Early development of apoplastic barriers and molecular mechanisms in juvenile maize roots in response to LaO nanoparticles.

The increasing occurrence of engineered nanoparticles (NPs) in soils may decrease water uptake in crops, followed by lower crop yield and quality. As one of the most common rare earth oxide NPs, lanthanum oxide (LaO) NPs may inhibit the relative expressions of aquaporin genes, thus reduce water uptake. In the present study, maize plants were exposed to different LaO NPs concentrations (0, 5, 50 mg L) for 72 h and 144 h right after the first leaf extended completely. Our results revealed that water uptake was reduced by LaO NPs through accelerating the development of apoplastic barriers in maize roots. The level of abscisic acid, determined by using ultra high performance liquid chromatography-tandem mass spectrometry, was increased upon LaO NPs exposure. Furthermore, ZmPAL, ZmCCR2 and ZmCAD6, the core genes specific for biosynthesis of lignin, were up-regulated by 3-13 fold in roots exposed to 50 mg L LaO NPs. However, ZmF5H was suppressed, indicating that lignin with S units could be excluded for the formed lignin in apoplastic barriers upon LaO NPs exposure. The level of essential component of apoplastic barriers - lignin was increased by 1.5-fold. The early development of apoplastic barriers was observed, and stomatal conductance and transpiration rate of LaO NPs-treated plants were significantly decreased by 63%-83% and 42%-86%, respectively, as compared to the control. The lignin enriched apoplastic barriers in juvenile maize thus led to the reduction of water uptake, subsequently causing significant growth inhibition. This is the first study to show early development of root apoplastic barriers upon LaO NPs exposure. This study will help us better understand the function of apoplastic barriers in roots in response to NPs, further providing fundamental knowledge to develop safer and more efficient agricultural nanotechnology.