Smart delivery of auxin: Lignin nanoparticles promote efficient and safer vascular development in crop species

Xylem is a key tissue responsible for transporting water and nutrients while also providing mechanical support to plants. Its development and function are tightly regulated by plant growth regulators (PGRs), particularly auxins. Recent advances in nanotechnology have introduced promising strategies to modulate plant physiological processes, including the use of nanoparticles that enable controlled PGR release. Among these, lignin nanoparticles (LNPs) are of special interest due to their biodegradability and high encapsulation efficiency. Here, we investigated the effects of foliar application of indole-3-acetic acid (IAA), delivered either in free form and nanoencapsulated within LNPs (LNP–IAA), on xylem tissue development in cherry tomato (a eudicot) and wheat (a monocot). Treatments were applied at 10 and 25 days after sowing, followed by daily qualitative and quantitative assessments of xylem tissue using light microscopy on stem cross-sections. To track the presence of LNPs within vascular tissues, confocal microscopy with fluorochrome-labeled LNPs was employed. Quantitative data were analyzed using generalized linear models (GLMs), correlation analysis, and principal component analysis (PCA). Our results demonstrate that nanoencapsulation preserves and modulates auxin activity, reducing phytotoxic effects observed at higher concentrations. Notably, empty LNPs also elicited biological responses, indicating intrinsic effects of the nanocarrier itself. The responses were species-specific and concentration-dependent, with the highest efficacy observed at 0.05 µg mL⁻¹ in cherry tomato and 100 µg mL⁻¹ in wheat. Application frequency and plant developmental stage also significantly affected the uptake and utilization of the delivered compound. Importantly, we confirmed the presence of LNPs in stem xylem tissue as early as 1 hour after foliar application in both species. Overall, this study highlights the potential of LNPs as sustainable nanocarriers for targeted modulation of vascular development, opening new avenues for xylem bioengineering and growth optimization across divergent plant lineages. • Lignin nanoparticles were detected in stem tissues as early as 1 hour after foliar application, and in wheat they also accumulated in adventitious roots involved in absorption and transport. • The first foliar application accelarated vascular meristem differentiation by two days, whereas the second enhanced xylem activity, producing detectable anatomical changes within 8 hours. • In cherry tomato, low LNP–IAA concentrations markedly increased stem xylem area, whereas in wheat, higher concentrations were more effective in promoting vascular cylinder expansion. • Unloaded lignin nanoparticles exhibited distinct, species-dependent biological effects, likely related to their chemical composition.