In this article we will discuss about the mechanism of restoration of fertility in CMS plants.
Restoration of fertility in nature is of two types, one is sporophytic and other is gametophytic. The sporophytic restorer functions before meiosis whereas gametophytic restorer acts after meiosis in pollen grain. As a consequence, a diploid plant act after meiosis in pollen grain and is heterozygous for a restorer will provide two types of pollen grain that carries the restorer and other do not.
A comprehensive study shows the complete restoration of fertility due to the participation of concerted action of several genes or only few genes. However, in most system, a single restorer is sufficient. By contrast, in T-maize and T-cytoplasm of onion, two restorers are indispensable for fertility restoration.
The restoration of fertility in CMS plants involves diverse mechanism. One of the possible mechanisms is physical loss of CMS-associated gene from the mitochondrial genome, in which they are associated and occur sterility. This phenomenon has been seen in Phaseolus due to the presence of nuclear gene ‘Fr’.
The exact functions of nuclear restorer often alters the expression of CMS-associated genes, which is ultimately end up in elimination of deleterious effects linked with their gene products. Some cases restoration of fertility is associated with processing of CMS gene transcript. In most cases, transcripts of CMS genes were altered more precisely, and the accumulation of specific transcript is altered.
One of the classic examples observed in T- cytoplasm maize, in the presence of Rf, fertility restorer. The accumulation of T-urf 13 transcript is 1.6 kb and 0.6 kb is increased considerably and accumulation of 13 kDa URF 13 proteins is considerably reduced. Fertility restoration can be possibly occurring due to post-transcriptional RNA editing.
The editing may alter the length of the CMS associated ORF by creating new initiation start codon (AUG) or stop codon (UAA, UAG or UGA). Editing process in mitochondrial sequence is C to U. It is also possible that tissue-specific editing resulted in deleterious effect only at micro-sporogenesis. Cloning of genes may be helpful in the sequence analysis of restorer genes which provide clones consequently to their functions.
To date only one restorer gene ‘rf2‘ has been cloned. The rf2 gene is indispensable for fertility restorer in T-cytoplasm maize. Several mechanisms have been proposed to explain the mechanism of their gene function. The protein- predicted by the rf2 sequence is a aldehyde dehydrogenase.
In the metabolic hypothesis, aldehyde dehydrogenase may be involved in the detoxification of acetaldehyde produced by ethanolic fermentation during pollen development. In this process, the role of RF2 protein (fertility restorer) would be either to detoxify this aldehyde or to catalyse their oxidation into compounds that are necessary in plants containing T-cytoplasm mitochondria.
In this ‘interaction hypothesis’ the Rf2 protein interact directly or indirectly with URf 13 and multifies its deleterious effect. For example, the RF2 protein could catalyse the oxidation of an aldehyde component of inner- mitochondrial membrane. Thereby altering the interaction between URF 13 and the membrane where it normally accumulates.