HCPro affects heterologous virus infection through salicylic acid and auxin pathways

Mixed infection by plant viruses is common in nature, but how a key viral protein of one virus affects the infection by heterologous viruses is not yet fully understood. The helper component proteinase (HCPro) is a widely studied RNA silencing suppressor encoded by viruses of the family Potyviridae. Here, we investigated the defence response of Nicotiana tabacum plants overexpressing HCPro of chilli veinal mottle virus (HCPro-OX) to tobacco mosaic virus and cucumber mosaic virus infection. We monitored the physiological and molecular changes of HCPro-OX plants in response to virus infection. The results showed that HCPro-OX plants under virus infection exhibited higher susceptibility at the early stage but stronger tolerance at the later stage compared to wild-type plants. The tolerance to heterologous virus infection of HCPro-OX plants corresponded to a lower level of reactive oxygen species accumulation and higher activities of several antioxidant enzymes. Reverse transcription-quantitative PCR assays showed that the expression of genes related to salicylic acid (SA) pathways was significantly upregulated, but the expression of genes related to the auxin pathways was downregulated at the late stage of virus infection in HCPro-OX plants compared to wild-type plants. By contrast, the situation in the early stage of virus infection was reversed. In addition, pretreatment with SA, the auxin naphthylacetic acid (NAA) and their respective inhibitors 1-aminobenzotriazole (ABT) and naphthalam (NPA) further confirmed the antagonistic effects of SA and NAA in the response of HCPro-OX plants to heterologous virus infection. Thus, our results demonstrate that HCPro affects heterologous virus infection through SA and auxin pathways.