**3. Biotechnology: tool for root vegetables production and improvement in Nigeria**

#### **3.1 Onions**

Onion being a crop that is propagated by vegetative means with high heterozygosity, the cultivars are often low in reproductive fertility, making the breeding of this diploid species a challenging effort. Onion has a relatively long breeding cycle and genetic complexity, as well as sensitivity to inbreeding depression which determines the conventional methods adopted for onion improvement. Traditional breeding methods for onion involve mass selection for disease/pest resistance, improving or maintaining quality traits such as a bulb color, shape and increasing yields and shelflife. This has given rise to the development of many good clones. According to [12], the yield potential of onion has remained relatively constant over centuries despite rigorous breeding efforts. Conventional breeding of diploids plants often involves screening and backcrossing of large number of plants in order to obtain the desired genotype. Selection of many desirable traits at the initial stage can be ineffective and/or time consuming. Hence, onion breeders have most times needed to screen a large number of seedlings (up to a million) to enable identification of a single clonal line that can pull through to the release of a successful cultivar. Biotechnology application has provided unparallel opportunities for plant production and quality improvement [13–15].

Possibilities for improving agronomically-important traits are limitless with the biotechnological applications like in vitro culture techniques, marker-assisted breeding technologies, genetic engineering, genome editing or a combination of all the novel gene technologies. To increase onion production and improvement through biotechnology, the following directions for research of this crop plant are suggested: (1) ploidy manipulation as an alternative technique to induce haploids in onions [16]. More haploid regenerants should be produced by ploidy manipulation to improve onion for breeding purpose; (2) embryo rescue techniques to enable successful intergeneric and interspecific hybridization as has been widely reported for other species [17]. Hybrid embryo developments have been developed through embryo rescue. These hybrids could have horticulturally- and agronomically- important traits; (3) protoplast regeneration and fusion have been used to improve a plant's agronomic and horticultural characteristics such as pests and diseases resistance [18]. This technique should be used to produce more somatic hybrids that are pests and diseases resistant. Somatic variation can lead to creation of additional genetic variation in onion. Tolerance to herbicides, environmental or chemical stresses have been developed via this technique [19]. Somatic hybridization is an alternative technique to overcome both intraspecific and interspecific cross incompatibility to a large extent [20], and this technique could be used to introduce horticulturally- and agronomically-important genes in onions.

As genetic diversity is the basic input for breeding programmes [21] an understanding of genetic diversity among Nigeria onion germplasm collection is imperative for onion breeding.

Ijeomah et al. [22] studied and revealed the genetic diversity of 10 cultivars of spring onions in Nigeria using one SSR and three ISSR markers. The four markers yielded a total of 26 polymorphic alleles with polymorphic information content (PIC) values ranging from 0.6402 to 0.7569. The resulting UPGMA dendrogram showed that the 10 cultivars studied formed two main clusters with one subgroup showing no genetic distance among them. This study indicated the efficiency of SSR and ISSR markers to estimate the extent of genetic polymorphisms of spring onion cultivars with potential utility towards the conservation and management of *Allium* species.

#### **3.2 Ginger**

Most ginger improvement efforts have been restricted to evaluation and selection of naturally-occurring clonal variants. Conventional crossing efforts have been largely ineffective as a result of rare flowering and poor seed setting. Efforts at evolving high yielding clones through mutation and polyploid breeding indicated lack of success [23, 24]. Furthermore, the seed stock (rhizomes for vegetative propagation) seriously suffers from fungal and bacterial diseases such as *Pseudomonas solanacearum* (bacterial wilt), *Fusarium oxysporum* (yellow leaf), *Pythium aphanidermatum* (soft rot), *Phyllosticta zingiberi* (leaf spot), leading to heavy crop losses [25, 26]. Underground rhizomes are usually used as vegetative propagules for ginger which accounts for its very low multiplication rate [25, 26]. *Nigeria Root Vegetables: Production, Utilization, Breeding, Biotechnology and Constraints DOI: http://dx.doi.org/10.5772/intechopen.106861*

Different explant types used for micropropagation of ginger and other related species include meristem, axilliary buds, shoot tips and aerial pseudostems, although the commonly used explants are rhizome buds and shoot tips which have been reported as responsive explants for large-scale micropropagation to generate pathogen-free propagules [27].

