Elucidation of diverse endophytic bacteria in tomato graft unions

Grafting is used in vegetable production to improve plant vigour and tolerance to soil-borne diseases, yet the biological properties of the graft union remain poorly understood. Here, we investigated culturable bacteria associated with tomato (Solanum lycopersicum L.) graft unions. Bacterial populations reached 105−107 colony‑forming units per gram of fresh weight (CFU g−1 FW) in graft union tissues. However, no culturable bacteria were detected from non-grafted stem samples collected from the graft union equivalent position at the corresponding sampling time (i.e. 8 days after grafting in grafted plants). The predominant bacterial genera isolated from graft unions were Microbacterium and Sphingomonas. All isolates tested negative for the hypersensitive reaction in tobacco, indicating their non-pathogenic nature. The isolates exhibited functional traits, including effects on seedling growth, phosphate solubilisation, production or degradation of N-acyl-homoserine lactones, and antibacterial activity against Ralstonia pseudosolanacearum. In addition, isolates were associated with reduced symptom development of bacterial wilt in tomato seedlings under controlled conditions. These results suggest that graft unions may provide a biologically active environment associated with the recovery of diverse culturable bacterial communities. Our findings provide insight into bacteria associated with tomato graft unions and their potential functional properties relevant to horticultural production.