Silicon's defensive role against biotic and abiotic stress: a review

Silicon enhances developmental processes and controls the enzymatic and functional properties of plants. It is considered a quasi-element in the earth's crust, absorbed and translocated to aerial parts through transpiration. Silicon reduces various plant stress conditions, with plants displaying both direct and indirect defensive mechanisms. Indirect defense involves the release of volatiles that attract the pathogen's natural enemies, while morphological, biochemical, and molecular impediments constitute direct defense. Both mechanisms are strengthened by silicon treatment. Silicon enhances the polymerization of silicic acid in intercellular spaces and beneath cuticles as phytoliths, establishing a defense against pathogens. Silicon activates multiple pathways, encouraging the accumulation of supplementary metabolites, thereby enhancing plant defenses against abiotic and biotic challenges. It also contributes significantly to defensive mechanisms mediated by phytohormones. Studies show that silicon positively affects plants during severe stress by modifying several metabolites. Phytohormones are essential to crop plants' biochemical and physiological functions under unfavorable environmental circumstances. Frontline phytohormones, such as auxin, cytokinin, ethylene, gibberellin, salicylic acid, abscisic acid, brassinosteroids, and jasmonic acid regulate abiotic stress tolerance pathways intrinsically linked with silicon. This review highlights silicon's functionality in various biotic and abiotic stresses.