Curdlan oligosaccharides induce salicylic acid-dependent defense responses in

Introduction: Long-term use of chemical pesticides has caused environmental and resistance-related concerns, highlighting the need for environmentally friendly plant immune inducers. This study aimed to prepare curdlan oligosaccharides (CDOS) and evaluate their immune-inducing activity in Arabidopsis thaliana. Methods: CDOS were prepared by enzymatic hydrolysis and purified by dialysis. Their structural characteristics were analyzed by TLC, FTIR, and ESI-MS. Antibacterial activity against Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), disease resistance in A. thaliana, ROS accumulation, defense-related enzyme activities, and immune-related gene expression were further investigated. Results: The purified CDOS preparation mainly consisted of oligosaccharides with a degree of polymerization of 2-3, with a total sugar content of 81.62% and a residual protein content of 3.25%. CDOS showed no direct antibacterial activity against Pst DC3000 but significantly enhanced resistance in A. thaliana, reducing bacterial colonization by approximately 50% at 50 mg/L. CDOS also promoted ROS accumulation, increased POD, SOD, CAT, and PAL activities, and upregulated the SA-associated marker genes PR1, PR2, and PR5, whereas the JA-associated genes VSP and PDF1.2 were not significantly altered. The protective effect was impaired in SA-pathway-deficient mutants but retained in the JA-pathway mutant. Discussion: These findings indicate that CDOS functions primarily as a plant immune elicitor rather than a direct antibacterial agent, and its protective activity is closely associated with SA-dependent defense responses.