Elemental bioimaging of Na distribution in roots of Arabidopsis thaliana using laser ablation-ICP-MS under cold plasma conditions

Increasing soil salinity is a major problem in global crop production. The detrimental effect of saline soils on plant growth results from ionic toxicity of the Na+ ion in combination with osmotic perturbation. However, the mechanisms of Na uptake in roots, its transport to other tissues as well as the distribution of Na+ in specific plant tissues and cell layers are still not fully understood. The hyphenation of laser ablation (LA) and inductively coupled plasma mass spectrometry (ICP-MS) provides a well-suited tool for elemental bioimaging in plant tissues. Here, we present a novel LA-ICP-MS method which allows the generation of meaningful images of the Na distribution in Arabidopsis thaliana roots at the cellular level. For this purpose, a sample preparation protocol was developed which maintains the native Na distribution of the sample while avoiding contamination with Na+ from other sources. The ICP-MS was optimized to provide a high sensitivity for Na by applying cold plasma conditions. Plasma cooling was achieved by reducing the plasma power and delivering an additional wet aerosol to the ICP. By this means, the limit of detection was reduced by a factor of two in comparison with the standard mode. Images of the Na+ distribution in roots indicative of a distinctive accumulation of Na+ in defined A. thaliana root cell layers are thus obtained with a spatial resolution of down to 4 μm. Our analysis provides the first evidence for sharp Na concentration differences in roots, which are established between different cell layers and result in maximal Na accumulation in the cortex cells of the root organ.