Contact-dependent killing: a prevalent yet overlooked microbial interaction pattern with significant implications for sustainable agriculture

Plant diseases present a substantial threat to global food security, and biocontrol offers a promising avenue for managing crop diseases sustainably in agriculture. The production and secretion of diverse diffusible antimicrobial chemicals represent well-established biocontrol principles. However, this review highlights a prevailing yet overlooked biocontrol mechanism termed as contact-dependent killing (CDK), employed by both Gram-negative and Gram-positive bacteria. We delineate the main types and discovery history of four CDK weapons: CdiAB, type IV, VI, and VII secretion systems. Moreover, we present case studies showing how well-established plant-beneficial bacteria employ CDK weapons to combat pathogens and/or promote plant growth. Notably, we summarize current progress in utilizing CDK for designing compatible biocontrol communities and exploring 'non-canonical' biocontrol agents from distinct environmental samples that act independently of diffusible antimicrobial chemicals. Finally, we underscore the promising prospects of CDK weapons in developing microbial biopesticides or biofertilizers, producing seed-coating agents, reassessing the overlooked antimicrobial activity of microbes commonly expressing activities against pests and weeds, and serving as a strategy for isolating low-abundance microorganisms in agricultural microbiomes. Integrating CDK weapons with other innovative strategies into agricultural practices holds significant promise for enhancing crop productivity, ensuring food security, and fostering environmental sustainability.