characteristic of CMS-S mitochondria were used as hybridization probes on Southern blots of BamHI and SalI digested mitochondrial DNA. The results obtained suggest that the nuclear, not the cytoplasmic, genotype of the parent plant affects the type of novel mitochondrial DNA organization found in the revertant. The DNA reorganization during reversion from CMS-S in tissue culture appears to be similar to that observed in spontaneous revertants obtained during the normal plant life-cycle. Unlike the situation for reversion from CMS-T, no common DNA sequence or reading frame appeared to be lost or disrupted in revertants.

Cytoplasmic reversion to fertility in cms-S maize need not involve loss of linear mitochondrial plasmids.

Cytoplasmic male sterility of the S type (cms-S) in maize is characterized by the presence of two autonomously replicating plasmid-like elements, S1 and S2. These plasmids have not been found in the mitochondrial genomes of normal (male-fertile) maize nor previously in male-fertile cytoplasmic revertants. This paper reports the discovery of spontaneous cytoplasmic reversion to fertility in cms-S maize not involving the loss of S1 and S2 plasmids. Data are presented showing that loss of the plasmids during cytoplasmic reversion is under nuclear influence and is not a characteristic of the S cytoplasm itself.

Cytoplasmic Reversion of cms-S in Maize: Association with a Transpositional Event.

Spontaneous reversion to fertility in S male-sterile cytoplasm of maize is correlated with the disappearance of the mitochondrial plasmid-like DNA's, S-1 and S-2, and changes in the mitochondrial chromosomal DNA. Hybridization data indicate that one of the plasmid-like DNA's, S-2, is prominently involved in the mitochondrial DNA rearrangements.