Cell death triggered by the P25 protein in Potato virus X-associated synergisms results from ER stress in Nicotiana benthamiana

51 p.-9 fig.-4 fig. supl.The synergistic interaction of Potato virus X (PVX) with a number of potyviruses results in systemic necrosis in Nicotiana spp. Previous investigations have indicated that the viral suppressor of RNA silencing (VSR) protein P25 of PVX triggers systemic necrosis in PVX-associated synergisms in a threshold-dependent manner. However, little is still known about the cellular processes that lead to this necrosis, and whether the VSR activity of P25 is involved in its elicitation. Here, we show that transient expression of P25 in the presence of VSRs from different viruses, including the HC-Pro of potyviruses, induces endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), which ultimately lead to ER collapse. However, the host RNA silencing pathway was dispensable for elicitation of cell death by P25. Confocal microscopy studies in leaf patches co-expressing P25 and HC-Pro showed dramatic alterations in ER membrane structures, which correlated with the up-regulation of bZIP60 and several ER-resident chaperones including the ER luminal binding protein (BiP). Overexpression of BiP alleviated the cell death induced by the potexviral P25 protein when expressed together with VSRs derived from different viruses. Conversely, silencing of UPR master regulator, bZIP60, led to an increase in cell death elicited by the P25/HC-Pro combination as well as by PVX-associated synergism. Besides its role as a negative regulator of P25-induced cell death, UPR partially restricted PVX infection. Thus, systemic necrosis caused by PVX-associated synergistic infections is likely the effect of an unmitigated ER stress following overaccumulation of a viral protein, P25, with ER-remodeling activity.This work was supported by the Ministry of Economy and Competitiveness of Spain (grant BIO2016-75619-R [AEI/FEDER, UE]) to T.C. and F.T., and the National Science and Engineering Research Council of Canada to P. M. E.A. was recipient of a FPU fellowship from the Spanish Ministry of Education, Culture and Sport (FPU12/01908).Peer reviewe