1.3.1 Mutagenesis
Mutagenesis is defined as the phenomenon in which sudden heritable changes occur within the genome of an organism. Its occurrence can be spontaneous or can be on exposure to different chemical, physical, or biological agents (Oladosu et al., 2016). In plant science, it has been considered a powerful strategy for bordering genetic variability in various species (Kumawat et al., 2019a). It has great significance, especially in crops where natural sources for the genetic variations are limited. Mutation breeding and plant mutagenesis assume a huge part in expanding the genetic variability for desired traits in various food crops (Chaudhary et al., 2019). In plant breeding programs, physical and chemical mutagens are effectively applied for the advancement of new varieties with improved characteristics (Kodym and Afza, 2003). Now, it’s a mainstay of modern plant breeding, alongside recombinant breeding and transgenic breeding (Shu et al., 2012). In plant science research, different mutagenesis approaches have been utilized to distinguish novel genes and their functional regulations.
In mutation breeding study, three known kinds of mutagenesis are used. The first is radiation-induced mutagenesis, in which mutations occur as a result of exposure to radiation (gamma rays, X-rays, or ion beams); the second is chemically induced mutagenesis; and the third is insertional mutagenesis (site-directed mutagenesis, a result of DNA insertions either through the genetic transformation and through the addition of T-DNA or the activation of transposable elements) (Forster et al., 2012). Induced mutagenesis is considered as one of the most effective tools for the detection and elucidation of key regulatory genes and molecular mechanisms. It is a promising methodology for delivering new varieties with improved agronomic traits, such as biofortification and higher stress tolerance (biotic and abiotic stresses) (Chaudhary et al., 2019).
Mutation breeding is a three-step process for direct release of improved crops, which is comprised of (i) inducing mutations which may take up to a year, (ii) screening for putative mutant candidates, and (iii) mutant varietal release. The foremost complicated and time-devouring step is mutant selection. Generally, several years are required to identify useful traits that are stable throughout the propagation cycles, and the third step, mutant varietal release, follows the standardized procedures of the country where the material is developed. This regularly requires multi-locational trials with farmer contribution (Jankowicz-Cieslak et al., 2017). Several years are ordinarily required to recognize valuable characteristics that are uniform through propagation cycles. Whereas the timing of this may shift, it more frequently requires a shorter duration than the selection and testing stage. The procedure gets to be longer and more complicated in case the selected mutants are utilized as pre-breeding material in hybridizations.
In molecular biology, scientific advancements have re-enhanced mutation breeding by making it more effective and productive than ever before. With new innovative technological developments, mutation screening by genotype became feasible. The common strategies, Targeting Induced Local Lesions IN Genomes (TILLING) and EcoTILLING, where mutagens are used to induce mutation randomly in the genome to cause a high density of triggered mutations, can specifically distinguish allelic changes in the genome (Wang et al., 2012; Kurowska et al., 2011). TILLING utilizes large offspring populations through chemical or irradiation mutagenesis, but only the gene of interest is studied instead of phenotypic screening (Jung et al., 2018). This involves substantial knowledge of the underlying genetic processes, which, for many agronomic characteristics, are notable today. Genotype-dependent mutation screens have been applied in all major crop species, and multiple mutants have been identified and recognized. Physical mutagens, such as fast neutron, UV, X-ray, and gamma radiations, and chemical mutagens, including N-methyl-N-nitrosourea (MNU), sodium azide, hydrogen fluoride (HF), methyl methanesulfonate (MMS), and ethyl methanesulfonate (EMS), have broadly been investigated over the last century. In addition, biological mutagens comprise Agrobacterium and transposon-based chromosomal integration. The mutation induced by EMS is a profoundly powerful technique and is therefore commonly used to develop improved crop varieties in crop breeding. Recently, in plant science, the use of fast neutron (FN) bombardment to create a mutagenized population has been gaining prominence. FN results in significant deletions from a few bases to a million bases (sometimes > 1 Mb) and a greater proportion of double lesions that are not repairable, as well as chromosome alterations in the genome. FN has been shown to be a very powerful mutagen in plants and the FN-treated lines are easily generated and deletion library is quickly assembled, which helps us to locate deletion mutant (Li et al., 2001). A random deletion library generated by FN mutagenesis lines may provide valuable and significant information for the reverse genetic approaches. Recently, Kumawat and colleagues (2019a) have highlighted the use of FN mutagenesis to build a resource of gene deletion lines. For functional genomics and even t...
