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

Aluminum oxide (AlO) 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 AlO NPs on the growth, metabolism of lignin and its monomeric composition in soybean plants. AlO NPs did not affect the length of roots and stems. However, at the microscopic level, AlO NPs altered the root surface inducing the formation of cracks near to root apexes and damage to the root cap. The results suggest that AlO NPs were internalized and accumulated into the cytosol and cell wall of roots, probably interacting with organelles such as mitochondria. At the metabolic level, AlO 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 AlO 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) 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 AlO NPs only. Altogether, these findings suggest that the structure and properties of the soybean cell wall were altered by the AlO NPs, probably to reduce its uptake and phytotoxicity.