Microbial volatile organic compounds in plant defense: mechanisms, signaling networks, and agricultural applications.

Microbial volatile organic compounds (mVOCs) are low-molecular-weight metabolites released by bacteria and fungi in the rhizosphere and phyllosphere. mVOCs have emerged as important mediators of plant-microbes' communication and systemic defense priming. This review covers current knowledge of the biochemical diversity of the mVOCs, encompassing major classes of mVOCs, and their mechanisms of action in plant defense. We delineate the molecular signaling cascades activated by key mVOCs such as 2,3-butanediol, acetoin, dimethyl disulfide, and geosmin, highlighting their convergence on jasmonic acid (JA), salicylic acid (SA), and ethylene (ET) phytohormone pathways, as well as their modulation of reactive oxygen species (ROS) signaling and MAPK cascades. Particular attention is given to the dual role of mVOCs as both direct antimicrobial agents against foliar and soil-borne pathogens and indirect defense elicitors that reconfigure plant transcriptional programs. Change in pattern of gene expression by the application of mVOCs in different plant species is also discussed. Cross-kingdom signaling dynamics, like mVOC-mediated modulation of root exudate composition and its effects on microbial community in the rhizosphere are discussed. We also discuss key challenges limiting field deployment, including mVOCs stability under different stresses, dose-dependent phytotoxicity, and complexity of the mVOCs consortia in natural soils. In addition, we propose future research directions integrating metabolomics, transcriptomics, and synthetic ecology to harness the full agronomic potential of the microbial volatile signaling.