Synergistic role of Fusarium solani IK-105 and humic acid in combating lead stress in tomato plants through physiological, biochemical and molecular modulation

• The study explores the combined effects of Fusarium solani IK-105 and humic acid (HA) in alleviating lead (Pb) stress in tomato plants. • IK-105 was characterized for plant growth-promoting traits and Pb resistance, showing potential for stress mitigation. • IK-105 and HA treatments improved plant growth, biomass, photosynthetic pigments, and physiological performance under Pb stress. • Pb uptake was restricted, while calcium (Ca) and magnesium (Mg) accumulation increased, enhancing stress tolerance. • Combined treatments modulated stress-related genes and phytohormone pathways, contributing to improved Pb stress resilience. To promote sustainable agriculture and eco-friendly agricultural practices, beneficial microorganisms and organic amendments like humic acid (HA) have been effectively used to enhance tolerance for heavy metals (HMs) in crops. In this study, we aim to isolate, identify, and characterize a novel endophytic fungal strain that exhibits significant plant growth-promoting (PGP) properties and has the potential to alleviate lead (Pb) toxicity in tomato plants. Fusarium solani IK-105 was selected for the current study from the 13 different fungal isolates due to its highest resistance to Pb stress and significant PGP traits, validated through various in vitro analyses. Pb stress severely disrupts the morphological, physiological, and growth attributes of tomato plants. However, the application of IK-105 and HA, particularly in combination, effectively mitigates the adverse effects of Pb stress by improving leaf area, water retention, and membrane stability. These treatments also enhance shoot length and weight by 34.79 % and 4.26 %, and root length and weight by 62.22 % and 5.4 %, respectively, under Pb stress compared to their non-stressed counterparts. Photosynthetic pigments, protein, sugar, and starch contents were significantly enhanced, while enzymatic and non-enzymatic antioxidants were significantly reduced with the application of IK-105 and HA under Pb stress conditions. Application of IK-105 and HA treatments significantly reduced endogenous abscisic acid (ABA), restricted Pb uptake, and enhanced Ca and Mg levels in tomato plants. These treatments modulated the expression of genes related to phytohormones and other signaling molecules associated with HMs stress. The findings of this study revealed the potential of IK-105 and HA as sustainable solutions to mitigate the environmental impacts of HMs, promote eco-friendly agriculture practices, and contribute to the remediation of contaminated regions.