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.
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.
S-type cytoplasmic male sterility (CMS-S) in maize is associated with high levels of a 1.6-kb RNA in mitochondria. This RNA contains two chimeric open reading frames (ORFs), orf355 and orf77. The previously described nuclear restorer-of-fertility allele Rf3 causes the processing of all transcripts that contain these chimeric ORFs. The Lancaster Surecrop-derived inbred line A619 carries a restorer that is distinct from Rf3 in that it selectively reduces only the CMS-S-specific 1.6-kb RNA. We have found that 10 additional Lancaster lines carry a single restoring allele traceable to either of two inbred lines, C103 and Oh40B. The C103 and Oh40B restorers are allelic to each other, but not to Rf3. Thus, this restoring allele, designated Rf9, represents a second naturally occurring CMS-S restorer in maize. Rf9 is a less effective restorer of fertility than is Rf3; its expression is influenced by both inbred nuclear background and temperature. Rf9 acts to reduce the amounts of orf355/orf77-containing linear mitochondrial subgenomes, which are generated by recombination of circular subgenomes with CMS-S-specific linear plasmids. The 1.6-kb RNA, which is transcribed only from linear ends, is correspondingly reduced.
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