Cadmium and Zn hyperaccumulation provide efficient constitutive defense against Turnip yellow mosaic virus infection in Noccaea caerulescens

To understand the role of Zn and Cd in anti-viral defence, Zn/Cd hyperaccumulator Noccaea caerulescens plants grown with deficient (0.3 µM), replete (10 µM) and excess (100 µM) Zn2+ and Cd (10 µM Zn2+ + 1 µM Cd2+) were infected with Turnip yellow mosaic virus (TYMV). Gas exchange and chlorophyll fluorescence kinetics analyses demonstrated direct TYMV effects on photosynthetic light reactions but N. caerulescens was more resistant against TYMV than the previously studied non-hyperaccumulator N. ochroleucum. Virus abundance and photosynthesis inhibition were the lowest in the high Zn and Cd treatments. RNAseq analysis of 10 µM Zn2+ plants revealed TYMV-induced upregulation of Ca transporters, chloroplastic ZTP29 and defence genes, but none of those that are known to be strongly involved in hyperaccumulation. Synchrotron µ-XRF tomography, however, showed that Zn hyperaccumulation remained strongest in vacuoles of epidermal storage cells regardless of infection. This was in contrast to N. ochroleucum, where apoplastic Zn drastically increased in response to TYMV. These results suggest that the antiviral response of N. caerulescens is less induced by the onset of this biotic stress, but it is rather a permanent resistant state of the plant. Real-time qPCR revealed upregulation of ferritin in Zn10 infected plants, suggesting Fe deprivation as a virus defence strategy under suboptimal Zn supply.