Synergistic effects of ectoine and biostimulants combinations on tomato seedling growth and heat stress resilience

• Synergistic Biostimulants Boost Tomato Growth and Heat Tolerance, with HCE showing strongest effects. • Transcriptomics show biostimulants enhance photosynthesis, carbon fixation, and hormone signaling. • Combined biostimulants optimize tomato resilience and yield under heat stress, aiding sustainable agriculture. Biostimulants play a crucial role in promoting plant growth and stress tolerance. In this study, we investigated the effects of single and combined applications of ectoine (E), myo-inositol (MI), corn steep liquor (CSL), and hydrogen-rich water (HRW) on the growth and heat tolerance of tomato seedlings. Our results demonstrated that the combined application of these biostimulants significantly enhanced plant growth and physiological characteristics. Among the treatments, HCE (HRW + CSL + E ) exhibited the most pronounced effects on root growth, increasing root length, volume, and surface area, while E and HE (HRW + E ) demonstrated the highest phenotypic index and stress resilience under heat stress conditions. Transcriptomic analysis revealed significant differences in differentially expressed genes (DEGs) among treatments. The HCE treatment exhibited the highest number of DEGs, particularly in pathways related to photosynthesis, carbon fixation, plant hormone signal transduction. Functional enrichment analysis showed that H and E treatments were primarily involved in plant-pathogen interactions and hormone signaling, whereas HE and HCE treatments were associated with carbon and energy metabolism. The combined application of biostimulants demonstrated synergistic effects, enhancing multiple physiological pathways that contribute to plant heat stress adaptation. These findings provide valuable insights of the molecular mechanisms underlying biostimulant-induced growth promotion and stress resilience in tomato seedlings. The results highlight the potential of biostimulant combinations for optimizing crop production in controlled environments, offering a promising strategy for sustainable and high-efficiency agriculture.