The Search for Resistance to Cassava Mosaic Geminiviruses: How Much We Have Accomplished, and What Lies Ahead

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.

Strategies for Enhanced Crop Resistance to Insect Pests

“…Multiple CRISPR protocols are available, including those with the capacity to generate site-specific indels (often yielding frameshift mutations), to replace or insert specific sequences, and (by using a deactivated Cas9) to suppress gene expression. In relation to insect pests, the first applications of CRISPR in crops confer resistance to insect-vectored viruses, especially the geminiviruses, which have DNA genomes (3, 53). CRISPR is also the technology of choice to produce new crop varieties in response to insect pest genotypes that break plant resistance mechanisms.…”

Cassava breeding and agronomy in Asia: 50 years of history and future directions

“…In the past two decades, with the advent of advanced molecular breeding and phenomics, significant progress has BS Breeding Science Preview Malik, Kongsil, Nguyễn, Ou, Sholihin, Srean, Sheela, Becerra López-Lavalle, Utsumi, Lu et al been made in cassava crop improvement and there have been several examples, such as developing disease resistance (Fondong 2017), reducing cyanide toxicity (Narayanan et al 2011), enhancing nutritional value (Bouis et al 2011), and improving starch yield and quality (Karlström et al 2016). These technologies have also increased fundamental knowledge related to breeding, planting, nutrition, and the processing of cassava (Becerra López-Lavalle 2017, Chapuis et al 2017, Ferguson et al 2012, Yuniwati et al 2015.…”

Recent Biotechnological Advances in the Improvement of Cassava

“…Thus, resistance breeding began in Ghana, Madagascar, Tanzania and elsewhere in Africa [40,41]. In the last 2 decades, use of genetic engineering to produce virus resistant cassava has gained considerable attention, especially with the discovery of RNA interference pathways [42].…”