Reduced RNA silencing suppressor activity of onion yellow dwarf virus HC-Pro with N-terminal deletion may be complemented in mixed infection with another potyvirus in garlic

Abstract

We investigated virus infection rates in Japanese garlic plants and found that a potyvirus, onion yellow dwarf virus (OYDV) was always associated with another potyvirus, leek yellow stripe virus (LYSV). When the OYDV HC-Pro genes from those viral isolates were PCR-amplified and sequenced, we found that all the HC-Pros isolated from Japanese garlic lacked a long stretch of the N-terminal amino acids (~ 100 residues) including the KITC motif, suggesting possible loss of viral aphid transmission. Our phylogenetic analysis revealed that HC-Pros were divided into three major groups and that they did not branch from a single common ancestor, suggesting different mechanisms for generation of the short HC-Pros from the intact ones. Because HC-Pro is a well-known RNA silencing suppressor (RSS), we then analyzed the RSS activity of HC-Pro using the conventional agroinfiltration assay with Nicotiana benthamiana and also with onion, which is a natural host of OYDV. The results demonstrated that the short HC-Pro had reduced RSS activity compared to the long HC-Pro while both type HC-Pros were well expressed in the agroinfiltrated tissues. In addition, the tested OYDV HC-Pros exhibited more efficient RSS activity in onion than in N. benthamiana. We presume that OYDV with reduced RSS activity of HC-Pro may take advantage of mixed infection with LYSV to survive.