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Tracer Technologies

The transport and allocation of molecules such as photoassimilates are critical for survival and performance of plants. Short lived radiotracers can be administered non-invasively to the plant and due to their radioactive properties, the radiotracer can be detected non-invasively from outside the plant, even if they are located deep within the plant body or within roots buried in the soil. This allows conclusions on transport properties and current allocation patterns in the plant.
For the detection of radiotracers we use two groups of instruments with different advantages and challenges:

Scintillation detectors

The basic approach consists of scintillation detectors with subsequent electronics placed along a linear transport pathway (e.g. photoassimilates within a stem) that will detect radioactivity from a source site to a sink (e.g. 11CO2 labelled leaf to root). From the radioactivity measured over time, parameters such as loss along the transport pathway and flow velocity can be calculated using a mathematical model (Bühler et al., 2014).

Positron Emission Tomography (PET)

PET allows for detection and three dimensional (3D) mapping of positron emitting radionuclides (e.g. 11C, 15O, 13N). The carbon isotope 11C allows for characterizing transport and allocation of photoassimilates in a dynamic manner for studying developmental changes and plant responses to environmental cues (Jahnke et al., 2009, de Schepper et al., 2013). Currently, three plant dedicated PET systems are in (or will come into) operation at IBG-2:

PlanTIS (Plant Tomographic Imaging System) is suited for small plant samples with a maximum field-of-view (FOV) of 65mm in diameter and a height of 100mm (Jahnke et al., 2009; Beer et al., 2010).

phenoPET includes last generation digital photon counter technology (Streun et al., 2014) and integrates all corrections for quantification of radioactivity in 3D. The system provides a FOV of 180mm in diameter and 190mm in height and will have much higher detection sensitivity than PlanTIS.

For assigning PET signal to individual roots or structures within belowground organs it is routinely co-registered with MRI (Fig. 1).

MRI-PETFigure 1 3D co-registration of MRI (grey) and PET (color) images of a sugar beet plant. The PET signal (relative distribution of radioactivity) is shown at different times after tracer application (pulse of 11CO2 on shoot level). Scale bars = 1cm (adapted from Jahnke et al. 2009)

Mathematical modeling for quantitative data analysis of tracer transport

Data obtained from tracer transport experiments typically need model based data analysis to retrieve a quantitative characterization of transport properties. For this purpose we developed compartmental tracer transport models (Bühler et al. 2014) which describe axial convection and diffusion as well as exchange of tracer between compartments, and are defined by partial differential equations (PDEs). Depending on the specific experimental situation, different models are used to fit the data. Special attention is on highly efficient implementation of the numerical procedures to solve the PDEs (Bühler et al. 2017). In our further development we focus on automated data processing and model based experimental design of PET measurements (Bühler et al. 2018).

 Fig. 2 Steps of data analysis of tracer transport experiments: 3D PET images are pre-processed to obtain time series of spatial tracer distribution; different compartmental models are fitted to these data; the best fit is used to characterize the transport properties of the plant.

Selected Publications

Jahnke, S., Menzel, M.I., Van Dusschoten, D., Roeb, G.W., Bühler, J., Minwuyelet, S., et al. (2009) Combined MRI-PET dissects dynamic changes in plant structures and functions. Plant Journal 59, 634-644. doi: 10.1111/j.1365-313X.2009.03888.x

Beer, S., Streun, M., Hombach, T., Buehler, J., Jahnke, S., Khodaverdi, M., et al. (2010) Design and initial performance of PlanTIS: a high-resolution positron emission tomograph for plants. Physics in Medicine and Biology 55, 635-646. doi: 10.1088/0031-9155/55/3/006.

De Schepper, V., Bühler, J., Thorpe, M., Roeb, G., Huber, G., Van Dusschoten, D., Jahnke, S., Steppe, K. (2013) 11C-PET imaging reveals transport dynamics and sectorial plasticity of oak phloem after girdling. Frontiers in Plant Science 4:200. doi: 10.3389/fpls.2013.00200

Bühler, J. von Lieres. E., Huber, G. (2014) A class of compartmental models for long-distance tracer transport in plants. Journal of Theoretical Biology 341: 131-142.

Bühler, J., Huber, G., von Lieres. E. (2017) Finite volume schemes for the numerical simulation of tracer transport in plants. Mathematical Biosciences 288: 14-20

Bühler, J., von Lieres. E., Huber, G. (2018) Model-based design of long-distance tracer transport experiments in plants. Frontiers in Plant Science 9: 773. doi: 10.3389/fpls.2018.00773