An aphid facultative symbiont suppresses plant defence by manipulating aphid gene expression in salivary glands

Aphids often carry facultative symbionts to achieve diverse advantages.Serratia symbiotica, one of facultative endosymbionts, increases aphid tolerance to heat. However, whether it benefits aphid colonization on host plants is yet to be determined. In the current study, we found thatAcyrthosiphon pisumharbouringS. symbioticahad longer feeding duration onMedicago truncatulathanSerratia-free aphids. Contrastingly,Serratia-free aphids triggered higher accumulation of reactive oxygen species (ROS), jasmonic acid and salicylic acid responsive genes and cytosolic Ca(2+)elevations thanSerratia-infected aphids. Transcriptomic analysis of salivary glands indicated that a histidine-rich Ca2+-binding protein-like gene (ApHRC) was expressed more highly in the salivary gland ofSerratia-infected aphids than that ofSerratia-free aphids. OnceApHRCwas silenced,Serratia-infected aphids also displayed shorter phloem-feeding duration and caused Ca(2+)elevation and ROS accumulation in plants. Our results suggest thatApHRC, a potential effector up-regulated byS. symbioticain the salivary glands, impairs plant defence response by suppressing Ca(2+)elevation and ROS accumulation, allowing colonization of aphids. This study has provided an insight into how facultative symbionts facilitate aphid colonization and adaptation to host plants.