Unravelling the role of grafting in tomato plants upon viroid infection

Tomato is one of the most important solanaceous crops due to its economic and nutritional relevance. In addition to their organoleptic properties, local tomato ecotypes serve as a valuable source of resistance/tolerance to pests. Indeed, tomato hosts a wide range of plant pathogens, including potato spindle tuber viroid (PSTVd; family Pospiviroidae), that elicits stunting and leaf curling symptoms. Grafting is an agricultural method employed as a sustainable approach to manage both biotic and abiotic stresses. Besides to boosting the performance of various horticultural crops, grafting has the potential to increase tomato plants' tolerance to viral infections. In the frame of the project DiVInGraft, we are investigating the effect of grafting in response to viroid infection in tomato plants. To this end, we used high-throughput imaging phenotyping to study the responses of grafted and non-grafted tomato plants to PSTVd infection. Two tomato varieties, Manduria (Ma) and UC82 (UC), tolerant and susceptible to viral infections, respectively, were tested. Non-grafted, self-grafted UC (UC/UC) and UC grafted onto Ma (UC/Ma) were mechanically inoculated with PSTVd or mock-inoculated. Quantitative data of morphological parameters were acquired at 9 time points up to 36 days post-inoculation (dpi). PCA analysis showed that grafting has a global effect on PSTVd infection, with a slightly positive effect of UC/Ma respect self-grafted tomato plants. Simultaneously, using the same experimental design, a parallel set of plants was prepared to extract RNA from leaf tissue at 15 dpi to be submitted for high-throughput sequencing (HTS) of small RNAs and messenger RNAs. The transcriptomic responses upon PSTVd infection, of the two tomato varieties and of the grafted and non-grafted plants were compared.Finally, the susceptibility of three Campanian tomato ecotypes to PSTVd infection was evaluated. Symptom observation and quantification of viroid accumulation showed that all tested ecotypes were susceptible to PSTVd. Specifically, San Marzano showed mild symptoms and reduced accumulation of PSTVd, unveiling a certain level of tolerance. In contrast, the ecotype Giallo determinato da serbo was particularly susceptible to PSTVd. These preliminary results demonstrate that phenotyping is a promising tool to evaluate tomato responses upon viroid infections and grafting. These responses may be further explored at molecular level by HTS data.