Bioactive chitin hydrolysates from chitinase-producing rhizobacteria enhance soybean tolerance to cobalt stress through pattern-triggered immunity activation and antioxidant modulation

Cobalt (Co), whose environmental release is escalating due to battery recycling and industrial activities, poses a growing threat to crop productivity despite its essential role as a micronutrient. Excessive Co disrupts cellular redox homeostasis, impairs photosynthesis, and markedly restricts plant growth; nevertheless, sustainable mitigation strategies remain limited. Although chitin-derived compounds have been suggested to enhance plant tolerance to certain abiotic stresses, the mechanisms underlying these responses and the potential advantages of combining chitin with chitinase-producing microorganisms remain unclear. Therefore, this study aims to clarify the physiological, biochemical, and molecular mechanisms underlying the ability of the chitinase-producing bacterium Paenibacillus illinoisensis JS80 and chitin to mitigate Co toxicity in soybean (Glycine max L.). Strain JS80 is a highly Co-tolerant rhizobacterium exhibiting chitinase activity of 144.55 mU mL^-1. Under Co toxicity, co-application of JS80 and chitin significantly enhanced biomass production and photosynthetic performance, and reduced the accumulation of reactive oxygen species (O₂^- and H₂O₂) and lipid peroxidation. These improvements were associated with the activation of pattern-triggered immunity, as evidenced by the upregulation of defense-related genes (GmCERK1, GmRLCK, GmCaMK, GmMAPKs, and GmWRKY), elevated salicylic acid levels, and elevated activities of antioxidant enzymes (SOD, CAT, GPx, and APx). These findings indicate how the co-application of the chitinase-producing bacterial strain JS80 and chitin activates abiotic stress signaling that interacts with biotic responses through shared signaling pathways. This interaction boosts Co stress tolerance and provides an effective, sustainable strategy to improve crop resilience under adverse environmental conditions.