Genetic Modification and Application in Cassava, Sweetpotato and Yams

*Prince Emmanuel Norman, Daniel K. Dzidzienyo, Kumba Yannah Karim and Aloysius A. Beah*

## **Abstract**

Cassava (*Manihot esculenta* Crantz), sweetpotato (*Ipomoea batatas*) and yams (Dioscorea spp.) are important root and tuber crops grown for food, feed and various industrial applications. However, their genetic gain potentials are limited by breeding and genetic bottlenecks for improvement of many desired traits. This book chapter covers the applications and potential benefits of genetic modification in breeding selected outcrossing root and tuber crops. It assesses how improvement of selected root and tuber crops through genetic modification overcomes both the high heterozygosity and serious trait separation that occurs in conventional breeding, and contributes to timely achievement of improved target traits. It also assesses the ways genetic modification improves genetic gain in the root and tuber breeding programs, conclusions and perspectives. Conscious use of complementary techniques such as genetic modification in the root and tuber breeding programs can increase the selection gain by reducing the long breeding cycle and cost, as well as reliable exploitation of the heritable variation in the desired direction.

**Keywords:** application, genetic modification, genetic gain, transgenic plants, roots and tubers

### **1. Introduction**

Root and tuber crops including cassava (*Manihot esculenta* Crantz), sweetpotato (*Ipomoea batatas*) and yams (Dioscorea spp.) are important crops with increasing food, feed and industrial applications in Sub-Saharan Africa and many other regions of the world [1–3]. These crops possess great potential to contribute to food, nutrition and income security of many livelihoods worldwide, but this potential is to be fully exploited. Variety development through breeding is among the activities targeted at unlocking the potential of these crops for food, feed and industrial applications [3].

Despite their importance, conventional breeding of root and tuber crops is limited by many challenges and heavily depends on the traditional techniques for exploitation of the existing variation. These challenges include high degree of genetic heterozygosity, genetic overloading, serious separation of progeny, few flowers, Irregularity in flowering time and flowering intensity, low pollen fertility, self-incompatibility, cross incompatibility, polyploidy, and low fruit set rate [3–6].

Genetic modification technologies are among many advances made to traditional breeding practices in plants, animals, and microbes to increase productivity and enhance food quality. Plant genetic modification is the oldest technique utilized in simple selection, where plants that exhibit desired characteristics are selected for continued propagation [7]. The advent of modern technology and various molecular analytical tools has improved upon simple selection for detection of elite plants expressing desired traits. Genetic modification is an important alternative and complementary technique for the genetic improvement of crops including roots and tubers. It is a powerful tool that can be used to introduce a number of genes with important agronomic traits, such as disease resistance, insect resistance, and high yield and quality. Genetic modification shows great potential for the genetic improvement of crops including roots and tubers and can compensate for the limitations of conventional breeding. The application of transgenic methods to cassava, sweetpotato and yam improvement programs is particularly important due to the difficulties associated with conventional breeding of these crops. However, an efficient plant regeneration system is imperative to achieve successful transformation [8].

Since the advent of genetic modification, rapid progress has been noted for cassava, sweetpotato and yam breeding programs through various of international non-profit organizations and scientists from developed countries. For instance, HarvestPlus and BioCassava Plus, have made remarkable achievements by transforming conventional breeding into molecular breeding [9, 10]. This book chapter focuses on genetic modification in selected root and tuber crops, applications, potential and future prospects for the genetic improvement of these economically important crops.

#### **1.1 Concepts of genetic and nongenetic modifications**

Genetic modification or transformation is the directed desirable transfer of gene or insertion of DNA from one organism to another along with the subsequent stable integration and expression of a foreign gene in the genome [7]. It also refers to the targeted manipulation of genetic material, and nontargeted, nontransgenic methods such as chemical mutagenesis and breeding applied to alter the genetic composition of plants, animals, and microorganisms. Genetic engineering refers to recombinant deoxyribonucleic acid (rDNA) methods that permit a gene from any species to be inserted and subsequently expressed in a food crop or other food product [7]. Although the process involving rDNA technology is not inherently hazardous, the products of rDNA technology may only be hazardous if inserted genes result in the production of hazardous substances.

Nongenetic engineering techniques of genetic modification such as embryo rescue involves placing of plant or animal embryos produced from interspecies gene transfer, or crossing, in a tissue culture environment to complete development [7]. Somatic hybridization of nongenetic engineering technique involves removal of the cell walls of a plant, forceful hybridization of cells and induction of mutagenesis. Irradiation or chemical mutagenesis is useful for the induction of random mutations in DNA [7]. The development of genetic modification approaches has enhanced an array of techniques that could be exploited to advance food production.

#### **2. Genetic modification technology**

The acquisition of new genes that confer selective merits is a relevant factor in genome evolution. Significant proportions of prokaryotic and eukaryotic genomes

#### *Genetic Modification and Application in Cassava, Sweetpotato and Yams DOI: http://dx.doi.org/10.5772/intechopen.101037*

originated from the exchange of genetic material among related or unrelated species through horizontal gene transfer (HGT). The HGT technique has been noted as one of the key sources of molecular variability and driver of evolution [11, 12]. This HGT often results in the occurrence of crown galls and the mechanism of HGT has been well understood and reported [12–14]. Moreover, Kyndt et al. [12] found that all the 291 tested accessions of cultivated sweetpotato contain one or more transfer DNA (T-DNA) sequences similar to the cultivated sweetpotato clone "Huachano", suggesting that an Agrobacterium infection occurred in evolutionary times. This finding also depicts the importance of plant–microbe interactions, and given that this crop has been eaten for millennia, it might change the negative mindset and paradigm governing the "unnatural" status of transgenic crops. The plant regeneration system of a typical root crop such as cassava was fully developed in the 1990s using somatic embryogenesis, shoot organogenesis from cotyledons of somatic embryos (somatic cotyledons), and friable embryogenic calli (FEC) (**Figure 1**). The key media components used for the establishment and optimization of the plant regeneration system have been well noted by Liu et al. [15]. The common transgenic techniques utilized for the genetic transformation of root and tuber crops are Agrobacterium-mediated gene delivery and particle bombardment. The explants used for transformation include somatic cotyledons and FEC.
