Modulation of Expression of PVY(NTN) RNA-Dependent RNA Polymerase (NIb) and Heat Shock Cognate Host Protein HSC70 in Susceptible and Hypersensitive Potato Cultivars

Potato virus Y (PVY) belongs to the genus Potyvirus and is considered to be one of the most harmful and important plant pathogens. Its RNA-dependent RNA polymerase (RdRp) is known as nuclear inclusion protein b (NIb). The recent findings show that the genome of PVY replicates in the cytoplasm of the plant cell by binding the virus replication complex to the membranous structures of different organelles. In some potyviruses, NIb has been found to be localized in the nucleus and associated with the endoplasmic reticulum membranes. Moreover, NIb has been shown to interact with other host proteins that are particularly involved in promoting the virus infection cycle, such as the heat shock proteins (HSPs). HSP70 is the most conserved among the five major HSP families that are known to affect the plant–pathogen interactions. Some plant viruses can induce the production of HSP70 during the development of infection. To understand the molecular mechanisms underlying the interactive response to PVY(NTN) (necrotic tuber necrosis strain of PVY), the present study focused on StHSC70-8 and PVY(NTN)-NIb gene expression via localization of HSC70 and NIb proteins during compatible (susceptible) and incompatible (hypersensitive) potato–PVY(NTN) interactions. Our results demonstrate that NIb and HSC70 are involved in the response to PVY(NTN) infections and probably cooperate at some stages of the virus infection cycle. Enhanced deposition of HSC70 proteins during the infection cycle was associated with the dynamic induction of PVY(NTN)-NIb gene expression and NIb localization during susceptible infections. In hypersensitive response (HR), a significant increase in HSC70 expression was observed up to 3 days post-inoculation (dpi) in the nucleus and chloroplasts. Thereafter, between 3 and 21 dpi, the deposition of NIb decreased, which can be attributed to a reduction in the levels of both virus accumulation and PVY(NTN)-NIb gene expression. Therefore, we postulate that increase in the expression of both StHSC70-8 and PVY(NTN)-NIb induces the PVY infection during susceptible infections. In contrast, during HRs, HSC70 cooperates with PVY(NTN) only at the early stages of interaction and mediates the defense response signaling pathway at the later stages of infection.