Processing of Airborne Green Leaf Volatiles for Their Glycosylation in the Exposed Plants

Structural and biochemical basis for regiospecificity of the flavonoid glycosyltransferase UGT95A1

Glycosylation is a predominant strategy plants use to fine-tune the properties of small molecule metabolites to affect their bioactivity, transport, and storage. It is also important in biotechnology and medicine as many glycosides are utilized in human health. Small molecule glycosylation is largely carried out by family 1 glycosyltransferases. Here, we report a structural and biochemical investigation of UGT95A1, a family 1 GT enzyme from Pilosella officinarum that exhibits a strong, unusual regiospecificity for the 3'-O position of flavonoid acceptor substrate luteolin. We obtained an apo crystal structure to help drive the analyses of a series of binding site mutants, revealing that while most residues are tolerant to mutations, key residues M145 and D464 are important for overall glycosylation activity. Interestingly, E347 is crucial for maintaining the strong preference for 3'-O glycosylation, while R462 can be mutated to increase regioselectivity. The structural determinants of regioselectivity were further confirmed in homologous enzymes. Our study also suggests that the enzyme contains large, highly dynamic, disordered regions. We showed that while most disordered regions of the protein have little to no implication in catalysis, the disordered regions conserved among investigated homologs are important to both the overall efficiency and regiospecificity of the enzyme. This report represents a comprehensive in-depth analysis of a family 1 GT enzyme with a unique substrate regiospecificity and may provide a basis for enzyme functional prediction and engineering.

Green leaf volatiles co-opt proteins involved in molecular pattern signalling in plant cells

The green leaf volatiles (GLVs) Z-3-hexen-1-ol (Z3-HOL) and Z-3-hexenyl acetate (Z3-HAC) are airborne infochemicals released from damaged plant tissues that induce defenses and developmental responses in receiver plants, but little is known about their mechanism of action. We found that Z3-HOL and Z3-HAC induce similar but distinctive physiological and signaling responses in tomato seedlings and cell cultures. In seedlings, Z3-HAC showed a stronger root growth inhibition effect than Z3-HOL. In cell cultures, the two GLVs induced distinct changes in MAP kinase (MAPK) activity and proton fluxes as well as rapid and massive changes in the phosphorylation status of proteins within 5 min. Many of these phosphoproteins are involved in reprogramming the proteome from cellular homoeostasis to stress and include pattern recognition receptors, a receptor-like cytoplasmic kinase, MAPK cascade components, calcium signaling proteins and transcriptional regulators. These are well-known components of damage-associated molecular pattern (DAMP) signaling pathways. These rapid changes in the phosphoproteome may underly the activation of defense and developmental responses to GLVs. Our data provide further evidence that GLVs function like DAMPs and indicate that GLVs coopt DAMP signaling pathways.

Acceptors and Effectors Alter Substrate Inhibition Kinetics of a Plant Glucosyltransferase NbUGT72AY1 and Its Mutants

One of the main obstacles in biocatalysis is the substrate inhibition (SI) of enzymes that play important roles in biosynthesis and metabolic regulation in organisms. The promiscuous glycosyltransferase UGT72AY1 from Nicotiana benthamiana is strongly substrate-inhibited by hydroxycoumarins (inhibitory constant Ki < 20 µM), but only weakly inhibited when monolignols are glucosylated (Ki > 1000 µM). Apocarotenoid effectors reduce the inherent UDP-glucose glucohydrolase activity of the enzyme and attenuate the SI by scopoletin derivatives, which could also be achieved by mutations. Here, we studied the kinetic profiles of different phenols and used the substrate analog vanillin, which has shown atypical Michaelis-Menten kinetics in previous studies, to examine the effects of different ligands and mutations on the SI of NbUGT72AY1. Coumarins had no effect on enzymatic activity, whereas apocarotenoids and fatty acids strongly affected SI kinetics by increasing the inhibition constant Ki. Only the F87I mutant and a chimeric version of the enzyme showed weak SI with the substrate vanillin, but all mutants exhibited mild SI when sinapaldehyde was used as an acceptor. In contrast, stearic acid reduced the transferase activity of the mutants to varying degrees. The results not only confirm the multi-substrate functionality of NbUGT72AY1, but also reveal that the enzymatic activity of this protein can be fine-tuned by external metabolites such as apocarotenoids and fatty acids that affect SI. Since these signals are generated during plant cell destruction, NbUGT72AY1 likely plays an important role in plant defense by participating in the production of lignin in the cell wall and providing direct protection through the formation of toxic phytoalexins.

Green Leaf Volatiles-The Forefront of Plant Responses Against Biotic Attack

Green leaf volatiles (GLVs) are six-carbon volatile oxylipins ubiquitous in vascular plants. GLVs are produced from acyl groups in the biological membranes via oxygenation by a pathway-specific lipoxygenase (LOX) and a subsequent cleavage reaction by hydroperoxide lyase. Because of the universal distribution and ability to form GLVs, they have been anticipated to play a common role in vascular plants. While resting levels in intact plant tissues are low, GLVs are immediately synthesized de novo in response to stresses, such as insect herbivory, that disrupt the cell structure. This rapid GLV burst is one of the fastest responses of plants to cell-damaging stresses; therefore, GLVs are the first plant-derived compounds encountered by organisms that interact with plants irrespective of whether the interaction is competitive or friendly. GLVs should therefore be considered important mediators between plants and organisms that interact with them. GLVs can have direct effects by deterring herbivores and pathogens as well as indirect effects by attracting predators of herbivores, while other plants can recruit them to prepare their defenses in a process called priming. While the beneficial effects provided to plants by GLVs are often less dramatic and even complementary, the buildup of these tiny effects due to the multiple functions of GLVs can amass to levels that become substantially beneficial to plants. This review summarizes the current understanding of the spatiotemporal resolution of GLV biosynthesis and GLV functions and outlines how GLVs support the basic health of plants.