Competition between plant and bacterial cells at the microscale regulates the dynamics of nitrogen acquisition in wheat (Triticum aestivum)

The ability of plants to compete effectively for nitrogen (N) resources is critical to plant survival. However, controversy surrounds the importance of organic and inorganic sources of N in plant nutrition because of our poor ability to visualize and understand processes happening at the root-microbial-soil interface. Using high-resolution nano-scale secondary ion mass spectrometry stable isotope imaging (NanoSIMS-SII), we quantified the fate of ¹⁵N over both space and time within the rhizosphere. We pulse-labelled the soil surrounding wheat (Triticum aestivum) roots with either ¹⁵NH₄⁺ or ¹⁵N-glutamate and traced the movement of ¹⁵N over 24 h. Imaging revealed that glutamate was rapidly depleted from the rhizosphere and that most ¹⁵N was captured by rhizobacteria, leading to very high ¹⁵N microbial enrichment. After microbial capture, approximately half of the ¹⁵N-glutamate was rapidly mineralized, leading to the excretion of NH₄⁺, which became available for plant capture. Roots proved to be poor competitors for ¹⁵N-glutamate and took up N mainly as ¹⁵NH₄⁺. Spatial mapping of ¹⁵N revealed differential patterns of ¹⁵N uptake within bacteria and the rapid uptake and redistribution of ¹⁵N within roots. In conclusion, we demonstrate the rapid cycling and transformation of N at the soil-root interface and that wheat capture of organic N is low in comparison to inorganic N under the conditions tested.

Application of nanoscale secondary ion mass spectrometry to plant cell research

Imaging resource flow in soil-plant systems remains central to understanding plant development and interactions with the environment. Typically, subcellular resolution is required to fully elucidate the compartmentation, behavior, and mode of action of organic compounds and mineral elements within plants. For many situations this has been limited by the poor spatial resolution of imaging techniques and the inability to undertake studies in situ. Here we demonstrate the potential of Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), which is capable of the quantitative high-resolution spatial imaging of stable isotopes (e.g. (12) C, (13) C, (14) N, (15) N, (16) O, (18) O, (31) P, (34) S) within intact plant-microbial-soil systems. We present examples showing how the approach can be used to investigate competition for (15) N-labeled nitrogen compounds between plant roots and soil microorganisms living in the rhizosphere and the spatial imaging of (31) P in roots. We conclude that NanoSIMS has great potential to elucidate the flow of isotopically-labeled compounds in complex media (e.g. soil) and opens up countless new opportunities for studying plant responses to abiotic stress (e.g. (18) O3, elevated (13) CO2), signal exchange, nutrient flow and plant-microbial interactions.

In situ mapping of nutrient uptake in the rhizosphere using nanoscale secondary ion mass spectrometry

Plant roots and microorganisms interact and compete for nutrients within the rhizosphere, which is considered one of the most biologically complex systems on Earth. Unraveling the nitrogen (N) cycle is key to understanding and managing nutrient flows in terrestrial ecosystems, yet to date it has proved impossible to analyze and image N transfer in situ within such a complex system at a scale relevant to soil-microbe-plant interactions. Linking the physical heterogeneity of soil to biological processes marks a current frontier in plant and soil sciences. Here we present a new and widely applicable approach that allows imaging of the spatial and temporal dynamics of the stable isotope (15)N assimilated within the rhizosphere. This approach allows visualization and measurement of nutrient resource capture between competing plant cells and microorganisms. For confirmation we show the correlative use of nanoscale secondary ion mass spectrometry, and transmission electron microscopy, to image differential partitioning of (15)NH(4)(+) between plant roots and native soil microbial communities at the submicron scale. It is shown that (15)N compounds can be detected and imaged in situ in individual microorganisms in the soil matrix and intracellularly within the root. Nanoscale secondary ion mass spectrometry has potential to allow the study of assimilatory processes at the submicron level in a wide range of applications involving plants, microorganisms, and animals.

A novel method for the study of the biophysical interface in soils using nano-scale secondary ion mass spectrometry

The spatial location of microorganisms and their activity within the soil matrix have major impacts on biological processes such as nutrient cycling. However, characterizing the biophysical interface in soils is hampered by a lack of techniques at relevant scales. A novel method for studying the distribution of microorganisms that have incorporated isotopically labelled substrate ('active' microorganisms) in relation to the soil microbial habitat is provided by nano-scale secondary ion mass spectrometry (NanoSIMS). Pseudomonas fluorescens are ubiquitous in soil and were therefore used as a model for 'active' microorganisms in soil. Batch cultures (NCTC 10038) were grown in a minimal salt medium containing 15N-ammonium sulphate (15/14N ratio of 1.174), added to quartz-based white sand or soil (coarse textured sand), embedded in Araldite 502 resin and sectioned for NanoSIMS analysis. The 15N-enriched P. fluorescens could be identified within the soil structure, demonstrating that the NanoSIMS technique enables the study of spatial location of microbial activity in relation to the heterogeneous soil matrix. This technique is complementary to the existing techniques of digital imaging analysis of soil thin sections and scanning electron microscopy. Together with advanced computer-aided tomography of soils and mathematical modelling of soil heterogeneity, NanoSIMS may be a powerful tool for studying physical and biological interactions, thereby furthering our understanding of the biophysical interface in soils.