Self-DNA as a chronic stress signal drives soil sickness in lisianthus by disrupting root integrity and facilitating pathogen invasion

Continuous cropping obstacle (CCO) severely limits the sustainable production of high-value cut flowers like lisianthus (Eustoma grandiflorum), primarily driven by species-specific negative plant-soil feedback (PSF). Although autotoxic compounds are implicated, the mechanisms behind this specificity remain elusive. This study investigated the role of extracellular self-DNA (sDNA) as a species-specific autotoxic stress in mediating negative PSF under a prolonged exposure scenario. This study first demonstrated that the growth-inhibitory effect of fragmented extracellular DNA is phylogenetically dependent, with conspecific DNA causing the strongest suppression, followed by DNA from closely related Gentiana species. Chronic exposure to lisianthus sDNA induced a concentration-dependent ecotoxic response, including significant reactive oxygen species (ROS) accumulation, disruption of root cellular architecture, and overall growth inhibition, with a half-maximal effective concentration (EC50) of 51.0 μg/mL (R2 = 0.816). Transcriptomic profiling further indicated that long-term sDNA exposure activated defense pathways akin to pathogen-associated molecular pattern (PAMP) responses (e.g., 'response to chitin', 'response to fungus', and 'defense response to fungus'), while concurrently repressing genes involved in primary metabolism. To investigate the paradox of sDNA acting as both a defense elicitor and a growth suppressor, a pathogen challenge assay was conducted. Pretreatment with sDNA for 24 h significantly enhanced the susceptibility of lisianthus to the soil-borne pathogen Fusarium oxysporum, suggesting that the sDNA-induced physiological damage may outweigh its potential defensive benefits. The present findings propose that sDNA drives negative PSF through a self-reinforcing cycle, in which monoculture-induced sDNA accumulation directly impairs root health and plant vigor, which in turn predisposes the plant to pathogen infection, further exacerbating root decay and sDNA release. This mechanistic cycle highlights a key pathway underpinning the persistent soil sickness in the continuous cropping system of lisianthus.