The Rootstock's Cotyledon-Regulated Fructokinase ClFRK1 Contributes to Graft Union Formation in Watermelon

Grafting is a traditional horticultural practice that enhances plant resilience against biotic and abiotic stresses. However, the influence of specific tissues, such as rootstock cotyledons, on graft union formation is not well understood. This study investigates the impact of rootstock cotyledon removal on graft healing in watermelon and its underlying mechanisms. Our results indicate that grafting with rootstock cotyledons (+C) consistently resulted in higher survival rates and better growth outcomes compared to grafting without rootstock cotyledons (-C). This effect was more pronounced in cultivated watermelon rootstocks, which have lower hypocotyl sugar content than wild watermelon rootstocks. Transcriptomic analysis revealed that cotyledon removal disrupted sugar metabolism and affected gene expression related to cell division and tissue development. A fructokinase, ClFRK1, was identified among the candidate genes positively correlated with graft survival rate and healing degree. Silencing ClFRK1 reduced callus proliferation, delayed graft healing and reduced survival rate. Conversely, fructose treatment increased ClFRK1 expression levels at the graft junction, which promoted callus proliferation and vascular reconnection. We propose a novel regulatory model for how ClFRK1 regulates graft union formation. These findings underscore new insights into the interactions and synergistic processes between the graft interface and non-grafted organs during graft union formation and also enrich our understanding of fructokinase.