Dissection of defense responses against viroid infection in grafted tomato plants

Grafting, besides boosting the performance of various horticultural crops, may counteract both biotic and abiotic stresses. While the potential to increase tomato plants' tolerance to viral infections has been shown in some tomato/virus combinations, whether and how grafting may interfere with infections by potato spindle tuber viroid (PSTVd; family Pospiviroidae), a severe patogen eliciting stunting and leaf curling symptoms in tomato, is not known. Here, the effect of grafting on the plant responses to viroid infection has been investigated by high-throughput imaging phenotyping and transcriptome analyses. Two tomato varieties, Manduria (Ma) and UC82 (UC), tolerant and susceptible to viral infections, respectively, were studied. Quantitative data of morphological parameters were acquired at 9 time points up to 36 days post-inoculation (dpi) from non-grafted, self-grafted UC (UC/UC) and UC grafted onto Ma (UC/Ma) mechanically inoculated with PSTVd or mock-inoculated. According to PCA analysis, grafting has a global effect on the phenotype of PSTVd-infected plants, with a positive effect especially in UC/Ma compared to self-grafted tomato plants. The results of a transcriptome analyses performed on a parallel set of grafted and ungrafted plants and focusing on early responses (15 days post inoculation) showed that the number of differentially expressed genes upon PSTVd infection is higher and more variegated in ungrafted (Ma, UC) than in grafted plants (UC/UC and UC/Ma), Moreover, at both 15 and 37 dpi, the viroid titer in UC grafted onto MA plants was significatively reduced with respect to UC/UC, UC and Ma plants. These preliminary results demonstrate that phenotyping couled with transcriptomic analyses is a promising tool to evaluate tomato responses upon viroid infections and grafting.