Abstract
The cassava mosaic disease (CMD), which occurs in all cassava growing regions of Africa and the Indian subcontinent, is caused by cassava mosaic geminiviruses (CMGs). CMGs are considered to be the most damaging vector-borne plant pathogens. So far, the most successful approach used to control these viruses has been the transfer of a polygenic recessive resistance locus, designated CMD1, from wild cassava to cassava cultivars. Further progress in harnessing natural resistance to contain CMGs has come from the discovery of the dominant monogenic resistance locus, CMD2, in some West African cassava cultivars. CMD2 has been combined with CMD1 through genetic crosses. Because of the limitations of the cassava breeding approach, especially with regard to time required to produce a variety and the loss of preferred agronomic attributes, efforts have been directed toward the deployment of genetic engineering approaches. Most of these approaches have been centered on RNA silencing strategies, developed mainly in the model plant Nicotiana benthamiana. Early RNA silencing platforms assessed for CMG resistance have been use of viral genes for co-suppression, antisense suppression or for hairpin RNAs-mediated gene silencing. Here, progress and challenges in the deployment of these approaches in the control of CMGs are discussed. Novel functional genomics approaches with potential to overcome some of the drawbacks of the current strategies are also discussed.
Highlights
Cassava, Manihot esculenta Crantz, was introduced in Africa by the Portuguese in the 16th century, and was initially grown in and around trading posts in the Gulf of Guinea in West Africa
Further studies showed that a short segment from AC1 that codes for 57 N-terminal amino acids of the African cassava mosaic virus (ACMV) Rep protein was sufficient to inhibit ACMV DNA replication, similar to observations made in plants containing the full-length AC1 (Hong and Stanley, 1995, 1996)
These results are supported by a recent study (Patil et al, 2016) showing that transient expression in N. benthamiana of post transcriptional gene silencing (PTGS) constructs targeting ACMV-CM AC1, AC2, and AC4 generated ACMV resistance, characterized by production of high levels of siRNA especially along AC2
Summary
Manihot esculenta Crantz, was introduced in Africa by the Portuguese in the 16th century, and was initially grown in and around trading posts in the Gulf of Guinea in West Africa. There are three objectives to decreasing losses caused by virus diseases: (1) decrease the proportion of plants that become infected; (2) delay infection to such a late stage of crop growth that losses become unimportant; (3) decrease the severity of damage sustained after infection has occurred (Thresh and Cooter, 2005). These objectives can be achieved through phytosanitation (involving quarantine measures, crop hygiene, use of virus-free planting material and eradication), changes in cropping practices (e.g., intercropping), use of pesticides to control vectors, and deployment of resistant or tolerant varieties (Thresh, 2003). The successes and failures of these CMG control approaches, as well as additional new opportunities offered by recent advances in genomics toward the containment of these viruses, are discussed in this review
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