Plant Volatile Methyl Salicylate Primes Wheat Defense Against the Grain Aphid by Altering the Synthesis of Defense Metabolites

Wheat (Triticum aestivum L.) is one of the most important staple crops all over the world. Its productivity is adversely affected by aphid infestation. Plant volatiles play a critical role in plant communication, inducing direct and indirect defenses against insect pests. However, little is known about the priming mechanism of key volatiles in wheat. To determine whether and how plant volatile induced defense priming in wheat against the grain aphid Sitobion avenae, a combination of insect bioassays, phytohormone and defense metabolite quantification, and transcriptome analyses were performed using an important aphid damage-induced plant volatile, methyl salicylate (MeSA). MeSA treatment primed wheat for enhanced accumulation of salicylic acid, flavonoid and benzoxazinoids (BXs), and increased resistance to S. avenae and attractiveness to an aphid parasitoid Aphelinus asychis. Supplementation with a BX (2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one) and two flavonoids (xanthohumol and isobavachalcone) in artificial diet impaired the survival, development and fecundity of S. avenae. Moreover, MeSA treatment induced wheat volatile emission especially MeSA. Functional investigation of odorant-binding proteins (OBPs) in A. asychis revealed that AasyOBP4 is responsible for the recognition of MeSA. Taken together, our results provide insights into the molecular mechanism of MeSA-mediated defense in wheat and propose MeSA as a phytoprotectant for crop protection and sustainable agriculture.