Separate pathways contribute to the herbivore-induced formation of 2-phenylethanol in poplar

Upon herbivory, the tree species western balsam poplar (Populus trichocarpa) produces a variety of phenylalanine-derived metabolites, including 2-phenylethylamine, 2-phenylethanol, and 2-phenylethyl-β-D-glucopyranoside. To investigate the formation of these potential defense compounds, we functionally characterized aromatic L-amino acid decarboxylases (AADCs) and aromatic aldehyde synthases (AASs), which play important roles in the biosynthesis of specialized aromatic metabolites in other plants. Heterologous expression in Escherichia coli and Nicotiana benthamiana showed that all five AADC/AAS genes identified in the P. trichocarpa genome encode active enzymes. However, only two genes, PtAADC1 and PtAAS1, were significantly upregulated after leaf herbivory. Despite a sequence similarity of about 96%, PtAADC1 and PtAAS1 showed different enzymatic functions and converted phenylalanine into 2-phenylethylamine and 2-phenylacetaldehyde, respectively. The activities of both enzymes were interconvertible by switching a single amino acid residue in their active sites. A survey of putative AADC/AAS gene pairs in the genomes of other plants suggests an independent evolution of this function-determining residue in different plant families. RNAi-mediated downregulation of AADC1 in grey poplar (P. × canescens) resulted in decreased accumulation of 2-phenylethylamine and 2-phenylethyl-β-D-glucopyranoside, while the emission of 2-phenylethanol was not influenced. To investigate the last step of 2-phenylethanol formation, we identified and characterized two P. trichocarpa short-chain dehydrogenases, PtPAR1 and PtPAR2, which were able to reduce 2-phenylacetaldehyde to 2-phenylethanol in vitro. In summary, 2-phenylethanol and its glucoside may be formed in multiple ways in poplar. Our data indicate that PtAADC1 controls the herbivore-induced formation of 2-phenylethylamine and 2-phenylethyl-β-D-glucopyranoside in planta, while PtAAS1 likely contributes to the herbivore-induced emission of 2-phenylethanol