Endodermal suberin restricts root leakage of cesium: a suitable tracer for potassium.

An urgent challenge within crop production is to maintain productivity in a world plagued by climate change and its associated plant stresses, such as heat, drought and salinity. A key factor in this endeavor is to understand the dynamics of root suberization, and its role in plant-water relations and nutrient transport. This study focuses on the hypothesis that endodermal suberin, acts as a physical barrier preventing radial potassium (K) movement out of the vascular tissues during translocation. Previous attempts to experimentally support this idea have produced inconsistent results. We developed a Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) method, allowing us to visualize the distribution of mineral elements and track K movement. Cesium (Cs), dosed in optimized concentrations, was found to be an ideal tracer for K, due to its low background and similar chemical/biological properties. In suberin mutants of Arabidopsis thaliana, we observed a positive correlation between suberin levels and K translocation efficiency, indicating that suberin enhances the plant's ability to retain K within the vascular tissues during translocation from root to shoot. In barley (Hordeum vulgare), fully suberized seminal roots maintained higher K concentrations in the stele compared to younger, less suberized root zones. This suggests that suberization increases with root maturity, enhancing the barrier against K leakage. In nodal roots, suberin was scattered towards the phloem in mature root zones. Despite this incomplete suberization, nodal roots still restrict outward K movement, demonstrating that even partial suberin barriers can significantly reduce K loss. Our findings provide evidence that suberin is a barrier to K leakage during root-to-shoot translocation. This understanding is crucial to maintain crop productivity in the face of climate change.

Application of ZnO Nanoparticles Encapsulated in Mesoporous Silica on the Abaxial Side of a Solanum lycopersicum Leaf Enhances Zn Uptake and Translocation via the Phloem.

Foliar application of nutrient nanoparticles (NPs) is a promising strategy for improving fertilization efficiency in agriculture. Phloem translocation of NPs from leaves is required for efficient fertilization but is currently considered to be feasible only for NPs smaller than a cell wall pore size exclusion limit of <20 nm. Using mass spectrometry imaging, we provide here the first direct evidence for phloem localization and translocation of a larger (∼70 nm) fertilizer NP comprised of ZnO encapsulated in mesoporous SiO2 (ZnO@MSN) following foliar deposition. The Si content in the phloem tissue of the petiole connected to the dosed leaf was ∼10 times higher than in the xylem tissue, and ∼100 times higher than the phloem tissue of an untreated tomato plant petiole. Direct evidence of NPs in individual phloem cells has only previously been shown for smaller NPs introduced invasively in the plant. Furthermore, we show that uptake and translocation of the NPs can be enhanced by their application on the abaxial (lower) side of the leaf. Applying ZnO@MSN to the abaxial side of a single leaf resulted in a 56% higher uptake of Zn as well as higher translocation to the younger (upper) leaves and to the roots, than dosing the adaxial (top) side of a leaf. The higher abaxial uptake of NPs is in alignment with the higher stomatal density and lower density of mesophyll tissues on that side and has not been demonstrated before.

What is missing to advance foliar fertilization using nanotechnology?

An urgent challenge within agriculture is to improve fertilizer efficiency in order to reduce the environmental footprint associated with an increased production of crops on existing farmland. Standard soil fertilization strategies are often not very efficient due to immobilization in the soil and losses of nutrients by leaching or volatilization. Foliar fertilization offers an attractive supplementary strategy as it bypasses the adverse soil processes, but implementation is often hampered by a poor penetration through leaf barriers, leaf damage, and a limited ability of nutrients to translocate. Recent advances within bionanotechnology offer a range of emerging possibilities to overcome these challenges. Here we review how nanoparticles can be tailored with smart properties to interact with plant tissue for a more efficient delivery of nutrients.