Binding mode of 1-methylcyclopropene, an ethylene antagonist in various copper models

Recent advances in understanding the regulation of plant secondary metabolite biosynthesis by ethylene-mediated pathways

Plants produce a large repertoire of secondary metabolites. The pathways that lead to the biosynthesis of these metabolites are majorly conserved in the plant kingdom. However, a significant portion of these metabolites are specific to certain groups or species due to variations in the downstream pathways and evolution of the enzymes. These metabolites show spatiotemporal variation in their accumulation and are of great importance to plants due to their role in development, stress response and survival. A large number of these metabolites are in huge industrial demand due to their potential use as therapeutics, aromatics and more. Ethylene, as a plant hormone is long known, and its biosynthetic process, signaling mechanism and effects on development and response pathways have been characterized in many plants. Through exogenous treatments, ethylene and its inhibitors have been used to manipulate the production of various secondary metabolites. However, the research done on a limited number of plants in the last few years has only started to uncover the mechanisms through which ethylene regulates the accumulation of these metabolites. Often in association with other hormones, ethylene participates in fine-tuning the biosynthesis of the secondary metabolites, and brings specificity in the regulation depending on the plant, organ, tissue type and the prevailing conditions. This review summarizes the related studies, interprets the outcomes, and identifies the gaps that will help to breed better varieties of the related crops and produce high-value secondary metabolites for human benefits.

Basis for high-affinity ethylene binding by the ethylene receptor ETR1 of Arabidopsis

The gaseous hormone ethylene is perceived in plants by membrane-bound receptors, the best studied of these being ETR1 from Arabidopsis. Ethylene receptors can mediate a response to ethylene concentrations at less than one part per billion; however, the mechanistic basis for such high-affinity ligand binding has remained elusive. Here we identify an Asp residue within the ETR1 transmembrane domain that plays a critical role in ethylene binding. Site-directed mutation of the Asp to Asn results in a functional receptor that has a reduced affinity for ethylene, but still mediates ethylene responses in planta. The Asp residue is highly conserved among ethylene receptor-like proteins in plants and bacteria, but Asn variants exist, pointing to the physiological relevance of modulating ethylene-binding kinetics. Our results also support a bifunctional role for the Asp residue in forming a polar bridge to a conserved Lys residue in the receptor to mediate changes in signaling output. We propose a new structural model for the mechanism of ethylene binding and signal transduction, one with similarities to that found in a mammalian olfactory receptor.

Spectroscopic and QM/MM studies of the Cu(I) binding site of the plant ethylene receptor ETR1

Herein, we present, to our knowledge, the first spectroscopic characterization of the Cu(I) active site of the plant ethylene receptor ETR1. The x-ray absorption (XAS) and extended x-ray absorption fine structure (EXAFS) spectroscopies presented here establish that ETR1 has a low-coordinate Cu(I) site. The EXAFS resolves a mixed first coordination sphere of N/O and S scatterers at distances consistent with potential histidine and cysteine residues. This finding agrees with the coordination of residues C65 and H69 to the Cu(I) site, which are critical for ethylene activity and well conserved. Furthermore, the Cu K-edge XAS and EXAFS of ETR1 exhibit spectroscopic changes upon addition of ethylene that are attributed to modifications in the Cu(I) coordination environment, suggestive of ethylene binding. Results from umbrella sampling simulations of the proposed ethylene binding helix of ETR1 at a mixed quantum mechanics/molecular mechanics level agree with the EXAFS fit distance changes upon ethylene binding, particularly in the increase of the distance between H69 and Cu(I), and yield binding energetics comparable with experimental dissociation constants. The observed changes in the copper coordination environment might be the triggering signal for the transmission of the ethylene response.