Insect-derived extra-oral GH32 plays a role in susceptibility of wheat to Hessian fly

The Hessian fly, Mayetiola destructor (Say), is an obligate parasite of wheat (Triticum aestivum) causing significant economic damage.The interaction of the insect with wheat operates on a gene-for-gene basis and triggers either a resistant or susceptible reaction in the plant. However, the molecular mechanisms of susceptibility leading to the establishment of the larvae are unknown. Larval survival on the plant requires the establishment of a steady source of readily available nutrition. Unlike other insect pests, the Hessian fly larvae have minute mandibles and cannot derive their nutrition by chewing tissue or sucking phloem sap. The evolutionary process that allows Hessian fly to hijack the host plant machinery during virulence is still a mystery. Our recent study reveals that the glycoside hydrolase MdesGH32 in the larvae is acquired via horizontal gene transfer. The virulent larvae secrete MdesGH32 extra-orally, that localizes within the host leaf tissue being fed upon. We demonstrate that MdesGH32 has strong inulinase and invertase activities aiding in the breakdown of the plant cell wall inulin polymer into monomers. Sucrose, the primary transport sugar in plants, is converted to glucose and fructose, resulting in the formation of a nutrient-rich tissue by the enzyme activity. Our finding elucidates the molecular mechanism of nutrient sink formation and establishment of susceptibility.