Bioengineered silicon nanoparticles in agroecosystems: molecular mechanisms, stress tolerance, and environmental implications.

The increasing incorporation of engineered nanomaterials into agro-environmental systems requires a balanced evaluation of their biological efficacy and potential environmental implications. Silicon (Si) is a quasi-essential element, which is not only essential for plant survival but also supports their growth under adverse conditions. The large size and tendency to form complexes with inorganic and organic compounds in the growth medium, the bioavailability of Si to plants is limited. These restrictions can be overwhelmed through the application of silicon nanoparticles (SiNPs), which typically range from 1 to 100 nm and exhibit better mobility, solubility, and bioavailability. SiNPs can be synthesized by physical, chemical, and biological methods; however, the biological approach has garnered appreciable response due to multiple ecological benefits, biocompatibility, reduced energy requirements, and cost-effectiveness. Biosynthesized SiNPs are taken up by the plants as silicic acid via transporters viz., Ls1, Ls2, and Ls6, and/or by endocytosis depending upon their size, physicochemical properties, and plant species. Following internalization, SiNPs are predominantly transported to the epidermal cell wall, where it forms silica-cuticle double layer, providing resistance to biotic and abiotic stresses. Accumulation of SiNPs improves plants' water use efficiency, nutrient uptake and assimilation, photosynthetic performance, and antioxidant-based defense system, thereby stimulating plant growth and productivity under normal and stressed conditions such as drought, heat, salinity, and heavy metals stress. At molecular level, SiNPs demonstrated activation of signaling pathways by inducing transient and tightly regulated reactive oxygen species (ROS) burst, which activates phytohormones and MAPK signaling cascade, eventually triggering expression of stress-responsive genes. Biosynthesized SiNPs hold immense potential in field of agriculture, however, their actual deployment is at infancy. A lack of comprehensive understanding of SiNP mediated stress alleviation in plants at molecular level coupled with variability in response, subjected to plant species, and morphological and physicochemical attributes of SiNPs, limits their large-scale application. Thus, elucidating precise molecular mechanism of biosynthesized SiNPs-mediated stress tolerance in plants is crucial. Moreover, integrating advanced biological approaches with emerging technologies can facilitate rational and sustainable implementation of nano-enabled agricultural practices.