The mirid bug Apolygus lucorum deploys a glutathione peroxidase as a candidate effector to enhance plant susceptibility

The mired bug Apolygus lucorum has become a major agricultural pest since the large-scale cultivation of Bt-cotton. It was assumed that A. lucorum, similar to other phloem sap insects, could secrete saliva that contains effector proteins into plant interfaces to perturb host cellular processes during feeding. However, the secreted effectors of A. lucorum are still uncharacterized and unstudied. In this study, 1,878 putative secreted proteins were identified from the transcriptome of A. lucorum, which either had homology with published aphid effectors or shared common features with plant pathogens and insect effectors. One hundred seventy-two candidate effectors were used to conduct cell death inducing/suppressing assays, and a putative salivary gland effector, Apolyguslucorum cell death inhibitor 6 (Al6), was characterized. The mRNAs of Al6 were enriched at feeding stages (nymph and adult), and especially, in salivary glands. Further studies revealed that the secreted Al6 encoded an active glutathione peroxidase that reduced ROS accumulation induced by INF1 or Flg22. Expression of the Al6 gene in planta altered insect feeding behaviour and promoted plant pathogen infections. Inhibition of cell death and enhanced plant susceptibility to insect and pathogens are dependent on glutathione peroxidase activity of Al6. Thus, this study concludes that a candidate salivary gland effector, Al6 functions as a GPx and suppresses ROS induced by the PAMP to inhibit PTI-induced cell death. The identification and molecular mechanism analysis of the Al6 candidate effector in A. lucorum will provide new insight into the molecular mechanisms of insect-plant interactions.,© The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Experimental Biology.