**5. Breeding approaches for QPM**

It was introduced into many breeding programs worldwide after the discovery of the nutritional benefits of the opaque-2 (op2) mutation, with a significant focus on the conversion of normal endosperm populations and inbred lines to op2 versions via a modified backcrossing-cum-recurrent selection process. At CIMMYT, QPM breeding strategies concentrate on pedigree breeding, whereby the best performing inbred lines, complementary in various characteristics, are crossed to create new segregating families. New inbred lines are formed from these segregating families [20].

## **5.1 Conventional and molecular breeding approaches in QPM**

Pixley and Bjarnason [21] reported that the consistency of proteins was very stable across environments, while QPM varieties were less stable in protein content and endosperm modifications. Pfunde and Mutengwa [22] reported that early maturing QPM inbred lines under drought stress could be used in a breeding programme as sources of early maturation, whereas early maturing single crosses could potentially be recommended in drought-prone areas for maize growers. The stability performance of CIMMYT tropical and subtropical elite QPM hybrids across stressed and non-stressed environments was analyzed [13]. In drought conditions with wide variability in grain yield and protein content among genotypes, the stress effect was comparatively large, indicating that the content of tryptophan and lysine is most stable across stressed and non-stressed environments. While drought tolerance screening has largely been performed for QPM varieties for the vegetative to flowering stages of development, very few studies have been done on tolerance to early drought stress at the seedling level. Drought tolerance has often been hated at the seedling stage of development in that it does not offer an indicator of a genotype's yielding ability under drought stress. Clearly, therefore, the correlation parameters that could relate early drought response to late drought stress tolerance need to be further investigated.

Henry *et al*. [23] studied the molecular structure of the opaque-2 gene and found that the molecular diversity in the transcriptional activator op2 was very high relative to that of other maize transcription factors. Multiple genes have been identified to regulate the quality of amino acids. In order to monitor the levels of a protein synthesis factor associated with lysine levels, at least three loci were involved and

#### *Breeding Maize for Food and Nutritional Security DOI: http://dx.doi.org/10.5772/intechopen.98741*

these were mapped on chromosomes 2, 4 and 7 [24]. Via marker-assisted backcross breeding, two QPM lines (CML 180 and CML 170) were selected as donors for introgression of the op2 allele into regular maize inbreds (CM 212 and CM 145) because the crosses between the donor QPM lines and non-QPM lines showed a 41% increase in tryptophan and a 30% increase in lysine over the original hybrid lines [23]. Therefore, modified marker-assisted back cross breeding is a potential way to produce QPM variants of standard maize inbreds with suitable endosperm features that can be combined to create QPM hybrids. Using inter-simple sequence repeat (ISSR) and random amplified polymorphic DNA (RAPD) markers. Nkongolo *et al*. [25] studied the degree of genetic variation and relatedness among and within QPM and non-QPM varieties. The findings showed that the genetic difference between QPM and non-QPM varieties and within them was high, while the genetic gap among them was minimal, giving the possibility of developing improved QPM hybrids. The use of molecular markers in QPM breeding programs shortens the selection process, making it more effective across environments during the production of enhanced genotypes. It is important to remember that it is considered that the latest generation of markers such as SNPs is comparatively more efficient and cheaper than older models (SSR, RAPD).

In CIMMYT, several QPM populations, inbreds, hybrids and pools were developed through conventional conversion breeding methods that could adapt to subtropical and tropical environments and are widely used in the production of QPM cultivars in several countries in Africa, Asia, and Latin America [26]. Two measures are involved in marker-assisted introgression using backcross breeding: (1) foreground selection: targeting gene by marker, and (2) background selection: targeting uniformly distributed markers for recurrent parental genome (RPG) recovery across the genome [27]. This is an effective way to transfer particular gene(s) to an otherwise superior variety or parental lines. By foreground selection, the detection of the gene of interest becomes accurate, while background selection accelerates the rate of RPG recovery with two backcrosses [28]. Simple access to accurate gene-based or linked markers based on PCR has made MAS an effective alternative. Microsatellite or Simple Sequence Repeat (SSR) markers are often the choice for their low cost, simplicity and effectiveness among the various types of DNA sequence-based markers available. Codominant, stable, hypervariable, abundant and evenly distributed SSR markers are distributed throughout plant genomes [29]. Several thousand SSRs in maize are mapped and accessible in the public domain. The availability of sufficient linked SSRs has provided a promising choice for marker-assisted introgression of o16 to further enhance the nutritional quality attributes of grain, in particular lysine and tryptophan in endosperm protein. In this context, associated SSRs, umc1141 and umc1149, were successfully used for introgression or pyramidization of o16 alone in the genetic context of o2. The improvement of the quality of proteins (tryptophan and lysine) by o16 over normal maize is comparable to the QPM genotypes based on o2 [30]. At Guizhou Institute of Upland Food Crops, Guizhou Academy of Agricultural Sciences, China, Marker Assisted Selection (MAS) was used to improve parental lines and derived hybrids by pyramiding o2 and o16 in maize adapted to temperate regions. A half-fold increase in lysine content has been reported among pyramid progenies of o2 and o16 [31].
