Defence-mediated phloem restriction of a plant virus facilitates insect transmission.

Plant viruses are causal agents of devastating diseases in crops and pose a threat to food security. Viruses transmitted by phloem-feeding insects are frequently restricted to the phloem; the mechanism determining this tissue tropism and its consequences for the viral cycle have remained elusive. Here we show that phloem restriction of tobacco curly shoot virus (TbCSV) depends on the allele of the C4 gene it carries. The Y35 allele produces a plasma membrane-associated C4 protein; TbCSV(Y35) is phloem-limited. The Y41 allele, however, produces two C4 variants, targeted to the plasma membrane and chloroplasts, respectively. Chloroplast-localized C4 interferes with salicylic acid-mediated defenses, which causes a destabilization of PENETRATION3 (PEN3), a subsequent decrease in callose deposition, and the escape of TbCSV(Y41) to the surrounding parenchyma cells. Interestingly, Y41 expands the host range of TbCSV, but Y35 is prevalent in nature. We determine that phloem restriction favors acquisition and transmission by the insect vector, conferring a competitive advantage to TbCSV(Y35). Importantly, PEN3 also determines phloem restriction of an RNA virus. In summary, we demonstrate that PEN3 activity and likely callose deposition confine viruses to the phloem, which favors viral spread by facilitating acquisition by the insect vector.