Abstract
Endophytic fungi have emerged as vital allies in enhancing plant resilience to abiotic stresses, offering significant potential for climate-smart agriculture (CSA). This mechanistic review synthesises physiological, biochemical, and molecular pathways through which these symbionts mitigate stress, emphasising mechanistic understanding over descriptive diversity. Key mechanisms include osmotic regulation, ion homeostasis, antioxidant defence, hormonal modulation, and epigenetic reprogramming. As demonstrated by studies on Trichoderma harzianum, Fusarium solani, and Piriformospora indica, such interactions allow plants to sustain photosynthesis, nutrient uptake, and growth when subjected to drought, salinity, heat, and heavy metal stress. Comparative insights highlighted lineage-specific strategies: Ascomycota display broad-spectrum regulation through metabolite production and hormonal control; Basidiomycota specialise in root-fungus signalling and resource acquisition; and Zygomycota contribute primarily to nutrient mobilisation and rapid colonisation. Collectively, these insights reveal that endophytes act as "hidden regulators" of plant stress resilience. Integration of fungal symbionts into CSA practices holds considerable potential for improving productivity, adaptation, and mitigation, particularly in stress-prone agroecosystems. Yet, as several sources have argued, key gaps remain including field performance is inconsistent across host genotypes, beneficial and pathogenic traits sometimes overlap, and inoculant mass production is still limited. Addressing these challenges will necessitate omics-driven innovations, efficient delivery methods, improved relationships between empirical biological research and on-farm application, and policy frameworks. Overall, this synthesis highlights endophytic fungi as essential partners in building resilient and sustainable food systems under changing climates.