Epigenetic bases of grafting-induced vigour in eggplant

In horticulture, grafting is a popular technique used to combine positive traits from two different plants. This is achieved by joining the plant top part (scion) onto a rootstock which contains the stem and roots. Despite its wide use, the biological mechanisms driving rootstock-induced alterations of the scion phenotype remain largely unknown. Given that epigenetics plays a crucial role during distance signalling in plants, we studied the genome-wide changes induced by DNA methylation in eggplant ( Solanum melongena ) plants grafted onto two interspecific rootstocks used to increase scion vigour. As a control, we compared any epigenetic effect found in such grafts to patterns detected in self-grafted plants. We found that vigour was associated with a specific change in scion gene expression and a genome-wide hypomethylation in CHH context. Interestingly, this hypomethylation correlated with the down-regulation of younger and potentially more active LTR retrotransposons (LTR-TEs), suggesting that graft-induced epigenetic modifications are associated to both physiological and molecular phenotypes in grafted plants. We propose that rootstocks can promote vigour by reducing DNA methylation in the scion genome, following similar principles found in some heterotic hybrids.