A Cuticular Protein in Aphid Saliva Suppresses Plant Defence by Inhibiting Cellulose Degradation

Aphids employ numbers of salivary proteins to subvert plant defences during phloem feeding. These salivary proteins can be secreted into plants, and localised to the mesophyll cells and the phloem, but few studies identify the effectors suppressing the apoplastic defence during feeding. This study found that a cuticular protein MpCP2 released from the green peach aphid Myzus persicae could stabilise cell wall cellulose by preventing its degradation. Salivary MpCP2 was secreted into the apoplast and bound with cellulose, which inhibited the enzymatic cleavage of cellulose into cellobiose. The reduced levels of cellobiose, a damage-associated molecular pattern (DAMP), alleviated downstream reactions of plants, including Ca²⁺ sparks, reactive oxygen species production, and transcriptional levels of defensive genes WRKY30, MYC2 and PAL. Knockdown of MpCP2 in aphids reduced the phloem ingestion time, and compromised the survival rate and fecundity of aphids. In contrast, overexpression of MpCP2 in plants prolonged aphid phloem feeding time and enhanced survival rate. These results reveal a novel mechanism that some effectors of piercing-sucking insects are utilised to reduce the releases of cell wall-derived DAMPs by binding with cellulose, thereby suppressing DAMP-triggered plant defences.