The biological functions of the multiple members of the xyloglucan endotransglycosylase/hydrolase (XTH) protein family are rather diverse: XTHs are cell wall remodeling enzymes that participate in plant growth and development, are involved in responses to various environmental stresses and interactions with pathogenic and symbiotic microorganisms. However, XTHs’ role upon viral infection remains poorly understood. Here we identified and characterized Nicotiana benthamiana XTH (NbXTH) which is involved in responses to viral infection. We demonstrated that NbXTH is a positive regulator of intercellular transport. NbXTH suppression leads to the inhibition of tobacco mosaic virus (TMV) local spread, resulting in the increased tolerance of N. benthamiana plants to TMV. Therefore, NbXTH could be regarded as a susceptibility factor.

A coordinated and generalized plant response to adverse environmental factors largely depends on the proper and finely-tuned regulation of intercellular transport via plasmodesmata (PD). However, the knowledge of the whole network of PD-controlling mechanisms is far from complete. Earlier, a cellular factor, Kunitz peptidase inhibitor-like protein (KPILP), that affects PD gating and plays a proviral role, was identified in Nicotiana benthamiana plants. Here we characterized its homolog from N. tabacum, NtKPILP, which is hardly detectable in leaves of intact plants, in contrast to roots, flowers and seeds where NtKPILP is highly expressed. However, its mRNA accumulation in leaves increases in response to various stresses, including viral infection. NtKPILP was demonstrated to affect chloroplast functioning. Using the virus-induced gene silencing approach, we have shown that NtKPILP downregulation negatively affects intercellular transport of macromolecules, inducing callose deposition at PD and suppressing beta-1,3-glucanase mRNA accumulation. Together, the obtained results indicate that NtKPILP is a viral infection-responsive cellular factor that is involved in PD permeability regulation, sharing thus the features of KPILPs from other Nicotiana species.