MYC2b-regulated tyramine-derived phenolamides in maize defense against Spodoptera frugiperda

Abstract

The fall armyworm Spodoptera frugiperda is a devastating pest of maize, capable of detoxifying the well-studied benzoxazinoid (BX) defenses. Whether maize deploys additional inducible defenses that complement BXs remains unclear. We performed targeted alkaloid metabolomics, absolute quantification, artificial-diet bioassays, transcript profiling, hormone quantification, and genetic analysis using a myc2a myc2b double mutant to investigate phenolamide-mediated defense against FAW in maize. Upon S. frugiperda herbivory, maize leaves accumulated tyramine-derived phenolamides to levels that far exceeded those of BXs at 48 h after infestation. The two most abundant phenolamides, N-p- coumaroyltyramine and N-feruloyltyramine, reached over 50 mg/g fresh weight. N-feruloyltyramine significantly suppressed larval growth and survival, whereas N-p-coumaroyltyramine showed no significant effect; the methoxy group of the feruloyl side chain enhanced anti-insect activity. Phenolamide synthesis genes maintained high expression at 48 h, while BX biosynthetic genes were significantly downregulated at 24 h, accounting for the late-stage quantitative advantage of phenolamides. Herbivory activated jasmonate signaling. with MYC2b showing much stronger induction than MYC2a. The myc2a myc2b double mutant showed that both paralogs are required for phenolamide accumulation and tyramine hydroxycinnamoyl transferase ( THT ) expression. Given its stronger herbivory-responsive induction, MYC2b was selected for mechanistic verification. Our findings establish that MYC2a/MYC2b-dependent tyramine-derived phenolamides, with MYC2b directly activating THT3 transcription, constitute an important anti-herbivore specialized chemical defense.