Effect of CeO ₂ nanomaterial surface functional groups on tissue and subcellular distribution of Ce in tomato (Solanum lycopersicum)

Using recent advances in X-ray microscopy, this study aimed to elucidate mechanisms of uptake, subcellular distribution, and translocation of functionalized CeO ₂ MNM (manufactured nanomaterials), having different charges, by tomato plants (Solanum lycopersicum cv Micro-Tom). We found that plant growth and Ce concentration in tissues were functions of surface charge and exposure concentration with root to shoot translocation being much greater for negatively charged CeO ₂ than positive or neutral CeO ₂ . Mechanisms of entry into roots and translocation within plants were examined using X-ray nano- and microprobes. There were dramatic differences in the tissue and subcellular distributions of Ce in plant roots exposed to dextran-coated CeO ₂ nanoparticles conjugated with positive, neutral and negative functional groups. Positively charged CeO ₂ remained mainly bound to the epidermis of the root with little present in the apoplast or cytoplasm. Negatively charged CeO ₂ was found in the cytoplasm throughout the root cross section, and negatively charged CeO ₂ was found within the apoplast in the cortex and both the apoplast and the cytoplasm in the vasculature. Neutral CeO ₂ likely entered through the gaps between epidermal cells being sloughed off during root growth and penetrated deeper into the interior of the roots (vasculature) via a combination of apoplastic and symplastic transport. Evidence of symplastic Ce transport was observed with the neutrally and negatively charged particles. We observed evidence of endocytosis as the mechanism for entry into the symplast allowing for entry into the xylem. This study provides critical information on how particle surface chemistry influences the biodistribution and cellular localization of nanomaterials in plants and is to date the highest resolution X-ray imaging of nanomaterials in plant cells.