Reasons for decreased aphid vector transmission of beet chlorosis virus from sugar beets co-infected with beet yellows virus

Co-infection of plants with two or more viruses can change infection parameters drastically, including transmission by insect vectors. Here we investigated the transmission by the green peach aphid (Myzus persicae) of beet yellows virus (BYV, Closteroviridae) and beet chlorosis virus (BChV, Solemoviridae) from co-infected sugar beet plants, a scenario that is frequently encountered in the field. Co-infection decreased transmission of BChV by 50 %, but had no impact on BYV transmission. RNAseq data indicated only a small decrease in accumulation of BChV in co-infected plants, which cannot account for lower BChV transmission. Localisation experiments in mono-infected plants by fluorescent in situ hybridization (FISH) showed that BChV and BYV were only detected in the phloem, as expected for phloem-restricted viruses. Co-infection did not result in phloem escape of either virus. In co-infected plants, BYV and BChV colocalised in ~40 % of infected phloem cells, the remainder of infected cells were mono-infected with BChV or BYV. In mono-infected plants, intracellular BChV label was diffuse in the cytoplasm, while BYV accumulated in spherical cytoplasmic inclusions. The BChV label was unchanged in cells from co-infected plants. A different picture was found for BYV distribution in co-infected plants. Only mono-infected cells showed the typical cytoplasmic BYV inclusions, whereas BYV in BYV/BChV co-infected cells was uniformly distributed in the cytoplasm. The meaning of this specific re-localisation of BYV in co-infected cells remains at present unclear. At the tissue level, BChV and BYV were detected in the phloem of the leaf midribs and the phloem of lower-order vessels in mono-infected plants. In contrast, BChV was mainly restricted to the leaf midrib phloem in co-infected plants and BYV accumulated preferentially in the phloem of secondary and lower-order vessels. Observation of aphids indicated that they preferred to settle on lower-order vessels rather than on the midribs of sugar beet leaves. Thus, the differential tissue localisation of BChV in mono-infected and co-infected plants might explain the lower transmission rates of BChV from co-infected plants. We are currently doing electrical penetration graph experiments and transcriptomics to better understand how co-infection lowers BChV transmission, causes exclusion of BChV from lower order phloem vessels, and results in intracellular re-localisation of BYV.