Stage-specific events in tomato graft formation and the regulatory effects of auxin and cytokinin

•Gene co-expression networks during graft formation are stage-specific.•The asymmetric response phase (3−72 hours after grafting) is the critical period for graft formation.•Auxin and cytokinin respond to grafting, above and below the graft junction, respectively, to promote the graft formation.•The accumulation of auxin above the graft junction prepares cells for mitosis, which promotes cell division to form callus.•Exogenous application of certain concentrations of IAA and 6-BA will promote graft formation. Grafting is widely used worldwide because of its obvious advantages, especially in solanaceous vegetable crops. However, the molecular mechanisms underlying graft formation are unknown. In this study, internode tissues from above and below the graft junction were harvested, and we performed weighted gene co-expression network analysis (WGCNA) to describe the temporal and spatial transcriptional dynamics that occur during graft formation in tomato. The wounding stress response involved in JA, ETH, and oxylipins mainly occurred at 1 h after grafting (HAG). From 3 to 12 HAG, the biological processes of snRNA and snoRNA modification and the gibberellin-mediated signaling pathway functioned both above and below the graft junction. However, auxin transport and signaling, DNA replication, and xylem and phloem pattern formation were restricted to the scion, whereas the cytokinin-activated signaling pathway and the cellular response to sucrose starvation was restricted to the rootstock. At 24−72 HAG, cell division occurred above the graft junction, and photosynthesis-related pathways were activated below the graft junction. The levels of auxin and cytokinin reached their maxima above and below the graft junction at 12 HAG, respectively. Exogenous application of certain concentrations of IAA and 6-BA will promote xylem and phloem transport capacity. The current work has analyzed the stage-specific events and hub genes during the developmental progression of tomato grafting. We found that auxin and cytokinin levels respond to grafting, above and below the graft junction, respectively, to promote the formation of xylem and phloem patterning. In addition, the accumulation of auxin above the graft junction induced cells to prepare for mitosis and promoted the formation of callus. In short, our work provides an important reference for theoretical research and production application of tomato grafting in the future.