Bacterial extracellular vesicles as emerging biostimulants in plant microbe signaling and stress tolerance

Bacterial extracellular vesicles (BEVs) are nanosized membrane-bound nanoparticles naturally secreted by Gram-negative and Gram-positive bacteria. These vesicles mediate intercellular communication through delivery of diverse bioactive cargo, including proteins, lipids, nucleic acids, metabolites, and signaling molecules. Initially recognized as virulence determinants in pathogenic bacteria, BEVs are increasingly recognized as multifunctional platforms with potential applications in sustainable agriculture. This review examines BEV biogenesis, cargo composition, plant uptake, and emerging roles in plant growth promotion, stress adaptation, and disease resistance. Emphasis is placed on BEV-mediated modulation of plant immunity through pattern recognition receptors (PRRs), pattern-triggered immunity (PTI), and induced systemic resistance (ISR). Potential roles of vesicle-associated phytohormones, ACC deaminase, siderophores, extracellular enzymes, and regulatory RNAs in nutrient acquisition, phytohormone homeostasis, and stress responses are critically evaluated. Emerging evidence linking BEVs to drought, salinity, heat, heavy metal, and nutrient-deficiency responses is examined, with emphasis on antioxidant defense, osmotic adjustment, ion homeostasis, and stress-responsive signaling. The review also discusses engineered BEVs as potential biostimulants, nano-biofertilizers, biocontrol agents, and precision delivery systems, including CRISPR-based engineering, synthetic vesicles, AI-guided cargo optimization, and smart nanoformulations. Major challenges include scalable production, standardization, biosafety, regulation, and field validation. However, critical knowledge gaps remain regarding BEV perception, cellular uptake, cargo delivery, and the causal basis of plant responses, together with uncertainties surrounding biosafety, reproducibility, and performance under field conditions. Collectively, BEVs represent promising biological nanocarriers for climate-resilient and sustainable agriculture, but further mechanistic, methodological, and field-based research is required to establish their reliability and practical agricultural potential.