**Abstract**

As global food security and staple food, maize has become one of the most widely used cereals for fundamental research. Several important discoveries are reported, some of which are technological processes being used to improve maize crops' dietetic, phenotypic, genotypic, and organoleptic properties. This chapter provides insight into improved technological techniques such as crossbreeding, genetic cloning, and functional genomics and how they improve the nutritional quality of maize crops. The use of these technological processes could be one of the sustainable strategies in meeting the dietary needs and livelihood of Africa, Mexico, and Latin America's growing populace.

**Keywords:** breeding, genomics, functional genomics, improved technological processes, maize nutritional quality

### **1. Introduction**

Maize (*Zea mays ssp.*) is one of the widely-spread staple cereals globally since its introduction to the New World by Christopher Columbus in the fifteenth century [1, 2]. Maize originated from central Mexico 7000–9000 years ago as a wild grass known as teosinte [3]. Today, maize is a cereal that serves as a significant food source in animal and human nutrition, playing an essential role in feeding the world. It is the most researched cereal due to its significant strategic role in social and economic development, mainly in Asia and Africa [4], impacting economic growth activities, including employment. Based on FAOSTAT [5] report, maize and its products contributed 6.5%, 30%, and 38% of food supply to Asia, the Americas, and Africa. Africa's farmland used for maize cultivation is 24% [5]. Although maize production has made a critical contribution to food security and poverty in many African countries, there have been persistent challenges, causing a low maize yield and poor crop nutritional quality.

Nevertheless, Otekunrin et al. [6] assert that maize production can still play an essential role in achieving poverty alleviation and zero hunger. They illuminated the importance of channeling the support from the agricultural ministry on educated maize farmers from an empirical study conducted in Ghana. They argue that knowledgeable farmers can effectively use new technologies. Technical efficiency, which is the ability to use available resources for maximum output, influences the choices for the strategies used for productivity improvement.

Maize evolved enormously, alienating itself from some key traits of teosinte. For example, teosinte has abundant branches and tillers, increased number of ears per plant, reduced number of kernels per ear (5–12 per ear for teosinte and several hundred for maize), and small kernels with a hardened fruit case (reviewed in [7]). About 40 years ago, Beadle [8] observed that after he planted a teosinte–maize F2 population consisting of 50,000 individuals, the frequency of parental types was ~1 in 500, then estimated that there were four or five major loci involved in maize domestication. Later, Doebley and Stec [9] mapped five major quantitative trait loci (QTLs) plus some minor-effect QTLs for key traits in which teosinte and maize differ. This result was consistent with Beadle's estimation and indicated that a small number of loci were responsible for the teosinte–maize morphological difference. Wright [10] investigated 774 genes and estimated that 2–4% of maize genes were selected during maize domestication and subsequent improvement. According to recently released gene annotations of high-quality maize genomes, modern maize contains ~39,000–42,000 protein-coding genes [11–14], indicating that ~800–1700 protein-coding genes (39,000 × 2% = 800; 42,000 × 4% = 1700) underwent selection during the process of domestication (**Figure 1**).

Recently, two researchers [16, 17] used chromatin interaction analysis by pairedend tag sequencing (ChIA-PET) technology to map genome-wide chromatin interactions. They revealed their connections to gene-expression regulation (**Figure 1**), including the chromatin interactions in which TB1, UB3, ZmCCT9, Vgt1 were involved. Likely, population-scale identification of chromatin interactions would allow the detection of many more important regulatory elements, providing several useful selection targets for improving future maize, which is essential in addressing poverty alleviation and zero hunger (sustainable development goals, 1 and 2, set by the United Nations in 2015 to achieve global food security by 2030) [18]. The current low in the maize crop yield calls for more comprehensive and more consistent crop production strategies. A newly introduced program named clustered regularly interspaced short

### **Figure 1.**

*Genomic sequence variants in the tb1 regions of teosinte and tropical and temperate maize lines [1]. (A) The red rectangles indicate the position of tb1, and the blue rectangles indicate the position of the hopscotch TE. This TE is the functional variant of tb1 and is absent in teosinte [14]. (B–D) the increased expression levels of representative selected genes (tb1 in B, ZmSWEET4c in C, ra1 in D) in modern elite maize lines compared with teosinte, the ancestor of maize. The expression profile was obtained by analyzing RNA-seq data generated by Lemmon et al. [15].*

palindromic repeats-associated protein (CRISPR-Cas) technology is widely used for plant genome editing. It can be hoped that the CRISPR-Cas system will accelerate the breeding of improved crop cultivars compared with conventional breeding and help to address the zero-hunger goal.

Although maize protein is high in the ratio of leucine to isoleucine, it lacks tryptophan and lysine, making it poor nutritionally. Also, threonine is found in a reduced quantity in common maize [19]. However, Mertz et al. [20] discovered an improved nutritional quality of maize mutant called *opaque* 2 (*O2*). This *O2* maize has 95% casein and 43% higher protein quality than common maize. Hence, efforts were made to integrate *O2* as a commercialized variety but hindered by processing and agronomic problems [21]. *O2* was characterized by the dull and chalky kernel, reduced grain yield, and susceptibility to stored grain pests and soft endosperm, making it unacceptable to farmers and consumers. Hence, quality protein maize (QPM) emerged. QPM is a genotype that incorporated modified *opaque 2.* QPM improved the poor keeping quality, deficient agronomic attributes of *opaque 2*, and the truncated nutritive value of normal endosperm; hence, it contained double tryptophan and lysine when juxtaposed with common maize endosperm [22]. Conventional breeding was used to develop QPM that possessed high lysine and complex endosperm characters by International Centre for Maize and Wheat Research (CIMMYT), Mexico, in 1993.

The breeding of QPM was introduced to improve the nutritional composition of protein in maize grain. Maize seeds contain an alcohol-soluble protein called zein [19]. QPM has a protein profile of 90% milk protein compared to the 40% milk biological value protein found in common maize [19]. Zein is more in the endosperm than in the embryo and constitutes 50–70% endosperm protein. Zeins are high in leucine, proline, and glutamine but are lack in tryptophan and lysine. Therefore, zein compositions are altered to enhance maize nutritional quality [19].

Maize products are shaped to make nutritious foods more available by using desirable characteristics and traits. Therefore, new varieties with high yields became the focus of maize breeders [23]. Information on the needs of maize users can be incorporated into the products' characteristics by the breeders. This information will increase the use of maize varieties and, most importantly, improve nutrition [24]. Worldwide, maize varieties vary genetically by hardness, sweetness, and grain's size and color, and this genetic variation results in diversity in nutritional properties of the whole maize grain. Maize endosperm is made up of 82% composition of maize kernel, which is mainly starch. Hence, the endosperm' protein profile of the maize kernel was improved by the QPM. The breeding of this QPM has been achieved through alteration in the recessive mutant allele of the *O2* gene, a specific set of amino acid modifier genes and pair of endosperm hardness modifier genes. The kernels can be flint, pop, waxy, floury, or dent and provide micronutrients and macronutrients. There is a high level of antioxidants and minerals in the aleurone; minerals and fiber in the pericarps; antioxidants, protein starch, and vitamins in the endosperm; vitamins like vitamin E, fat, and minerals are rich in the germs [25]. The primary compounds in the kernels are cellulose and lignin, while secondary compounds are hemicellulose, β-glucans, and arabinoxylans. In maize, the presence of phytochemicals (anthocyanins, phlobaphenes, carotenoids, phenolic acids, nonpolar and polar lipids) prevents diseases and strengthens health.
