Molecular insights into viroid-induced defense mechanisms in grafted tomato plants

Grafting may enhance the performance and potential resilience against biotic and abiotic stresses in various horticultural crops. While certain tomato-virus combinations have demonstrated increased tolerance to viral infections through grafting, its impact on infections caused by potato spindle tuber viroid (PSTVd; family Pospiviroidae)—a severe pathogen responsible for stunting and leaf curling in tomato—remains unclear. Here, the influence of grafting on tomato plant responses to PSTVd infection was investigated using high-throughput imaging phenotyping and transcriptomic analyses. The tomato cultivars Manduria (Ma), known for its tolerance to viral infections, and UC82 (UC), more susceptible were tested. Following mechanical inoculation with PSTVd or mock treatment, morphological parameters were quantitatively assessed across nine time points up to 36 days postinoculation (dpi) in non-grafted, self-grafted UC (UC/UC), and UC grafted onto Ma (UC/Ma) plants. Principal component analysis (PCA) revealed that grafting has a global effect on the phenotype of PSTVd-infected plants. Transcriptomic profiling at 15 dpi indicated a higher and more diverse number of differentially expressed genes in ungrafted plants (Ma, UC) than in grafted counterparts (UC/UC, UC/Ma), with gene ontology analyses indicating that plants response to grafting and to PSTVd largely overlap. Furthermore, viroid titers measured at both 15 and 37 dpi were significantly lower in UC/Ma plants compared to UC/UC, UC, and Ma, suggesting a suppressive effect of UC/Ma grafting on viroid accumulation. These preliminary findings highlight the potential of integrating phenotyping and transcriptomic approaches to unravel tomato defense mechanisms against viroid infections and to evaluate the protective role of grafting