Phytoliths as Structural, Physiological Integrators of Multifunctional Defense and Climate Resilience in Grasses

Grasses (Poaceae) are the foremost silicon (Si) accumulators among terrestrial plants. Plants take up Si as monosilicic acid, Si(OH)4, which polymerizes in cells and tissues such as the epidermis, xylem and phloem to form phytogenic (biogenic) silica, SiO2 · nH2O, deposited as phytoliths. This review, based on a documented literature search with explicit inclusion and exclusion criteria, evaluates evidence that phytoliths protect grasses against herbivores, fungal and bacterial pathogens and nematodes, and against drought, heat, salinity, metal toxicity, UV-B radiation and mechanical stress, and it examines combined stresses under climate change. We distinguish phytolith-specific evidence from studies that supplied soluble Si or Si nanoparticles, and we grade the evidence for each proposed mechanism. Physical mechanisms (abrasion, mechanical and apoplastic barriers, reduced non-stomatal water loss) are the best supported. Biochemical and hormonal mechanisms, including jasmonic acid, salicylic acid and ethylene signaling, remain hypotheses because Si(OH)4 is chemically inert and much of the evidence is correlative. Elevated CO2 can lower foliar Si concentration in some grasses, which may weaken protection, but effects on total Si accumulation and function are poorly known. Phytolith-occluded carbon is a potential but uncertain carbon sink. We identify research priorities and practical implications for crop, pasture and carbon management.