Priming with broccoli extract mitigates salinity stress in tomato through enhanced water and Na⁺/K⁺ homeostasis

Salinity is a major abiotic stress that severely limits crop productivity. In recent years, plant-based biostimulants have emerged as sustainable tools to enhance plant growth and improve stress resilience. However, their physiological and molecular mechanisms of action-particularly when applied as seed priming-remain largely unexplored. To evaluate the biostimulant potential of a broccoli (Brassica oleracea L. var. italica) extract on tomato (Solanum lycopersicum L.), plants were grown under controlled and saline conditions. Treatments were applied at the seed stage (priming), and adult plants were assessed for biomass accumulation, water status (including RWC and water potential), mineral nutrient composition, phenolic content, and the expression of key genes involved in ion transport (SlHKT1.2, SlNHX4, SlSOS1) and water regulation (SlPIP2;1, SlTIP2;1). The extract significantly enhanced biomass accumulation at both seedling and adult stages. Under salt stress, extract-treated plants maintained better water status, restricted Na⁺ translocation to the shoot, and showed improved nutrient use efficiency. This was associated with increased expression of SlSOS1 in both roots and shoots, and upregulation of SlHKT1.2 specifically in aerial tissues, as well as the modulation of key aquaporins, suggesting coordinated control of ionic and water homeostasis. Here we demonstrate that a broccoli-derived extract applied as a seed priming agent induces stress memory and confers enhanced physiological and molecular resilience to salinity in tomato. These findings provide novel insights into the action of plant-based biostimulants and highlight their potential as sustainable tools for improving crop performance under abiotic stress.