Fig. 1
From: Establishment and Advances of Third-Generation Hybrid Rice Technology: A Review
Three-line system (the first-generation hybrid rice) and two-line system (the second-generation hybrid rice) for rice breeding. (A) The three-line system. The cytoplasmic male sterility (CMS) line, which carries the mitochondrial CMS gene, but does not have a functional nuclear restorer gene (Rf), is propagated by cross-pollination by the maintainer line. The Rf genes encode proteins that are targeted to mitochondria to inhibit the CMS gene, thus restoring the fertility of the F1 hybrid. (B) The two-line system. The environment-sensitive genic male sterility (EGMS) line carries a recessive nuclear EGMS gene (mt). The restorer line carries the wild-type EGMS gene (WT). Under restrictive conditions (such as long day length, high temperature, or low humidity), the EGMS line is male-sterile and can cross with the restorer line to produce F1 hybrid seeds. Under permissive conditions (such as short day length, low temperature, or high humidity), the EGMS line can produce functional pollen and self-pollinate. The restorer line can be any normal rice germplasm that can self-reproduce