Molecular characterizations and functional analyses of the novel salivary effector SaApo AI from the cereal aphid Sitobion avenae

Salivary proteins are 'scouts' in plant-herbivore interactions, influencing plant immune responses and insect fitness. Apolipoproteins, a kind of lipid metabolism-associated proteins, have been repeatedly found in insect salivary glands, but their exact functions are still elusive.

The highly expressed apolipoprotein SaApo AI in salivary glands of the English grain aphid, Sitobion avenae (Fabricius), was identified and characterized. Phylogenetic analyses showed that Apo AIs from Hemiptera were highly conserved. SaApo AI was found to be a novel aphid-specific salivary effector. Expression of SaApo AI was lower in S. avenae individuals fed on artificial diets than on wheat seedlings. SaApo AI was found to inhibit plant hypersensitive responses, and the function of its signal peptide was verified. Transient expression of SaApo AI revealed that SaApo AI was localized to the cell membrane and nucleus. Based on SaApo AI-silencing assays, SaApo AI might be able to improve feeding, survival and reproduction of S. avenae. The secretion of SaApo AI could inhibit the accumulation of hydrogen peroxide to promote feeding of S. avenae on wheat. Salicylic acid (SA) signaling in wheat fed by SaAPO AI-silenced aphids was suppressed, suggesting the capacity of SaApo AI to modulate the SA-jasmonic acid cross-talk, and corresponding wheat defenses.

SaApo AI is a novel salivary effector that could modulate wheat immune responses, and enhance the fitness of S. avenae on wheat. Our results provide insights into understanding of mechanisms for salivary apolipoprotein-mediated plant-aphid interactions, and a potential molecular target for wheat aphid control through RNAi technologies. © 2026 Society of Chemical Industry.