An optimum fragment (explant) size is required for initiating successful tissue cultures. Sathyagowri and Seran [28] reported that rhizome buds of 0.5 cm in length were best for initiating ginger in vitro culture and shoot multiplication among the different tested explant sizes of 0.5, 1.0 and 2 cm long. The establishment of clean in vitro culture ginger from rhizome explants can be a daunting task as these underground explant is laden with pathogens resulting in contamination of cultures. As a general rule, absence of browning and freedom from contamination are criteria for the explants' survival for subsequent shoot multiplication. Surface sterilization of explants is commonly carried out with disinfectants such as ethanol (C2H5OH), sodium hypochlorite (NaOCl) and mercuric chloride (HgCl2). Ginger aseptic cultures have been obtained by surface sterilization of the rhizome buds explants with 0.1% HgCl2 solution for 10–20 min [16], turmeric [29]. Rout et al. [30] established a surface sterilization protocol for ginger sprouting buds explants using 2% (v/v) Teepol for 15 min followed by 0.2% (w/v) HgCl2 solution for 25 min and several changes of sterile distilled water. Contamination-free in vitro culture of ginger can also be optimally achieved by sterilizing the rhizome buds with 20% Clorox for 20 min [27]. Although bacterial contamination can be a common challenge with the undgerground ginger rhizome bud explants, this problem can be overcomed by incorporating antibiotics to the initial culture medium.

Maintaining optimum environmental parameters such as light and temperature should also be of utmost consideration to ensure optimal growth of cultures; the cultures being generally incubated under photoperiod regime of 16 h light/day and 8 h dark/night with cool, white fluorescent light, 60–70% relative humidity and temperature of 25 ± 2°C in culture room [26, 30]. MS meadia [31] containing 0–5 mg/l cytokinin (BAP or kinetin) eiher used alone or in combination with auxin (NAA or IAA or IBA) are commonly used for multiple shoots induction and subsequent regeneration of ginger plantlets (**Table 1**) [27, 32]. Nkere and Mbanaso [33] investigated the optimal concentrations of phytohormones for in vitro ginger culture, and found a combination of 0.05 mg/l NAA and 4.0 mg/l BAP yielding the highest mean shoot multiplication rate of 4.25 (**Table 1**). Although shoot survival is a key factor in in vitro propagation, a treatment combination of 1.0 mg/l BAP and 0.05 mg/l NAA recorded a relatively high survival rate of 80% and resulted in approximately 4-fold mean shoot multiplication rate in ginger as shown in **Table 1** [33]. Sharma and Singh [34] indicated that kinetin (Kn) outperformed BAP for vegetative bud multiplication. However, [25] reported that BAP at 17.76 μM (4.0 mg/l) yielded a 4-fold multiplication rate after the second subculture (**Table 1**). Balachandran et al. [35] also found using BAP alone resulted in better shoot multiplication than when BAP was combined with Kn in turmeric and ginger clonal propagation. Considering performance of the explants and the need to lower the cost associated with micropropagation, media containing the lowest concentrations of NAA (0.05 mg/l) and BAP (1.0 mg/l) which resulted in a very good survival rate of 80% and about 4-fold mean shoot multiplication rate [33] was recommended for in vitro ginger micropropagation. Zeatin was also reported as being more effective for microrhizome induction of 'Bentong' ginger, although its effectiveness on shoot multiplication was poor compared with BAP [32],


#### **Table 1.**

*Studies on in vitro propagation of ginger directly from rhizome explants.*

with zeatin's less effectiveness for shoot multiplication relative to BAP likely attributed to its high oxidative cleavage with prolonged incubation.
