Introduction
Cytoplasmic male sterility (CMS), a state in which plants are unable to produce functional pollen, is widespread among higher plants. The CMS system is a valuable tool in the production of hybrid seeds in self-pollinating crop species, in a number of vegetable crops. Hybrids often exhibit hybrid (also known as heterosis) and hybrid offspring show excellent growth characteristics relative to the parental lines. CMS systems can be of considerable value in promoting efficient hybrid seed production.
In plants, male sterility can be caused by either mitochondrial genes with coupled nuclear genes or nuclear genes alone; the resulting conditions are known as cytoplasmic male sterility (CMS) and genic male sterility (GMS), respectively. CMS and GMS facilitate hybrid seed production of many crops, thus allowing breeders to take advantage of the yield increase associated with hybrid vigor. In CMS, the layers of interaction between mitochondrial and nuclear genes control its male specificity, occurrence, and restoration of fertility.\cite{Chen_2014}
The CMS-based hybrid seed technology uses a three-line system that requires three different breeding lines: CMS line, maintainer line, and restore line (Figure 1). The CMS line has a male sterile cytoplasm with a gene causing CMS (hereinafter, CMS gene) and lacks a functional nuclear nuclear restorer of fertility and is used as the female parent. The maintainer line has a normal fertile cytoplasm, but contains the same nuclear genome as the CMS line and therefore serves as a male parent for the proliferation of the CMS line. The restore line has a functional Rf gene and therefore serves as the male parent to cross with the CMS line to produce F1 hybrid seeds. In F1 plants, the Rf gene restores male fertility, and the combination of nuclear genomes from the CMS line and the restorer line creates hybrid vigor.\cite{Chen_2014}