Abstract
Cytoplasmic male sterility (CMS) in plants is usually associated with the expression of specific chimeric regions within rearranged mitochondrial genomes. Maize CMS-S plants express high amounts of a 1.6-kb mitochondrial RNA during microspore maturation, which is associated with the observed pollen abortion. This transcript carries two chimeric open reading frames, orf355 and orf77, both unique to CMS-S. CMS-S mitochondria also contain free linear DNA plasmids bearing terminal inverted repeats (TIRs). These TIRs recombine with TIR-homologous sequences that precede orf355/orf77 within the main mitochondrial genome to produce linear ends. Transcription of the 1.6-kb RNA is initiated from a promoter within the TIRs only when they are at linear ends. Reversions of CMS-S to fertility occur in certain nuclear backgrounds and are usually associated with loss of the S plasmids and/or the sterility-associated region. We describe an unusual set of independently recovered revertants from a single maternal lineage that retain both the S plasmids and an intact orf355/orf77 region but which do not produce the 1.6-kb RNA. A 7.3-kb inversion resulting from illegitmate recombination between 14-bp microrepeats has separated the genomic TIR sequences from the CMS-associated region. Although RNAs containing orf355/orf77 can still be detected in the revertants, they are not highly expressed during pollen development and they are no longer initiated from the TIR promoter at a protein-stabilized linear end. They appear instead to be co-transcribed with cytochrome oxidase subunit 2. The 7.3-kb inversion was not detected in CMS-S or in other fertile revertants. Therefore, this inversion appears to be a de novo mutation that has continued to sort out within a single maternal lineage, giving rise to fertile progeny in successive generations.
Highlights
Unlike the small mitochondrial genomes of animals, in which the order of the genes along the genomes tends to be conserved, the large mitochondrial genomes of seed plants usually exhibit rearrangements among populations within a single species [1]
We show that an inversion has altered the location of the genomic copy of the terminal inverted repeats (TIRs) relative to orf355orf/77, so that recombination between the S plasmids and the TIR no longer produces a linear end near orf355/orf77
One lineage of Wf9 plants carrying the Cytoplasmic male sterility (CMS)-S subtype VG was unusual in that its descendants exhibited a relatively high rate of cytoplasmic reversion to fertility, with fertile plants appearing in sequential generations
Summary
Unlike the small mitochondrial genomes of animals, in which the order of the genes along the genomes tends to be conserved, the large mitochondrial genomes of seed plants usually exhibit rearrangements among populations within a single species [1]. Rearranged mitochondrial genomes are often seen in plants that exhibit maternally inherited male sterility. Recombination can occur between terminal inverted repeats (TIRs) at the end of each S plasmid and TIR sequences that precede orf355/orf in the main mitochondrial genome of CMS-S [5] (Figure 1A). This produces linear ends of mtDNA from which transcription of a 1.6-kb RNA initiates [6]. The male-sterile phenotype is correlated with high expression levels of the 1.6-kb mitochondrial transcript during microspore biogenesis [7], [9], [6]. A constitutively expressed 2.8-kb RNA in CMS-S mitochondria carries the orf355/orf region, but this RNA is not associated with the CMS phenotype [7], [6]
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