Lignin-induced growth inhibition in soybean exposed to iron oxide nanoparticles

Plants are occasionally exposed to environmental perturbations that limit their growth. One of these perturbations is the exposure to and interaction with various nanoparticles (NPs) that are discarded continuously into the environment. Hitherto, no study has been carried out evaluating the effects of iron oxide (γ-Fe ₂O ₃) NPs on soybean growth and lignin formation, as proposed herein. For comparative purposes, we also submitted soybean plants to non-nanoparticulate iron (FeCl ₃). Exposure of the plants to γ-Fe ₂O ₃ NPs increased cell wall-bound peroxidase (POD) activity but decreased phenylalanine ammonia lyase (PAL) activity due, probably, to the negative feedback of accumulated phenolic compounds. In contrast, FeCl ₃ decreased cell wall-bound POD activity. Both γ-Fe ₂O ₃ NPs and FeCl ₃ increased the lignin content of roots and stems. However, significant lignin-induced growth inhibition was noted only in stems after exposure to NPs, possibly due to changes in lignin monomer composition. In this case, γ-Fe ₂O ₃ NPs decreased the guaiacyl monomer content of roots but increased that of stems. The high levels of monomer guaiacyl in stems resulting from the action of γ-Fe ₂O ₃ NPs decreased syringyl/guaiacyl ratios, generating more highly cross-linked lignin followed by the stiffening of the cell wall and growth inhibition. In contrast, FeCl ₃ increased the contents of monomers p-hydroxyphenyl and syringyl in roots. The observed increase in the syringyl/guaiacyl ratio in plant roots submitted to FeCl ₃ agrees with the lack of effect on growth, due to the formation of a less condensed lignin. In brief, we here describe that γ-Fe ₂O ₃ NPs and FeCl ₃ act differently in soybean plants.