Continuous monitoring of plant sodium transport dynamics using clinical PET

Abstract

Background: The absorption, translocation, accumulation and excretion of substances are fundamental processes in all organisms including plants, and have been successfully studied using radiotracers labelled with ¹¹C, ¹³N, ¹⁴C and ²²Na since 1939. Sodium is one of the most damaging ions to the growth and productivity of crops. Due to the significance of understanding sodium transport in plants, a significant number of studies have been carried out to examine sodium influx, compartmentation, and efflux using ²²Na- or ²⁴Na-labeled salts. Notably, however, most of these studies employed destructive methods, which has limited our understanding of sodium flux and distribution characteristics in real time, in live plants. Positron emission tomography (PET) has been used successfully in medical research and diagnosis for decades. Due to its ability to visualise and assess physiological and metabolic function, PET imaging has also begun to be employed in plant research. Here, we report the use of a clinical PET scanner with a ²²Na tracer to examine ²²Na-influx dynamics in barley plants (Hordeum vulgare L. spp. Vulgare-cultivar Bass) under variable nutrient levels, alterations in the day/night light cycle, and the presence of sodium channel inhibitors.

Results: 3D dynamic PET images of whole plants show readily visible ²²Na translocation from roots to shoots in each examined plant, with rates influenced by both nutrient status and channel inhibition. PET images show that plants cultivated in low-nutrient media transport more ²²Na than plants cultivated in high-nutrient media, and that ²²Na uptake is suppressed in the presence of a cation-channel inhibitor. A distinct diurnal pattern of ²²Na influx was discernible in curves displaying rates of change of relative radioactivity. Plants were found to absorb more ²²Na during the light period, and anticipate the change in the light/dark cycle by adjusting the sodium influx rate downward in the dark period, an effect not previously described experimentally.

Conclusions: We demonstrate the utility of clinical PET/CT scanners for real-time monitoring of the temporal dynamics of sodium transport in plants. The effects of nutrient deprivation and of ion channel inhibition on sodium influx into barley plants are shown in two proof-of-concept experiments, along with the first-ever 3D-imaging of the light and dark sodium uptake cycles in plants. This method carries significant potential for plant biology research and, in particular, in the context of genetic and treatment effects on sodium acquisition and toxicity in plants.

Keywords: 22Na transport dynamics; Barley; Diurnal transport; Inhibitor effect; Nutrient effect; Positron emission tomography.

Fig. 1

Dynamic images of Experiment 2 to capture ²²Na movement in linear scale. Images were scaled identically. Red arrow shows the reference source. At each time point, Voxel values of slices containing plants were summed to generate these images. Voxel size is 1.59 mm × 1.59 mm. In each timeframe, from left to right: high nutrient plants grouping without BaCl₂, high nutrient plants grouping with BaCl₂, low nutrient plants grouping without BaCl₂ and low nutrient grouping with BaCl₂

Fig. 2

Nutrient effect on ²²Na uptake in plants without BaCl₂. The red and blue curves denote high-nutrient and low-nutrient plants without BaCl₂, respectively. Dark shading indicates night times. Expt.1 Roots: a Radioactivity ratio. b Rate of change of the radioactivity ratio. Expt.2 Roots: c Radioactivity ratio. d Rate of change of the radioactivity ratio. Expt.1 Leaves: e Radioactivity ratio. f Rate of change of the radioactivity ratio. Expt.2 Leaves: g Radioactivity ratio. h Rate of change of the radioactivity ratio

Fig. 3

Nutrient effect on ²²Na uptake in plants with BaCl₂. The purple and green curves denote high-nutrient and low-nutrient plants with BaCl₂, respectively. Dark shading indicates night times. Expt.1 Roots: a Radioactivity ratio. b Rate of change of the radioactivity ratio. Expt.2 Roots: c Radioactivity ratio. d Rate of change of the radioactivity ratio. Expt.1 Leaves: e Radioactivity ratio. f Rate of change of the radioactivity ratio. Expt.2 Leaves: g Radioactivity ratio. h Rate of change of the radioactivity ratio

Fig. 4

BaCl₂ effect on ²²Na uptake in low-nutrient plants. The green and blue curves denote low-nutrient plants with and without BaCl₂, respectively. Dark shading indicates night times. Expt.1 Roots: a Radioactivity ratio. b Rate of change of the radioactivity ratio. Expt.2 Roots: c Radioactivity ratio. d Rate of change of the radioactivity ratio. Expt.1 Leaves: e Radioactivity ratio. f Rate of change of the radioactivity ratio. Expt.2 Leaves: g Radioactivity ratio. h Rate of change of the radioactivity ratio

Fig. 5

BaCl₂ effect on ²²Na uptake in high-nutrient plants. The purple and red curves denote high-nutrient plants with and without BaCl₂, respectively. Dark shading indicates night times. Expt.1 Roots: a Radioactivity ratio. b Rate of change of the radioactivity ratio. Expt.2 Roots: c Radioactivity ratio. d Rate of change of the radioactivity ratio. Expt.1 Leaves: e Radioactivity ratio. f Rate of change of the radioactivity ratio. Expt.2 Leaves: g Radioactivity ratio. h Rate of change of the radioactivity ratio

Fig. 6

Comparison of PET images reconstructed a without and b with CT-based Attenuation Correction. The images without correction contain significant inaccuracies in the voxel localized radioactivity measures

Fig. 7

Experimental setup. a Four plant setups in two larger containers with the reference source, b Experimental setup in the scanner with the light source on, c Siemens Biograph128 mCT PET/CT scanner

Fig. 8

Coronal plane CT slices through a stems, b leaves, and c a plane above the level of leaves. Voxel size is 0.98 mm × 0.98 mm

Fig. 9

High contrast PET projection images with Regions of Interest overlaid (red boxes) for a Experiment 1, and b Experiment 2. Projections are given by the summation of all voxel intensities in the through-plane direction. The ROIs are 3D rectangular prisms, with identical spatial extent in the through-plane direction. c Axial PET image overlaid on CT image