Studies on differential behavior of cassava mosaic geminivirus DNA components, symptom recovery patterns, and their siRNA profiles

Basavaprabhu L. Patil, Claude M. Fauquet

Cassava mosaic disease caused by cassava mosaic geminiviruses (CMGs) with bipartite genome organization is a major constraint for production of cassava in the African continent and the Indian sub-continent. Currently, there are eleven recognized species of CMGs, and several diverse isolates represent them, with vast amount of sequence variability, reflecting into diversity of symptom severity/phenotypes. Here, we make a systematic effort to study the infection dynamics of several species of CMGs and their isolates. Further, we try to identify the genomic component of CMGs contributing to the manifestation of diverse patterns of symptoms and the molecular basis for the differential behavior of CMGs. The pseudo-recombination studies carried out by swapping of DNA-A and DNA-B components of the CMGs revealed that the DNA-B component significantly contributes to the symptom severity. Past studies had shown that the DNA-A component of Sri Lankan cassava mosaic virus shows monopartite feature. Thus, the ability of DNA-A component alone, to replicate and move systemically in the host plant with inherent monopartite features was investigated for all the CMGs. Geminiviruses are known to trigger gene silencing and are also its target, resulting in recovery of the host plant from viral infection. In the collection of several different CMG species and isolates we had, there was a vast variability in their recovery and non-recovery phenotypes. To understand the molecular basis of this, the origin and distribution of virus-derived small interfering RNAs were mapped across their genome and across the CMG-infected symptomatic Nicotiana benthamiana.