Volatile compound-mediated plant-plant interactions under stress with the tea plant as a model

Plants respond to environmental stimuli via the release of volatile organic compounds(VOCs),and neighboring plants constantly monitor and respond to these VOCs with great sensitivity and discrimination.This sensing can trigger increased plant fitness and reduce future plant damage through the priming of their own defenses.The defense mechanism in neighboring plants can either be induced by activation of the regulatory or transcriptional machinery,or it can be delayed by the absorption and storage of VOCs for the generation of an appropriate response later.Despite much research,many key questions remain on the role of VOCs in interplant communication and plant fitness.Here we review recent research on the VOCs induced by biotic(i.e.insects and pathogens)and abiotic(i.e.cold,drought,and salt)stresses,and elucidate the biosynthesis of stress-induced VOCs in tea plants.Our focus is on the role of stress-induced VOCs in complex ecological environments.Particularly,the roles of VOCs under abiotic stress are highlighted.Finally,we discuss pertinent questions and future research directions for advancing our understanding of plant interactions via VOCs.

Subfunctionalization of a monolignol to a phytoalexin glucosyltransferase is accompanied by substrate inhibition

Uridine diphosphate-dependent glycosyltransferases(UGTs)mediate the glycosylation of plant metabolites,thereby altering their physicochemical properties and bioactivities.Plants possess numerous UGT genes,with the encoded enzymes often glycosylating multiple substrates and some exhibiting substrate inhibition kinetics,but the biological function and molecular basis of these phenomena are not fully understood.The promiscuous monolignol/phytoalexin glycosyltransferase NbUGT72AY1 exhibits substrate inhibition(Ki)at 4 mM scopoletin,whereas the highly homologous monolignol StUGT72AY2 is inhibited at 190 mM.We therefore used hydrogen/deuterium exchange mass spectrometry and structure-based mutational analyses of both proteins and introduced NbUGT72AY1 residues into StUGT72AY2 and vice versa to study promiscuity and substrate inhibition of UGTs.A single F87I and chimeric mutant of NbUGT72AY1 showed significantly reducedscopoletin substrate inhibition,whereas its monolignolgly cosylation activity was almost unaffected.Reverse mutations in StUGT72AY2 resulted in increased scopoletin glycosylation,leading to enhanced promiscuity,which was accompanied by substrate inhibition.Studies of 3D structures identified open and closed UGT conformers,allowing visualization of the dynamics of conformational changes that occur during catalysis.Previously postulated substrate access tunnels likely serve as drainage channels.The results suggest a two-site model in which the second substrate molecule binds near the catalytic site and blocks product release.Mutational studies showed that minor changes in amino acid sequence can enhance the promiscuity of the enzyme and add new capabilities such as substrate inhibition without affecting existing functions.The proposed subfunctionalization mechanism of expanded promiscuity may play a role in enzyme evolution and highlights the importance of promiscuous enzymes in providing new functions.