Aluminum oxide nanoparticles affect the cell wall structure and lignin composition slightly altering the soybean...

Aluminum oxide (Al₂O₃) nanoparticles (NPs) are among the nanoparticles most used industrially, but their impacts on living organisms are widely unknown. We evaluated the effects of 50-1000 mg L^-1 Al₂O₃ NPs on the growth, metabolism of lignin and its monomeric composition in soybean plants. Al₂O₃ NPs did not affect the length of roots and stems. However, at the microscopic level, Al₂O₃ NPs altered the root surface inducing the formation of cracks near to root apexes and damage to the root cap. The results suggest that Al₂O₃ NPs were internalized and accumulated into the cytosol and cell wall of roots, probably interacting with organelles such as mitochondria. At the metabolic level, Al₂O₃ NPs increased soluble and cell wall-bound peroxidase activities in roots and stems but reduced phenylalanine ammonia-lyase activity in stems. Increased lignin contents were also detected in roots and stems. The Al₂O₃ NPs increased the p-hydroxyphenyl monomer levels in stems but reduced them in roots. The total phenolic content increased in roots and stems; cell wall-esterified p-coumaric and ferulic acids increased in roots, while the content of p-coumaric acid decreased in stems. In roots, the content of ionic aluminum (Al⁺³) was extremely low, corresponding to 0.0000252% of the aluminum applied in the nanoparticulate form. This finding suggests that all adverse effects observed were due to the Al₂O₃ NPs only. Altogether, these findings suggest that the structure and properties of the soybean cell wall were altered by the Al₂O₃ NPs, probably to reduce its uptake and phytotoxicity.