4.2 Plasma membrane ROS and viruses
The silencing of NbTRXh2 , a PM-localized protein, has been associated with increased accumulation of Bamboo mosaic virus (BaMV) in the inoculated leaves, suggesting that NbTRXh2 acts as a negative regulator of BaMV movement. Further, NbTRXh2 interacts with BaMV’s triple gene block protein 2 (TGBp2), although the precise implications for regulating cell-to-cell movement have not been fully explored (Chen et al., 2018). Interestingly, an alteration in the homeostasis of HMG1/2a nucleo-protein in N. benthamiana led to reduced levels ofNbTRXh2 and enhanced systemic infection of BaMV (Alazem et al., 2020). This effect, however, was absent in the inoculated leaves. Supporting this notion is the role of RBOHD in potato defense against PVY. RBOHD is responsible for ROS production at PM and in the apoplast. In the potato-PVY pathosystem, when Ny-1 potato plants (Ny-1 is a PVYN-strain specific R -gene) with silenced RBOHD (shRBOHD) were infected with the PVYN strain, the PVYN accumulated less in the shRBOHD -transgenic plants compared to NahG-plants (Lukan et al., 2020). Despite shRBOHD plants accumulating high levels of SA, they allowed for more extensive viral trafficking to systemic leaves compared to NahG plants. This led to the conclusion that RBOHD-generated ROS are not involved in the induction of SA biosynthesis in the potato-PVY pathosystem. Instead, it was observed that theUGT76D1 gene, which plays a role in SA biosynthesis, was upregulated in shRBOHD plants, providing an explanation for the elevated SA content in these plants. The notable presence of high SA levels in the shRBOHD plants, coupled with the increased viral trafficking to systemic leaves, raises the question of whether the ROS-mediated effect on cell-to-cell movement relies on the SA pathway, at least within the context of the potato-PVY pathosystem. It should also be noted that SA is reported to induce callose accumulation at PD and reduce intercellular trafficking in a PDLP5-dependet manner (Lee et al., 2011), raising questions about the sites of action of SA with relation to regulating PD.