The Role of CAX1 and CAX3 in Elemental Distribution and Abundance in Arabidopsis Seed

The ability to alter nutrient partitioning within plants cells is poorly understood. In Arabidopsis (Arabidopsis thaliana), a family of endomembrane cation exchangers (CAXs) transports Ca2+ and other cations. However, experiments have not focused on how the distribution and partitioning of calcium (Ca) and other elements within seeds are altered by perturbed CAX activity. Here, we investigate Ca distribution and abundance in Arabidopsis seed from cax1 and cax3 loss-of-function lines and lines expressing deregulated CAX1 using synchrotron x-ray fluorescence microscopy. We conducted 7- to 10-μm resolution in vivo x-ray microtomography on dry mature seed and 0.2-μm resolution x-ray microscopy on embryos from lines overexpressing deregulated CAX1 (35S-sCAX1) and cax1cax3 double mutants only. Tomograms showed an increased concentration of Ca in both the seed coat and the embryo in cax1, cax3, and cax1cax3 lines compared with the wild type. High-resolution elemental images of the mutants showed that perturbed CAX activity altered Ca partitioning within cells, reducing Ca partitioning into organelles and/or increasing Ca in the cytosol and abolishing tissue-level Ca gradients. In comparison with traditional volume-averaged metal analysis, which confirmed subtle changes in seed elemental composition, the collection of spatially resolved data at varying resolutions provides insight into the impact of altered CAX activity on seed metal distribution and indicates a cell type-specific function of CAX1 and CAX3 in partitioning Ca into organelles. This work highlights a powerful technology for inferring transport function and quantifying nutrient changes.