Cytoplasmic male sterility (CMS) is the maternally inherited inability of plants to produce viable pollen, whereas plant growth and female fertility is normal [1, 2]. There are three major groups of CMS in maize, CMS-C, CMS-T, and CMS-S, which is defined according to the sterilizing factors in mitochondria and the corresponding main restorers in nucleus. Maize CMS is an economically valuable trait used for the production of hybrid seeds. CMS-T system was first used in three-line system technology to produce hybrid maize, which accounted for about 85% of U.S. hybrid seed until the 1970 epidemic of southern corn leaf blight [3, 4]. Since only one single major restorer (Rf3) is required for pollen fertility in the S-system, breeder switched to CMS-S to produce hybrid maize.
CMS-S maize have a clear elucidated mitochondria-nucleus interaction system, where the sterilizing factor orf355 initiates microspore degeneration while a single main restorer Rf3 cleaves orf355 transcripts to restore fertility. Mechanically, CMS-S mitochondria have two linear plasmids called S1 and S2, which have exactly the same terminal inverted repeats (TIRs) as that in CMS-associated region. These plasmids actively recombine with the circular genome to linearizing the mitochondrial DNA (mtDNA) [5–8]. The sterilizing factor orf355 locates just at the near end of the linearized mtDNA and expresses at bi-cellular stage of microspores development, finally leading to the gametophytic sterility of maize [9, 10]. In the presence of Rf3, the 1.6-kb transcripts containing orf355 are cleaved via posttranscriptional modification, leading to the reduction of transcripts level and fertility restoration .
However, CMS-S maize is a relative unstable system where revertants are reported to arise in real time through genetic mutation . Although the reversions usually take place in a single floret or a small sector in the tassel, this would potentially reduce the purity of hybrid seeds and hinder the utilization of CMS-S. Spontaneous fertility reversions usually take place in absence of main restorer Rf3. For example, nuclear-encoded restorer Rf9 does not cause the cleavage of 1.6-kb transcripts but decreases the abundance of the linearized transcription mtDNA template . In addition, loss of the free S1 and S2 plasmids of mitochondria blocks the rearrangement with the sterility-associated region, causes fertility reversion in the CMS-S maize . In some case, mtDNA rearrangement disrupts the sterility associated region such that the sterilizing factor either was lost or cannot be expressed. For example, a 7.3-kb inversion in mtDNA separates the TIR sequences from the CMS-associated region in some special revertants. As a result, orf355 is no longer transcribed from mtDNA linear ends but co-transcribed with cox2. Although orf355 transcripts can still be detected in the plant, they are not highly expressed and no longer initiated the sterility of the plant . Furthermore, the restorer-of-fertility lethal 1 (rfl1) mutant disrupts mitochondrial gene expression and the accumulation of α-subunit of the ATP synthase (ATPA), which link the functional plasticity of mitochondria with the spontaneous reversion in CMS-S .
Maize mitochondrial genome is about 500kb in size, containing a suite of relatively conserved protein-coding genes within different cytotypes . However, the relative placement of the genes and the intergenic spacer regions within the mitochondrial genome vary extensively among different subgroups of maize . A major reason for the highly variable structural organization of the maize mtDNA is the abundance of recombination-active repeated sequences . The shuffling of mtDNA sequences by recombination plays a role in evolution, changing gene organizations and creating gene chimeras . All the three types of CMS in maize, as well as CMSs reported in other species to date, are caused by chimeric ORFs that generated from mtDNA rearrangement. Mitochondrial rearrangements associated with the loss of portions of essential genes cause poor growth or lethality in maize [18, 19]. On the other hand, rearrangements affecting only noncoding regions of mtDNA are usually neutral, but sometimes cause different kind of phenotype, such as fertility reversion in CMS-S maize. Maize mitochondrial genomes have a special type of DNA sequence that acquired from plastid, which accounts for 4.4% of CMS-S mitochondrial genome . The plastid-derived DNA sequences change rapidly in content and location among five maize mitochondrial genomes . However, the relationship about plastid-derived DNA sequence and recombination is not clear.
On the other hand, mitochondria is a dynamic organelle that modulate its function and biogenesis in respond to fluctuating energy demand triggered by developmental signals and environmental stimuli . This key property is referred as mitochondrial robustness, and has important implication for understanding why severe mutations of mitochondrial can be compatible with life or only manifest in specific tissue. Study in animal suggests that the tissue-specific control coefficients of different respiratory chain complexes counteract the influence of mitochondrial, probably vice versa . Plant male sterility might be the outcome of inability of mitochondria to meet the increasing energy demand of microspore development , but the relationship between mitochondrial robustness and sterility determination is largely unknown. Here we described a special mitochondrial genome of CMS-S subtype that retained an intact orf355/orf77 region but lost the second copy of nad1- exon1. This Stoichiometric change reduced the mitochondrial biogenesis and genes transcription in anther tissue, which render low spontaneous reversion rate of the sterile plant.