Grafting Alleviates the Single and Combined Salt and <scp> <i>Phytophthora capsici</i> </scp> Stress in Tomato by Activating Salicylic Acid‐Mediated Defence Pathway

Abiotic and biotic stresses threaten crop yield. Grafting promotes the resistance of vegetables to environmental stresses. However, rootstocks with enhanced resistance to multiple stressors are scarce and potential mechanisms remain obscure. Here, we screened a tomato rootstock 'QZ71' with resistance to salt, saline-alkali, drought stress, Fusarium oxysporum and Phytophthora capsici root rot. Compared with the self-root grafted cultivar 'STG2', rootstock grafting showed increased resistance to single and combined salt and P. capsici stress. Salt alone had a greater effect than P. capsici infection alone, and the adverse impact was worsened under combined stresses. Moreover, grafting-enhanced resistance was related to maintaining K⁺/Na⁺ homeostasis, increasing defensive enzyme activities and lignin biosynthesis and activating the expression of their associated genes. Importantly, grafting significantly increased SA content and the expression of SA signalling marker genes, including SlNPR1 and SlPR-1. Conversely, SA-deficient mutant NahG rootstock grafting obviously attenuated the resistance by inhibiting the expression of ion transporters, defence enzymes, lignin synthesis and SA signalling marker genes. Together, our findings demonstrated, for the first time, that the excellent tomato rootstock QZ71 enhanced resistance to single and combined salt and P. capsici stress by activating the SA signalling pathway, highlighting the importance of rootstock selection in improving crop performance.