**3. Molecular breeding and marker-assisted selection (MAS)**

Traditional plant breeding involves the iterative practice of selecting both parents and their offspring based on desirable characteristics. The significance of molecular breeding is substantial in the contemporary world, especially in developing countries where a small proportion of the population is engaged in agriculture. This minority group bears the demanding responsibility of providing sustenance for the majority of the country's population [35]. This achievement is feasible because plant breeding

#### *Enhancing Maize (*Zea mays *L.) Crop through Advanced Techniques: A Comprehensive Approach DOI: http://dx.doi.org/10.5772/intechopen.114029*

has effectively enhanced crop yields without the need to expand the cultivated land or the workforce engaged in agriculture. Achieving this objective can be readily accomplished through crop enhancement via molecular breeding techniques. Molecular breeding employs various approaches such as the identification of simple traits or QTLs within breeding lines/populations, the integration of genes from breeding lines or wild relatives, gene pyramiding, marker-assisted recurrent selection (MARS), and Marker-assisted backcross (MABC).

Molecular MAS, often referred to as marker-aided selection or MAS is an indirect approach to nomination or selection wherein a specific trait is targeted through the use of a marker [29]. Within the context of MAS, a marker is located in the vicinity of a gene responsible for controlling the trait, thereby signifying the presence of a desirable allele when the marker is detected [36]. Knowing the alleles in key loci allows for the creation of optimal allele combinations to enhance the agronomic value of the genotype. Marker-assisted selection is commonly used for resistance gene pyramiding, which can provide complete resistance for several plant generations until it's challenged by pathogen strains. Achieving gene pyramiding for resistance becomes exceedingly challenging through classical breeding methods for certain traits like pathogen-induced disease resistance when dominant resistance genes are present [35]. There are primarily three categories of genetic markers. The first comprises visible or morphological markers, which are characteristics or phenotypic traits. The second category includes biochemical markers, which involve enzyme allelic variations referred to as isozymes. The third category consists of molecular or DNA markers, which unveil points of variation in the DNA sequence [37, 38]. For a nucleotide sequence to be useful as a molecular marker in molecular breeding, it usually requires polymorphism within its sequence. These variations in nucleotide sequences are unveiled through molecular methods like restriction fragment length polymorphism (RFLP), amplified fragment length polymorphism (AFLP), random amplified polymorphic DNA (RAPD), single nucleotide polymorphism (SNP), microsatellite or simple sequence repeat polymorphism (SSRP), sequence-tagged site (STS), single-strand conformation polymorphism (SSCP), and cleavable amplified polymorphic sequences (CAPS) among others [39]. Utilizing this marker set relies on extensive prior research. This typically involves various research stages for each trait, commencing with QTL mapping, progressing to fine mapping, and ultimately culminating in positional cloning [40]. For a successful MAS program, essential components include dependable markers, a robust DNA extraction method, well-constructed genetic maps, swift and efficient data handling, an interpretation of marker and trait connections, and knowledge to access tools for high-throughput marker detection [41].

MABC represents a specialized example of MAS, wherein the process of backcrossing is aided by molecular markers to expedite the selection of the recurrent parent and enhance genome recovery speed. The MABC technique has found extensive application in eliminating undesirable traits, such as susceptibility to insects and diseases, as well as anti-nutritional factors, from widely adopted high-yielding varieties by introducing QTLs or genes of interest from donor parents [42, 43]. Using DNA markers in a breeding program recommended a variety of benefits. For instance, DNA marker-based screening facilitates early selection for traits due to the evaluation of plant genotypes at the seedling stage or even from seeds, that may only manifest in adult plants, such as male sterility, fruit or grain quality, and photoperiod sensitivity. It expedites the selection of alleles that are difficult to assess phenotypically, especially for environmentally sensitive traits, simplifying and enhancing

the breeding process. Individual plant selection, which may be impractical through phenotypic means, becomes feasible when relying on marker information. The issue of low heritability becomes inconsequential when using marker-based selection. Additionally, in traits with intricate inheritance patterns, it becomes possible to select each genetic component contributing to the trait independently. Using molecular markers, multiple characters that typically exhibit epistatic interactions can be preserved and ultimately stabilized. Moreover, the preservation of recessive genes does not necessitate progeny testing in every generation since homozygous and heterozygous plants can be differentiated using (codominant) markers, as explained by Lema [32]. Molecular markers have an important role in enhancing maize's genetic traits, such as addressing the intricate inheritance patterns related to drought tolerance [44–46], improving nutrient utilization [46–52], and enhancing diseases, parasitic and insect pests plant resistance in maize [17, 53–56]. Additional details can be found in the works of Hossain et al. [57] and Muntean et al. [33].
