**4.6 Delayed post-harvest physiological deterioration**

Postharvest physiological deterioration (PPD) limits the storability and utilization of cassava. The PPD is a physiological and biochemical decay process caused by an oxidative burst in storage root cells of cassava [90]. The PPD phenomenon has a close relationship with reactive oxygen species (ROS) [90]. Analysis of the proteins and enzymes influencing PPD using the cDNA-AFLP technique showed that most of the proteins and enzymes are involved in signal transduction, ROS, cell wall repair, programmed cell death, metabolite transport, signal transduction, and a series of biological processes [91]. The upregulation or downregulation of key enzymes or factors in the PPD pathway by the overexpression or RNAi might effectively slow or decrease the PPD activity [91]. Study of the temporal and spatial expressions of genes related to ROS production and scavenging in cassava PPD, and the functional verification of key genes, indicates the possibility of interference of the PPD process by the regulation of ROS-scavenging activities [15]. These genes are involved in the regulation of glutathione-peroxidase cycle, the ascorbateglutathione cycle, and the peroxidase-oxidoreductase cycle [15]. The genes also combine with superoxide dismutase to generate univalent, bivalent, and trivalent overexpression or RNAi vectors to transform cassava. Secondary metabolites of PPD such as scopoletin and diterpenoids, are synthesized in deteriorated roots [92]. The interference with the biosynthesis of the secondary metabolites might also influence PPD. Morante et al. [93] noted the discovery of germplasms of radiation mutants that suppress the occurrence of PPD. These results provide gene resources for the amelioration of PPD through traditional and molecular breeding techniques.

### **5. Future prospects of genetic modification for root and tuber improvement**

Pests and diseases are among key factors affecting yield and quality of root and tuber crops. The progress made in genetic modification of these crops could open up many avenues to produce disease resistant varieties, through pathogen-derived resistance strategies, that would not be possible using conventional breeding approaches alone. In yams, host plant resistance to anthracnose has been suggested as a more viable alternative to control yam anthracnose disease (YAD) than use of chemical fungicides [94]. However, studies reveal lack of genotypes resistant to the disease [94]. Thus, the most efficient strategy for YAD control is possibly the development of disease resistant plants using the transgenic approach. These approaches could include the expression of genes encoding elicitors of defense response [95], genes encoding plant, fungal or bacterial hydrolytic enzymes [96] and antimicrobial peptides (AMPs) [97]. Most AMPs are non-toxic to plant and mammalian cells, with a broad-spectrum antimicrobial activity against fungi and bacteria.

Use of nematode resistant yam varieties can be an effective strategy in controlling the disease, however, there are no resistant varieties to nematodes. The application of transgenic approach could serve as a viable alternative for improvement of the nematode resistance of yam. Several transgenes have been noted to confer plant resistance to both tropical and temperate plant parasitic nematodes [98]. Cystatins inhibit nematode digestive cysteine proteinase activity, thereby suppressing the growth and multiplication of these pests [99]. Cystatin is one of the transgenes that has been successfully applied to control plant nematodes. The cystatins transgene confers improved resistance to a range of nematodes in different crops including potato, sweetpotato, rice, tomato, and plantain [100–105]. The transgene has displayed proven efficacy under field conditions [102]. Such an approach could be exploited for genetic enhancement of resistance of yam against nematodes in the near future.

In sweetpotato, genetic modification studies show that the suppression of βcarotene hydroxylase (CHY-β), which catalyzes the hydroxylation steps of both β-carotene into β-cryptoxanthin and β-cryptoxanthin into zeaxanthin, significantly increased the β-carotene and total carotenoid content in transgenic cultured cells [106]. Moreover, suppression of lycopene β-cyclase (LCY-β), which catalyzes the cyclization steps of lycopene to produce β-carotene, resulted in increased total carotenoid content [107]. Transgenic sweetpotato plants overexpressing an Or homolog, IbOr, showed increased carotenoid contents compared to non-transformed control plants [108]. These findings indicate that the CHY-β and LCY-β are key enzymes of carotenoid biosynthesis in sweetpotato that could be exploited as good targets for molecular breeding.

In many plants, transcription factors belonging to MYB-family are implicated in the control of a tissue-specific accumulation of anthocyanin. Mano et al. [109] investigated gene expression analysis using purple-fleshed clones and transformation using sweetpotato leaves and calli. Found that one of the MYB-type transcription factors in sweetpotato, IbMYB1, regulates anthocyanin accumulation in storage roots. The overexpression of this gene successfully induces anthocyanin accumulation in the storage roots of an orange-fleshed sweetpotato cultivar, resulting in higher radical scavenging activity [110]. Comparison of the structure of the IbMYB1 genes between high-anthocyanin content and non-anthocyanin cultivars revealed distinct IbMYB1 copies of IbMYB1-2a and IbMYB1-2b, shared only in the high-anthocyanin cultivars and their common ancestor 'Yamagawamurasaki' [111]. The PCR fragment amplified from IbMYB1-2a and IbMYB1-2b cosegregated with the pigmentation of the storage roots in the F1 progenies of high-anthocyanin cultivars, suggesting the usefulness of the PCR fragment as a selection marker for high-anthocyanin lines [111].

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

Transgenic sweetpotato plants overexpressing IbMYB1 were detected to exhibit an elevated total polyphenol level [110]. The gene expression of phenylalanine ammonia lyase (PAL), cinnamate 4-hydroxylase (C4H), and 4-hydroxycinnamoyl-CoA ligase (4CL), involved in the early steps of both anthocyanin and caffeoylquinic acids (CQA) biosynthesis of the transgenic plants, was upregulated. Tanaka et al. [111] also noted a suppressed expression of these genes in a whitefleshed mutant of 'Ayamurasaki'. Padda and Picha [112] detected variations in the CQA content in the storage roots of non-purple fleshed cultivars, leading to the speculation that, in addition to a co-regulation of the anthocyanin content, a specific regulation mechanism of CQA content exists.
