What is mutation and its importance in plant sciences
Mutation is an alteration in the nucleotide sequence of the genome of an organism, virus, or extra chromosomal DNA. Mutations may also result from insertion or deletion of segments of DNA due to mobile genetic elements.
Mutation in plants
It is the process of exposing seeds to chemicals or radiation in order to generate mutants with desirable traits to be bred with other cultivars. Plants created using mutagenesis are sometimes called mutagenic plants or mutagenic seeds.
Classes of mutation
Four classes of mutations are
- Spontaneous mutations (molecular decay)
- Mutations due to error-prone replication bypass of naturally occurring DNA damage (also called error-prone translesion synthesis)
- Errors introduced during DNA repair
- Iinduced mutations caused by mutagens
Mechanisam of mutation
A mutation is any change within the genome of an organism that is not brought on by normal recombination and segregation. Causative agents are many, but include exposure to mutagenic agents such as radiation or certain chemicals, and mistakes made during normal cell division and replication. Most of these act upon the genome at random and are occurring all the time. These mutations are usually benign and go unnoticed in the organism due to the many cellular mechanisms that protect against these sorts of genetic mistakes. Mutations that are not caught by DNA repair mechanisms in the cell can go on to affect the organism and be present in future progeny.
In Luria and Delbruck’s classic experiment on bacterial resistance, they demonstrated that mutations occurred within the population at random and go unnoticed until some sort of selective pressure is placed upon the population.
Spontaneous mutations are those that occur without human intervention
These types of mutation happen randomly and the cause of them therefore not easily traceable. We do know, however, several things that cause mutations or increase their frequency. This includes the activity of mutagens found in nature such as solar radiation or reactive chemicals such as depurinators or free radicals. Mistakes during the replication of DNA during mitosis or unequal crossing-over events during meiosis can go un-repaired in a cell, leading to mutant progeny cells. It is hard to tell at what rate these occur, because the cellular machinery typically catches these mistakes. Occasionally, however, some mistakes make it permanently into the organism’s genome.
Types of mutation
At the most basic level, there are only a handful of classifications of mutations regardless of causative event. Deletions and insertions involve the removal or addition of segments of DNA respectively. These segments can range from individual base-pairs to several thousand base-pairs long. Substitutions occur when a particular base is replaced with one of the other three nucleotide bases.
Inversions are instances where a segment of chromosome is rotated and replaced in the opposite direction that the segment was facing. The last major classification is a reciprocal translocation. This involves the excision of segments from two non-homologous chromosomes. These excised portions are then inserted into the other chromosome. Chromosome A will gain the segment from Chromosome B and Chromosome B gains the segment from chromosome A.
Importance of variation in plants breeding
Variation is the source from which plant breeders are able to produce new and important cultivars. Alleles of varying forms at given loci in a population can be selected and fixed within a new individual or line. We depend on recombination and independent assortment of favorable alleles to produce new and unique individuals from which to select and produce the lines that will serve as our cultivars. With tens of thousands of genes within each crop species genome, the possibilities seem limitless. This, however, is not so clearly the case.
Consider that variation within a population can be exploited by selecting individuals with new combinations of desirable traits or alleles. As discussed in other chapters on genetic diversity of crop plants, most crop species, which have been selectively bred for centuries, have large portions of their genome essentially fixed. This means then, that the portions of DNA, and therefore number of traits available for re-shuffling is reduced. This reduction, however ominous it may seem, has still allowed us to make significant gains in crop yield and quality in recent decades.
Importance of mutation in plants In self fertilizing species
Breeding mutant traits is fairly straightforward in crops that are capable of self-fertilization. Because many mutations are recessive, after mutagenic treatment, the material should be self-fertilized and advanced to at least the M2 before phenotypic screening. At this point plants will be segregating for the recessive mutant trait. Positive mutant identifications should be kept for future selection In cross fertilizing species Cross-fertilizing species raise some difficulties. Because species which are predominantly cross-fertilizing typically exhibit significant inbreeding depression, the necessary self-fertilizations to identify mutants in the population result in reduced plant vigor due to the genetic background and not necessarily the mutations. This compounds the difficulty of successfully identifying mutations. Dominant mutations can be identified, but these occur very rarely.
In vegetatively propagated species
When attempting to effect mutation in vegetatively propagated species such as sugarcane or banana, it is important to note the chimeric nature of mutagenic treatment. All cells exposed to the mutagen will not necessarily incur mutations, but those that do incur mutations, will give rise to cells exhibiting the mutation. For this reason it is important to treat parts of the plant that will give rise to either seed or vegetative propagules. Identification and propagation of the necessarily large numbers of plants to identify successful mutants is difficult for many vegetatively propagated plants, however, once one is identified, the mutation is fixed in the cloned progeny.
- In seed propagated species
- In main crops
- In wheat
By the application of Eradication and chemical mutagens mutation being used to introduced in wheat varieties. By this process the resistance variety NP836 was derived from NP 799 at India agricultural research institute (IARI), New Delhi. This is done by application gamma ray on NP 799 variety of wheat. Thus NP 836 is a mutant variety.
Mutation breeding in rice is very common in south and south-east Asia. In rice certain chemical mutafgenens has been used to produce polyploid varieties of rice and hybridised with the diploids producing high yielding and resistant varieties.The high yielding varieties of rice produced by mutation breeding is P 500.28. This variety is obtained from the T-1145 variety at Bose’s institute, Calcutta.
Mutation beading achieved evolving improved variety in cotton. A caiety named indore-2 was developed from “Malwa Upland -4” by X-ray treatment.
Both eradication and chemical mutagens are used to induce maturation in sugarcane. In sugarcane nodal buds are exposed to radiation in field and the mutant buds or tillers are selected in F1 and F2 generations through artificial crosses.
Mutations also introduce in potato crop through mutation breeding. It aims at the production of early harvesting varieties and high yielding varietyThese includes eradication and chemical mutagenic products and through cross breeding. This is done in flowering or by exposing the seed tubers.
In horticultural plants
About 50% of the present day crop and horticultural plants have developed in nature through polyploidy, e.g., Wheat, Rice, Maize, Gram, Cotton, Potato, Sugarcane, Banana, Pineapple, Apple, Pear, etc.
In ornamental plants
A large number of mutations have been induced in ornamental plants in order to enhance their beauty, longer life and fragrance, e.g., Dahlia, Rosa, Chrysanthemum, Pa- paver.
Cauliflower, Cabbage, Brussel’s Sprout and Knol Kohl are all mutants developed from Wild Cabbage.
Sumera Akram, Department of Agronomy, University of Agriculture Faisalabad