A New Type of Satellite associated with Cassava Mosaic Begomoviruses

Abstract

Cassava mosaic disease (CMD), which is caused by single-stranded DNA begomoviruses, severely limits cassava production across Africa. A previous study showed that CMD symptom severity and viral DNA accumulation increase in cassava in the presence of a DNA sequence designated as SEGS-2 (sequence enhancing geminivirus symptoms). We report here that when SEGS-2 is co-inoculated with African cassava mosaic virus (ACMV) onto Arabidopsis thaliana, viral symptoms increase. Transgenic Arabidopsis with an integrated copy of SEGS-2 inoculated with ACMV also display increased symptom severity and viral DNA levels. Moreover, SEGS-2 enables Cabbage leaf curl virus (CaLCuV) to infect a geminivirus resistant Arabidopsis accession. Although SEGS-2 is related to cassava genomic sequences, an earlier study showed that it occurs as episomes and is packaged into virions in CMD-infected cassava and viruliferous whiteflies. We identified SEGS-2 episomes in SEGS-2 transgenic Arabidopsis. The episomes occur as both double-stranded and single-stranded DNA, with the single-stranded form packaged into virions. In addition, SEGS-2 episomes replicate in tobacco protoplasts in the presence, but not the absence, of ACMV DNA-A. SEGS-2 episomes contain a SEGS-2 derived promoter and an open reading frame with the potential to encode a 75-amino acid protein. An ATG mutation at the beginning of the SEGS-2 coding region does not enhance ACMV infection in Arabidopsis. Together, the results established that SEGS-2 is a new type of begomovirus satellite that enhances viral disease through the action of a SEGS-2 encoded protein that may also be encoded in the cassava genome.

Cassava mosaic disease in South and Southeast Asia: current status and prospects

“…The satellite molecule when co-inoculated with Cabbage leaf curl virus could infect an ecotype of A. thaliana, recalcitrant to geminivirus infection. This confirms its status as a satellite molecule capable of enhancing CMD symptoms (Ndunguru et al, 2016;Aimone et al, 2021b).…”

“…In addition to this, increased variations were also observed in coding regions and predicted to have deleterious effects on protein structure, owing to relaxed selection pressure over half a dozen vegetative passages. The effect of temperature changes or the presence of SEGS did not have a significant impact on the viruses (Aimone et al, 2021b). Cassava mosaic geminivirus is vectored by the whitefly, Bemisia tabaci (Fauquet and Stanley, 2003).…”

Population diversity of cassava mosaic begomoviruses increases over the course of serial vegetative propagation

“…Some begomovirus satellites have been associated with elevated levels of viral DNA and suppression of host DNA methylation and silencing pathways [ 88 ], and as such have the potential to impact viral diversity. Although it is not known if SEGS-1 or SEGS-2 function in a similar manner, they have several features in common with satellites [ 46 ]. We were unable to detect an effect of either SEGS-1 or SEGS-2 on viral nucleotide diversity in coinoculation experiments with CMBs (Table S4-1).…”

“…Subsequently, two DNAs, SEGS-1 and SEGS-2 ( s equences e nhancing g eminivirus s ymptoms), were shown to produce similar symptoms when coinoculated with CMBs into both resistant and susceptible cassava cultivars [ 45 ]. SEGS-2, which occurs as episomes in CMB-infected cassava, virions and whiteflies, is thought to be a novel satellite [ 46 ]. SEGS-1 also forms episomes in CMB-infected cassava but is likely derived from a cassava genomic copy [ 45 ].…”