11C-PET imaging reveals transport dynamics and sectorial plasticity of oak phloem after girdling

Carbon transport processes in plants can be followed non-invasively by repeated application of the short-lived positron-emitting radioisotope (11)C, a technique which has rarely been used with trees. Recently, positron emission tomography (PET) allowing 3D visualization has been adapted for use with plants. To investigate the effects of stem girdling on the flow of assimilates, leaves on first order branches of two-year-old oak (Quercus robur L.) trees were labeled with (11)C by supplying (11)CO2-gas to a leaf cuvette. Magnetic resonance imaging gave an indication of the plant structure, while PET registered the tracer flow in a stem region downstream from the labeled branches. After repeated pulse labeling, phloem translocation was shown to be sectorial in the stem: leaf orthostichy determined the position of the phloem sieve tubes containing labeled (11)C. The observed pathway remained unchanged for days. Tracer time-series derived from each pulse and analysed with a mechanistic model showed for two adjacent heights in the stem a similar velocity but different loss of recent assimilates. With either complete or partial girdling of bark within the monitored region, transport immediately stopped and then resumed in a new location in the stem cross-section, demonstrating the plasticity of sectoriality. One day after partial girdling, the loss of tracer along the interrupted transport pathway increased, while the velocity was enhanced in a non-girdled sector for several days. These findings suggest that lateral sugar transport was enhanced after wounding by a change in the lateral sugar transport path and the axial transport resumed with the development of new conductive tissue.We thank the Research Foundation – Flanders (FWO) for the PhD funding granted to Veerle De Schepper, the scientific research committee (CWO) of the Faculty of bioscience engineering (UGent) to support the research visit of Veerle De Schepper at the Forschungszentrum Jülich and the Special Research Fund (B.O.F.) of Ghent University for the post-doc funding granted to Veerle De Schepper

Observation of root shrinkage and reduced root water uptake

Observation of root shrinkage and reduced root water uptakeDagmar van Dusschoten and Yannik MüllersIBG-2, Plant Sciences, Forschungszentrum Jülich, Germany.The importance of root-soil water contact was recognized in the 1970s and ‘80s as a main factor in the efficiency of root water uptake (RWU). Several studies indicated that roots shrink when the soil becomes dry while even exhibiting a day-night rhythm [1]. However, over the years the main focus had shifted to the reduction of the soil water potential upon drying and the accompanying reduction of the soil water conductivity to explain reduced RWU upon soil drying. Also, considerable efforts have been made to identify the importance of aquaporins for root conductivity. Recently, it was demonstrated in an indirect manner that root shrinkage is indeed a main factor in water uptake efficiency [2]. Based on this we devised some experiments to investigate this topic with two complementary, non-invasive technologies which observe both root function and root structure.We performed time series of root water uptake measurements on three different plant species (Zea mays, Vicia faba and Helianthus annuus) using the Soil Water Profiler (SWaP) [3] while the plants were depleting available soil water, yielding a soil water content gradient with most of the water remaining at the bottom. Simultaneously, the profile of RWU shifted to deeper soil layers as particularly the upper soil layers became drier. After this series the root system was scanned using Magnetic Resonance Imaging (MRI) two times, once directly after the SWaP measurements and once after re-watering. This combination of MRI measurements showed that roots themselves had lost considerable amounts of water during soil water depletion which was more pronounced at the top than at the bottom. The uptake pattern by the roots also showed that uptake is primarily reduced in the root zones exhibiting the largest water loss. This indicates a strong correlation between the two observations and is a good indicator that root-soil contact is important. Root diameter reduction clearly is an under-researched topic which deserves more attention both from an experimental and a modeling point of view. [1] M.G. Huck, B. Klepper and H.M. Taylor, Plant Physiol. (1970), 45. [2] C.M. Rodriguez‐Dominguez, T.J. Brodribb, New Phytol. (2020) 225. [3] D. van Dusschoten et. al, Plant Physiol (2020), 183

Imaging belowground dynamics with MRI and PET

The development of a root system which is adequate for the respective growing conditions of a plant is critical for survival, performance and yield. Furthermore for “Root Crops” where the yield-relevant organ is developing belowground, the processes leading to amount and quality of the product at harvest also happen among the plants hidden half. The opaque nature of soil prevents direct observation and while a number of approaches for observing 2D root development such as rhizotrons have been applied successfully, roots naturally develop in interaction with the 3D soil environment and form complex 3D structures. Therefore the ability to deep-phenotype the 3D structure and function of roots and other belowground structures non-invasively yields a high potential for gaining new insights into root development, its regulation and responses to stress. Magnetic resonance imaging (MRI) is a technique that allows for visualization and quantification of root system architecture traits in soil such as root length and mass but also of internal structures of belowground storage organs. Positron emission tomography (PET) using short-lived radiotracer 11CO2 provides additional imaging of the photoassimilate distribution and flow characteristics can be extracted with a model-based analysis. We show here application of both techniques for visualization and quantification of root system architecture, anatomy and photoassimilate allocation

Investigating belowground dynamics with Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET)

Investigating belowground dynamics with Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET)Ralf Metzner*, Dagmar van Dusschoten and Siegfried JahnkeInstitute of Bio- and Geosciences IBG 2: Plant Sciences, Forschungszentrum Jülich GmbH, Germany*Presenting author: [email protected] development of a root system adequate for supplying a plant with water and nutrients under dynamic growing conditions is critical for survival, performance and yield. Particularly for “Root Crops” where the storage organs are developing belowground, the dynamics of the carbon storage of the roots are also highly relevant. The opaque nature of soil prevents direct observation and while a number of approaches for observing 2D root development such as rhizotrons have been applied successfully, roots naturally develop in interaction with the 3D soil environment and form themselves complex 3D structures. Therefore the ability to deep-phenotype the 3D structure and function of roots and other belowground structures non-invasively yields a high potential for gaining new insights into root development, its regulation and responses to stress. Here we present two approaches that allow this kind of investigation: Magnetic resonance imaging (MRI) allows for visualization and quantification of root system architecture traits in soil such as root length and mass, but also of internal structures of storage organs. Positron emission tomography (PET) using short-lived radiotracer 11C provides additional 3D imaging of the photoassimilate distribution. Photoassimilate flow characteristics can be extracted from these data with a model-based analysis. We show here application of both techniques for repeated visualization and quantification of root system architecture, anatomy and photoassimilate allocation of a number of species and developmental stages, including barley, pea and sugar beet