Chitosan-Derived Carbon Dots Enhance Root Rot Resistance in Isatis Tinctoria L. by Enhancing Plant Defense Responses

Activating plant defense systems to protect against biotic stresses represents a promising strategy. Chitosan and its derivatives are considered excellent plant elicitors due to their efficacy and natural origin. However, the preparation of water-soluble chitosan oligosaccharide fragments or nanoparticles often involves complex processing and costly chemicals, hindering their broader application. In this study, chitosan-derived carbon dots (CDs) are prepared using a facile one-pot hydrothermal method. The resulting CDs exhibit a quasi-spherical morphology with an approximately diameter of 2-5 nm. Notably, nitrogen self-doping conferres chitosan CDs with a positive zeta potential. These characteristics facilitate CDs absorption, transport, and degradation within the plant. Crucially, the CDs activate the plant defense system, significantly enhancing resistance to root rot and reducing the disease incidence rate from 95% to 30%. Metabolomic and transcriptomic analyses elucidate the underlying mechanism: 1) alleviation of oxidative stress through increased antioxidant enzyme activities (e.g., activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD)); 2) stimulation of phenolic compound production (phenylalanine, ferulic acid, and isoquercitrin), thereby reinforcing the cell wall; and 3) activation of the defense system, regulating the production of defense-related phytohormones (salicylic acid and jasmonic acid). These findings highlight the potential of chitosan-derived CDs as environmentally friendly plant elicitors for enhancing disease resistance.