Corn male fertility restoration

​Several plant species feature mitochondrial genetic mutations that result in lack of pollen production, allowing hybrid production even in normally self-pollinated species such as sorghum and rice.  In each case, production of viable hybrid seed requires a method to produce hybrids that do produce viable pollen.  In all cases of cytoplasmic male sterility (CMS), that restoration of pollen viability has come from the chromosomal genetics in which dominant, fertility restorer (Rf) genes are involved.
 
Corn is known to have three main types of CMS, designated as CMS-T, CMS-C and CMS-S.  Each involve different mutations in the mitochondrial DNA but all leading to inability to adequately process the transformation of carbohydrates into usable form of energy (ATP) needed for pollen production.  These mutations create the wrong protein to function as an enzyme in this pathway within the mitochondria.  These genes are referred to as ORF genes.
 
Restoration of pollen production appears to be related to the proteins produced in the cells associated with specific Rf genes.  These proteins are believed to be close in structure to the missing protein in the CMS mitochondria, allowing the carbohydrate transformation to ATP to continue.  Each of the CMS versions ORF gene products are different and thus appropriate restorer genes are different.  Restoration of CMS-T requires two Rf genes, designated as Rf1 and Rf2.  Restoration of CMS-C is associated with Rf4.  CMS-S is restored by the gene Rf3.
 
Corn breeders cross inbred designated to be female parents by crossing into related inbreds of the desired CMS after the inbred has been shown to not have the appropriate Rf genetics.  Some inbreds are not possible to get complete sterility, probably because they include some Rf genetics.  If the male of a hybrids does not already restore fertility to a specific CMS, the appropriate Rf gene is crossed into the inbred.  CMS-T was desirable because it always gave complete male sterility.  CMS-C and CMS-S occasionally, in some environments and some genetics, produce a small amount of pollen and therefore could result in some selfing in seed production fields.  Seed producers usually remove tassels to prevent this problem.
 
Corn, having male flower parts easily removed from the plants designated as the seed parent, can produce hybrids without CMS.  It is done mostly for cost and labor efficiently.  For mass production of hybrid Rice or Sorghum seed, CMS and Rf genes are essential.