Cyanide-Mediated Signaling in Plants

Dear Colleagues,

Hydrogen cyanide (HCN) is a low-molecular-weight molecule that is highly reactive. Due to its reactivity and its abundance in the earliest atmosphere, the participation of HCN in the origin of ribonucleotides, lipids and amino acids is more than possible. It is well-known that HCN is toxic, mainly because it affects the function of the mitochondrial cytochrome c oxidase, blocking the electron transfer in mitochondrial oxygenic respiration, although it also affects photosynthetic enzymes in chloroplasts.

Despite its notorious toxicity, HCN is found in organisms of all kingdoms, where it is produced in diverse ways. Cyanogenic plants produce high concentrations of HCN through the degradation of cyanogenic glucosides and cyanolipids, and they liberate HCN when they are in contact with predatory herbivores to cope with them. In non-cyanogenic plants, HCN is produced exclusively during the biosynthesis of ethylene and the antipathogenic molecule camalexin. Microorganisms of the rhizosphere also produce HCN from the amino acid glycine in an oxidative reaction catalyzed by the enzyme cyanide synthase.

HCN functions are diverse and sometimes controversial or unknown. In plants, HCN and cyanogenic compounds have a protective role. It has been suggested that HCN produced by the root microbiome plays such a role, where it is considered a biocontrol agent, although other roles have been suggested. Independently of its toxic capacity, HCN itself also plays an important role in biological processes such as dormancy break and germination, root development and plant response to pathogens. Thus, HCN function in plants deserves special attention because it drives a change of the concept of HCN as only a poison to also being a signaling molecule.

Dr. Irene García
Dr. Luis C. Romero González
Guest Editors

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• hydrogen cyanide
• signaling
• biotic and abiotic stress
• reactive oxygen species
• biocontrol

Title: Tissue-specific expression of HCN and its metabolic precursors in Trifolium repens
Authors: Keerath Bhachu; Hind Emad Fadoul; Marc T. J. Johnson
Affiliation: Department of Biology, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada; Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON M5S 3B2, Canada
Abstract: Cyanogenesis is the process by which a plant releases hydrogen cyanide (HCN), a potent toxin, following tissue damage. The cyanogenic phenotype in white clover (Trifolium repens L.) is maintained by the coexistence of two alleles, Ac and Li, encoding CYP79D15 and linked metabolic genes required for the production of cyanogenic glucosides (i.e., linamarin and lotaustralin), and their hydrolyzing enzyme linamarase, respectively. We investigated the variability in the expression of Ac, Li, and their putative homologues across leaf, floral, and root tissues of cyanogenic and acyanogenic white clovers, followed by transcriptomic analysis to confirm the genetic basis and quantify the expression levels of Ac and Li. The Ac phenotype was found to be expressed only in the leaves, but not the floral or root tissues of the plant. The presence/absence of the Li phenotype in the leaves showed a perfect correlation with the presence/absence of this phenotype in the floral tissue. Li expression was detected in the roots, regardless of the presence of linamarase in the leaf tissue, consistent with a paralogous gene copy resulting in spatial variation in enzyme expression. Floral HCN expression was detected in eight of 441 field acquisitions sampled from 24 locations across the GTA, indicating expression of Ac, Li and HCN in flowers is rare but sometimes present. This study is the first detailed study of the tissue-specificity of HCN expression in white clover, a plant of agricultural significance. The variability in the expression of these secondary metabolites of cyanogenesis across tissues lays the groundwork for future studies of how white clover fine-tunes its defenses to conserve energy and resources, and ultimately to maintain fitness. Keywords: Cyanogenesis, cyanogenic glucosides, linamarase, Trifolium repens, transcriptomic