The potential of using biotechnology to improve cassava: a review

Abstract

The importance of cassava as the fourth largest source of calories in the world requires that contributions of biotechnology to improving this crop, advances and current challenges, be periodically reviewed. Plant biotechnology offers a wide range of opportunities that can help cassava become a better crop for a constantly changing world. We therefore review the state of knowledge on the current use of biotechnology applied to cassava cultivars and its implications for breeding the crop into the future. The history of the development of the first transgenic cassava plant serves as the basis to explore molecular aspects of somatic embryogenesis and friable embryogenic callus production. We analyze complex plant-pathogen interactions to profit from such knowledge to help cassava fight bacterial diseases and look at candidate genes possibly involved in resistance to viruses and whiteflies-the two most important traits of cassava. The review also covers the analyses of main achievements in transgenic-mediated nutritional improvement and mass production of healthy plants by tissue culture and synthetic seeds. Finally, the perspectives of using genome editing and the challenges associated to climate change for further improving the crop are discussed. During the last 30 yr, great advances have been made in cassava using biotechnology, but they need to scale out of the proof of concept to the fields of cassava growers.

Keywords: Artificial TALs; Cassava viruses whiteflies CBB; Climate change; Effector binding element EBE; Ensifer-mediated transformation; Genome editing; Nutritional improvement; Propagation; Somatic embryogenesis; Synthetic seeds; Transgenic cassava.

Figure 1.

(a) Transgenic somatic embryos and (b) plant of cassava cv. 60444 transformed with Ensifer adaherens OV14, expressing GUS. Note the formation of nodules on roots (arrows). This event was one of the three obtained for which a Southern blot (c), confirmed the presence of single copy insertions (first and third lanes) as well as multicopies of the T-DNA (second lane; fourth lane is control transgenic plant).

Figure 2.

Flowchart for the production of cassava plants from naked or encapsulated somatic embryos (synthetic seeds). The initial source of explants must be certified, disease-free, in vitro plants (upper left corner), from which axillary buds are dissected to induce primary somatic embryos (primary SE) or friable embryogenic callus (FEC). SEs can then be used with an artificial coat (encapsulated SE; scale in cm) or without it (naked SE) for producing plants in vitro. Encapsulated SEs are equivalent to synthetic seeds which, as described in the text, may be used for short-term storage of cassava germplasm. It is unknown if synthetic seeds tolerate below-freezing temperatures, which would be ideal for the long-term storage of germplasm.

Figure 3.

Flowchart for decision making on propagation methods for cassava planting material production. The success of diagram procedures connected by red lines depends solely on the initial material certification as disease free. The lack of such certification results in lack of confidence in the system and may result in the distribution of unhealthy planting material in farmers’ fields. One possibility to ensure clean starting material is that gene banks provide certified plants in vitro. Any cassava seed system (blue arrows) must integrate in-vitro platforms with macro-propagation schemes to offer high-quality abundant planting material continuously to the end-users.