L-Canavanine, a Root Exudate From Hairy Vetch (Vicia villosa) Drastically Affecting the Soil Microbial Community and Metabolite Pathways

Arbuscular mycorrhizal fungi and Trichoderma longibrachiatum alter the transcriptome of Vicia villosa in response to infection by the fungal pathogen Stemphylium vesicarium

BACKGROUND

Leaf spot caused by Stemphylium vesicarium is a severe disease of Vicia villosa and first reported in 2019. Arbuscular mycorrhizal fungi (AMF) and Trichoderma are common beneficial microorganisms in soil that enhance plant resistance to pathogens. This study established a greenhouse experiment to examine the physiological and transcriptomic changes of V. villosa that were co-inoculated with the AMF Sieverdingia tortuosa and Trichoderma longibrachiatum to determine their effects on the development of resistance to disease.

RESULTS

Infection by the pathogen reduced the shoot biomass of V. villosa. Individual inoculation or co-inoculation with AMF and T. longibrachiatum reduced the severity of disease and promoted defense-related reactions, such as the production of salicylic acid (SA), activity of phenylalanine ammonia lyase and chitinase. Inoculation of Trichoderma alone or in combination with AMF significantly increased the content of SA of the diseased V. villosa by 12.23% and 12.80%, respectively. Treatment with AMF alone significantly increased the chitinase activity of susceptible V. villosa by 6.4% compared with V. villosa only infected with S. vesicarium. Gene ontology terms that related to plant disease resistance, such as upregulated "Defense response", "Peroxidase activity", and "Signal acceptor activity", were significantly enriched in diseased plants that had been inoculated with S. tortuosa and T. longibrachiatum. However, they were not significantly enriched in susceptible plants that had not been inoculated with S. tortuosa and T. longibrachiatum. The expression of the genes that were involved in the Kyoto Encyclopedia of Genes and Genomes pathways "Isoflavonoid biosynthesis" and "Flavone and flavonol biosynthesis" and were related to disease defense was upregulated.

CONCLUSION

Both of T. longibrachiatum and AMF exhibit significant potential in managing leaf spot disease caused by S. vesicarium in V. villosa. The mechanism includes the increased SA content as well as the expression of pathogen defense-related genes in plant. T. longibrachiatum alone and combined with AMF resulted in a significant increase in SA levels. Furthermore, AMF also significantly up-regulated the expression of NPR1-related genes, which are integral to systemic acquired resistance. Our findings underscore the efficacy of T. longibrachiatum and AMF as potential biological control agents, providing a promising strategy for the management of leaf spot disease in V. villosa.

A standalone editing protein deacylates mischarged canavanyl-tRNAArg to prevent canavanine incorporation into proteins

Error-free translation of the genetic code into proteins is vitally important for all organisms. Therefore, it is crucial that the correct amino acids are loaded onto their corresponding tRNAs. This process is highly challenging when aminoacyl-tRNA-synthetases encounter structural analogues to the native substrate like the arginine antimetabolite canavanine. To circumvent deleterious incorporation due to tRNA mischarging, editing mechanisms have evolved. However, only for half of the tRNA synthetases, editing activity is known and only few specific standalone editing proteins have been described. Understanding the diverse mechanisms resulting in error-free protein synthesis is of great importance. Here, we report the discovery of a protein that is upregulated upon canavanine stimulation in bacteria that live associated with canavanine-producing plants. We demonstrate that it acts as standalone editing protein specifically deacylating canavanylated tRNAArg. We therefore propose canavanyl-tRNAArgdeacylase (CtdA) as systematic name. Knockout strains show severe growth defects in canavanine-containing media and incorporate high amounts of canavanine into the proteome. CtdA is frequently found under control of guanidine riboswitches, revealing a functional connection of canavanine and guanidine metabolisms. Our results are the first to show editing activity towards mischarged tRNAArg and add to the puzzle of how faithful translation is ensured in nature.

Antibacterial and Antibiofilm Effects of Allelopathic Compounds Identified in Medicago sativa L. Seedling Exudate against Escherichia coli

In this study, the allelopathic properties of Medicago sativa L. (alfalfa) seedling exudates on the germination of seeds of various species were investigated. The compounds responsible for the allelopathic effects of alfalfa were identified and characterized by employing liquid chromatography ion mobility high-resolution mass spectrometry. Crude exudates inhibited the germination of seeds of all various plant species tested. Overall, nine compounds in alfalfa were identified and quantified. The most predominant compounds were a hyperoside representing a flavonoid glucoside, the non-proteinogenic amino acid canavanine, and two dipeptides, identified as H-Glu-Tyr-OH and H-Phe-Glu-OH. The latter corresponds to the first finding that dipeptides are exuded from alfalfa seedlings. In addition, the antibacterial and antibiofilm activities of alfalfa exudate and its identified compounds were elucidated. Both hyperoside and canavanine revealed the best antibacterial activity with minimum inhibitory concentration (MIC) values that ranged from 8 to 32 and 32 to 256 µg/mL, respectively. Regarding the antibiofilm action, hyperoside and canavanine caused a decline in the percentage of E. coli isolates that possessed a strong and moderate biofilm-forming potential from 68.42% to 21.05% and 31.58%, respectively. Studies on their inhibiting effects exhibit that these major substances are predominantly responsible for the allelopathic and antimicrobial effects of the crude exudates.

The Importance of Microorganisms for Sustainable Agriculture-A Review

In the face of climate change, progressive degradation of the environment, including agricultural land negatively affecting plant growth and development, endangers plant productivity. Seeking efficient and sustainable agricultural techniques to replace agricultural chemicals is one of the most important challenges nowadays. The use of plant growth-promoting microorganisms is among the most promising approaches; however, molecular mechanisms underneath plant-microbe interactions are still poorly understood. In this review, we summarized the knowledge on plant-microbe interactions, highlighting the role of microbial and plant proteins and metabolites in the formation of symbiotic relationships. This review covers rhizosphere and phyllosphere microbiomes, the role of root exudates in plant-microorganism interactions, the functioning of the plant's immune system during the plant-microorganism interactions. We also emphasized the possible role of the stringent response and the evolutionarily conserved mechanism during the established interaction between plants and microorganisms. As a case study, we discussed fungi belonging to the genus Trichoderma. Our review aims to summarize the existing knowledge about plant-microorganism interactions and to highlight molecular pathways that need further investigation.

Canavanine utilization via homoserine and hydroxyguanidine by a PLP-dependent γ-lyase in Pseudomonadaceae and Rhizobiales

Canavanine, the δ-oxa-analogue of arginine, is produced as one of the main nitrogen storage compounds in legume seeds and has repellent properties. Its toxicity originates from incorporation into proteins as well as arginase-mediated hydrolysis to canaline that forms stable oximes with carbonyls. So far no pathway or enzyme has been identified acting specifically on canavanine. Here we report the characterization of a novel PLP-dependent enzyme, canavanine-γ-lyase, that catalyzes the elimination of hydroxyguanidine from canavanine to subsequently yield homoserine. Homoserine-dehydrogenase, aspartate–semialdehyde–dehydrogenase and ammonium–aspartate–lyase activities are also induced for facilitating canavanine utilization. We demonstrate that this novel pathway is found in certain Pseudomonas species and the Rhizobiales symbionts of legumes. The findings broaden the diverse reactions that the versatile class of PLP-dependent enzymes is able to catalyze. Since canavanine utilization is found prominently in root-associated bacteria, it could have important implications for the establishment and maintenance of the legume rhizosphere.

A novel degradation pathway enables rhizosphere-associated bacteria to utilize canavanine.