Spatial accumulation of salicylic acid is in effector-triggered immunity of potato against viruses regulated by RBOHD

While activation of resistance (R) proteins has been intensively studied, the mechanisms acting downstream of R protein activation remain elusive. We studied effector-triggered immunity (ETI) conditioned by the potato Ny-1 gene against potato virus Y. Transcriptomic, ultrastructural and biochemical analyses of four consecutive tissue sections in and surrounding the foci of viral infection at three different lesion developmental stages revealed processes that are spatiotemporally regulated. The transcriptional response in the cell death zone and surrounding tissue is dependent on SA. For some of the genes, spatiotemporal regulation is completely lost while for others, the regulation is different in SA-deficient line indicating multiple connections between hormonal signaling modules. The induction of RBOHD NADPH oxidase expression, together with expression of Thioredoxin H gene, occurs specifically in the border region of the lesion during ETI. In plants with silenced RBOHD, ETI is perturbed and virus spread is not arrested at the site of infection. Expression of RBOHD is under the control of SA. On the other hand, RBOHD activity is required for spatial regulation of SA accumulation. We identified an UDP-glucosyltransferase, encoding an enzyme involved in feedback activation of SA biosynthesis, that is derepressed at the border of the lesion in RBOHD silenced plants. Altogether, we revealed a novel aspect of viral ETI, the RBOHD-SA feedback loop required for its spatial regulation. SIGNIFICANCE To get towards sustainable resistance in the field, it is important to understand mechanisms of immune signaling. We here show novel aspects of spatial regulation in effector-triggered immunity (ETI) against potato virus Y. A detailed spatiotemporal analysis in and surrounding the foci of viral infection, on ultrastructural, biochemical and gene expression levels revealed that redox state maintenance is tightly spatiotemporally regulated. We also show the interconnection of salicylic acid and RBOHD through a feedback loop, regulated through UDP-glucosyltransferase, revealing the mechanism of this spatial regulation. The proper function of this loop is essential for successful ETI leading to viral arrest.