Cyclic and non-ribosomal microbial metabolites in the biological control of plant pathogens

Abstract

Cyclic and non-ribosomal microbial metabolites are important bioactive secondary metabolites with significant potential in sustainable plant disease management. These metabolites are primarily produced by beneficial microorganisms including Bacillus, Pseudomonas and Streptomyces through non-ribosomal peptide synthetase (NRPS)-mediated biosynthetic pathways. This narrative review summarises the structural diversity, biosynthesis, mechanisms of action, detection methods and agricultural applications of cyclic and non-ribosomal microbial metabolites in the biological control of plant pathogens. Among these metabolites, surfactins, iturins, fengycins, bacillomycins and amphisins are the most extensively studied compounds due to their broad-spectrum antimicrobial activities. These metabolites exhibit strong inhibitory effects against major phytopathogens including Fusarium, Alternaria, Rhizoctonia and Magnaporthe oryzae, with disease suppression efficiencies ranging from 40 to 90 % under in-vitro and in-planta conditions. Iturin and fengycin have demonstrated 65 to 90 % inhibition against Fusarium species, whereas surfactin- and fengycin-producing strains have been reported to reduce rice blast severity by 40 to 70 %. Their amphiphilic structures enable membrane permeabilisation, pore formation, inhibition of spore germination and disruption of pathogen growth. In addition to direct antimicrobial activity, these metabolites promote biofilm formation, rhizosphere colonisation and induced systemic resistance mediated through jasmonic acid, ethylene and salicylic acid dependent pathways. Despite promising biocontrol potential, challenges related to large-scale production, formulation stability and field-level consistency remain major limitations for commercialisation.