Abstract
Cassava is the main staple food for over 800 million people globally. Its production in eastern Africa is being constrained by two devastating Ipomoviruses that cause cassava brown streak disease (CBSD); Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), with up to 100% yield loss for smallholder farmers in the region. To date, vector studies have not resulted in reproducible and highly efficient transmission of CBSV and UCBSV. Most virus transmission studies have used Bemisia tabaci (whitefly), but a maximum of 41% U/CBSV transmission efficiency has been documented for this vector. With the advent of next generation sequencing, researchers are generating whole genome sequences for both CBSV and UCBSV from throughout eastern Africa. Our initial goal for this study was to characterize U/CBSV whole genomes from CBSD symptomatic cassava plants sampled in Kenya. We have generated 8 new whole genomes (3 CBSV and 5 UCBSV) from Kenya, and in the process of analyzing these genomes together with 26 previously published sequences, we uncovered the aphid transmission associated DAG motif within coat protein genes of all CBSV whole genomes at amino acid positions 52–54, but not in UCBSV. Upon further investigation, the DAG motif was also found at the same positions in two other Ipomoviruses: Squash vein yellowing virus (SqVYV), Coccinia mottle virus (CocMoV). Until this study, the highly-conserved DAG motif, which is associated with aphid transmission was only noticed once, in SqVYV but discounted as being of minimal importance. This study represents the first comprehensive look at Ipomovirus genomes to determine the extent of DAG motif presence and significance for vector relations. The presence of this motif suggests that aphids could potentially be a vector of CBSV, SqVYV and CocMov. Further transmission and ipomoviral protein evolutionary studies are needed to confirm this hypothesis.
Highlights
Vector transmission associated motifs in PotyviridaeMost plant-infecting viruses are transmitted to their host plants by vectors
Eight samples were subjected to sequencing on an Illumina Hiseq2000, which resulted in raw reads ranging from 13,839,232 to 22,613,080 for each sample
Cassava brown streak virus has a DAG motif in the coat protein of all published sequences, including the 3 new whole genomes added in this study, while Ugandan cassava brown streak virus (UCBSV) does not
Summary
Most plant-infecting viruses are transmitted to their host plants by vectors This interaction between vector and virus varies in duration and specificity, but some common themes have emerged. Plant viruses encode proteins on the surface of the virion that are essential for transmission and in some cases additional non-structural helper proteins act to bridge the virion to the vector binding site. The most plausible explanation for HC-Pro requirement in potyvirus vector transmission is that it acts as a “bridge” between aphid stylet and virus particle [5]. It has been hypothesized that to bridge virions to aphid alimentary tract, the HC pro acts as a dimer, KITC motif interacts with the aphid stylet and there is PTK to DAG interaction [6, 7]
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