Two viral proteins translated from one open reading frame target different layers of plant defence

Multi-layered defence responses are activated upon pathogen attack. Viruses utilize a number of strategies to maximize the coding capacity of their small-sized genomes and produce viral proteins for infection, including suppression of host defence. Here, we uncover translation leakage as one of these strategies: two viral effectors, cC4 and mC4, encoded by tomato golden mosaic virus (TGMV) are translated from two in-frame start codons and function cooperatively to suppress defence. cC4 localizes in chloroplasts, to which it recruits NbPUB4 to induce ubiquitination of the outer membrane; as a result, this organelle gets degraded, and chloroplast-mediated defences are abrogated. However, chloroplast-localized cC4 induces the production of singlet oxygen (1O2), which in turn promotes the translocation of the 1O2 sensor NbMBS1 from the cytosol to the nucleus, where it activates the expression of the CERK1 gene. Importantly, an anti-viral effect exerted by CERK1 is countered by mC4, localized at the plasma membrane. mC4, similarly to cC4, recruits NbPUB4 and promotes the ubiquitination and subsequent degradation of CERK1, suppressing membrane-based receptor-like kinase-dependent defences. Importantly, this translation leakage strategy seems to be conserved in multiple viral species and is related with host range. This finding suggests that stacking of different cellular anti-viral responses could be an effective way to abrogate viral infection and engineer sustainable resistance to major crop viral diseases in the field.