Cytoplasmic male sterility (CMS) system for F1 hybrid seed production. The 3-line system requires a CMS line, containing sterile cytoplasm (S) and a nonfunctional (recessive) restorer (rf ) gene or genes; a maintainer line, containing normal cytoplasm (N) and a nuclear genome identical to that of the CMS line; and a restorer line, with normal (N) or sterile (S) cytoplasm and a functional (dominant) restorer (Rf ) gene or genes. The CMS line is propagated by crossing with the maintainer line; the maintainer and restorer lines can produce seeds by self-pollination. The CMS line is crossed with the restorer line to produce male-fertile hybrids.  \cite{Chen_2014}