Secondary metabolites of Pseudomonas and Bacillus species in plant disease management

Abstract

Beneficial Pseudomonas spp. and Bacillus spp. are ubiquitous in soils and colonize the plant microbiome, including the rhizosphere, phyllosphere, and endosphere. Their beneficial effects on host plants are due to a wide range of secondary metabolites that act through several mechanisms, including direct antibiosis (antifungal, antibacterial, antiviral, and antinematicidal), competition for nutrients, promotion of plant growth and the induction of systemic resistance (ISR) in host plants. The genus Pseudomonas includes a variety of species with beneficial effects on plant health that play an important role in sustainable agriculture. The major secondary metabolites involved in biocontrol are phenazine-1-carboxylic acid, phenazine-1-carboxamide, pyrrolnitrin, pyoluteorin, 2,4-diacetylphloroglucinol, nunamycin, nunapeptin, brasmycin, braspeptin, sessilins, orfamides, HCN and various volatile organic compounds. Competition for nutrients, such as iron uptake, is based on siderophores such as pyoverdine and pyochelin. Plant-growth promotion is mediated by 1-aminocyclopropane-1-carboxylate deaminase, indoleacetic acid, abscisic acid, gibberellic acid, and cytokinins, as well as vitamins niacin, pantothenic acid, thiamine, riboflavin, and biotin. Endospore-forming Bacillus species also inhibit phytopathogens, including all of the above mechanisms underlined by a wide range of structurally diverse compounds. Direct inhibition of vast variety of phytopathogens is achieved by compounds such as volatiles, bacteriocins, and lipopeptides, i.e., iturins, surfactins, fengycins, and kurstakins. Indirect effects are performed by phytostimulating compounds such as auxins, gibberellins, cytokinins and abscisic acid, which positively influence plant growth and development. They also induce systemic resistance in the host plant through the production of volatiles. Expression of defense-related proteins is induced in plants by lipopeptides, polyketides, and volatiles that activate the jasmonic acid, salicylic acid, or ethylene signaling pathways. They also have the ability to inhibit quorum sensing of various phytopathogens. As a result, higher biomass and nutritional quality, improved nutrient mobilization and uptake, enhanced tolerance to biotic and abiotic stresses, and shaping of the microbiota are all positive effects that healthy plants acquire from Pseudomonas and Bacillus beneficial strains. This review aims to highlight the most important aspects of Pseudomonas spp. and Bacillus spp. secondary metabolites in the control of phytopathogens and their beneficial properties for the host plant. Bioformulations based on their metabolites could replace synthetic pesticides and enable sustainable agricultural practices.