Molecular Basis of Cytoplasmic Male Sterility | Genetics

In this article we will discuss about the molecular basis of cytoplasmic male sterility in nature.

Cytoplasmic male sterility (CMS) has been noticed over 150 plant species. CMS is a condition where plant is unable to produce functional pollen. Exploration of CMS in the produc­tion of male sterile lines eliminates hand emasculation. CMS can arise spontaneously from breeding lines due to wide array of crosses.

Several crop plants are known to have cytoplasmic male sterility like rice, sorghum, wheat, sunflower, carrot, beet, onion, petunia and phaseolus etc. Several cases of CMS is con­tinued to exist inadvertently in breeding line until mutation or segregation process removes restorer genes. Interspecific hybridization is an attractive approach which involves protoplast fusion and production of cybrids generating cytoplasmic male sterile plants. In addition, wide crosses have been implicated to generate CMS system in certain family.

One of the prominent symptoms appears during CMS is the premature degeneration of tapetum layer of the anther. Tapetum is a nutritive tissue, involved in the development of pollen. Therefore, tapetal degeneration would cause CMS. In Texas (T) cytoplasmic maize, the morphological symptom appears immediately after meiosis.

Destruction of mitochondria takes place in the degenerated tapetal cells. CMS is often associated with chimeric mitochondrial open reading frame (ORF). The transcripts of the ORF are translated into a protein that seems to interfere with mitochondrial function and pollen development.

Disfunction in the mitochondrial genome is responsible for CMS. A specific CMS associ­ated genes has been identified in mitochondrial genome. Application of RFLP and other com­parative physical mapping technique helps in the identification of CMS-associated genes in the mt DNA of larger plant.

Identification of genes responsible for CMS in maize initially posed several problems due to numerous RFLR The gene T-urf 13, was identified in the mitochondrial DNA associated with CMS. The identification and sequence analysis of T-specific transport led to the conclusion that it contains two ORF, T-urf 13 and orf 221. The CMS responsive T-urf 13 encodes a 13-kDa mitochondrial pore-forming protein (URF 13) (Fig. 21.1).

Disruption of pollen development by the expression of a mitochondrial gene was proved by transgenic plants. The CMS associated mitochondrial DNA sequence from common bean, orf 239, was introduced into tobacco nuclear gene.

Transformants containing the orf 239 exhibits a semi or complete male-sterile phenotype. The protein production of orf 239 27 kDa in size, present only in the reproductive tissue led to the conclusion that it is the gene responsible for the pollen disruption probably by interfere with callose deposition.

Similarly, in Brassica napus, there are CMS-associated ORF linked to male sterility. Physical mapping reveals to the discovery of the CMS-associated orf 224/atp 6 locus. In this plant, Rf P₁ or Rf P₂ is sterility restorer. In wheat for example, orf 256 are the chimeral gene, present in Triticum aestivum in the mitochondrial genome of CMS. Wheat that arose via interspecific cross between T. aestivum carries orf 256 like T. timopheevi.