In vivo quantitative imaging of photoassimilate transport dynamics and allocation in large plants using a commercial positron emission tomography (PET) scanner.

Abhijit A Karve,Benjamin A Babst,Dohyun Kim,David Alexoff,Michael J Schueller,Richard A Ferrieri

doi:10.1186/s12870-015-0658-3

Abstract

BackgroundAlthough important aspects of whole-plant carbon allocation in crop plants (e.g., to grain) occur late in development when the plants are large, techniques to study carbon transport and allocation processes have not been adapted for large plants. Positron emission tomography (PET), developed for dynamic imaging in medicine, has been applied in plant studies to measure the transport and allocation patterns of carbohydrates, nutrients, and phytohormones labeled with positron-emitting radioisotopes. However, the cost of PET and its limitation to smaller plants has restricted its use in plant biology. Here we describe the adaptation and optimization of a commercial clinical PET scanner to measure transport dynamics and allocation patterns of 11C-photoassimilates in large crops.ResultsBased on measurements of a phantom, we optimized instrument settings, including use of 3-D mode and attenuation correction to maximize the accuracy of measurements. To demonstrate the utility of PET, we measured 11C-photoassimilate transport and allocation in Sorghum bicolor, an important staple crop, at vegetative and reproductive stages (40 and 70 days after planting; DAP). The 11C-photoassimilate transport speed did not change over the two developmental stages. However, within a stem, transport speeds were reduced across nodes, likely due to higher 11C-photoassimilate unloading in the nodes. Photosynthesis in leaves and the amount of 11C that was exported to the rest of the plant decreased as plants matured. In young plants, exported 11C was allocated mostly (88 %) to the roots and stem, but in flowering plants (70 DAP) the majority of the exported 11C (64 %) was allocated to the apex.ConclusionsOur results show that commercial PET scanners can be used reliably to measure whole-plant C-allocation in large plants nondestructively including, importantly, allocation to roots in soil. This capability revealed extreme changes in carbon allocation in sorghum plants, as they advanced to maturity. Further, our results suggest that nodes may be important control points for photoassimilate distribution in crops of the family Poaceae. Quantifying real-time carbon allocation and photoassimilate transport dynamics, as demonstrated here, will be important for functional genomic studies to unravel the mechanisms controlling phloem transport in large crop plants, which will provide crucial insights for improving yields.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0658-3) contains supplementary material, which is available to authorized users.

Highlights

Important aspects of whole-plant carbon allocation in crop plants occur late in development when the plants are large, techniques to study carbon transport and allocation processes have not been adapted for large plants
In order to simultaneously administer 11CO2 to the plants and study 11C transport and allocation with a commercial Positron emission tomography (PET) scanner we developed a portable handheld 11CO2 pulsing system to deliver 11CO2 from the cyclotron to the PET scanner [22], and an externally illuminated plexiglass chamber that enclosed the whole plant to maintain environmental control, to keep the plant in position as it moved through the PET scanner, and to provide secondary containment of 11CO2, much like a fume hood (Fig. 1)
In large grasses like sorghum the aerial part of the plant consists of a cylindrical stem whose diameter is about 3 cm, leaving most of the imaging field of view (FOV) filled with air

Summary

Introduction

Important aspects of whole-plant carbon allocation in crop plants (e.g., to grain) occur late in development when the plants are large, techniques to study carbon transport and allocation processes have not been adapted for large plants. It has been shown that growing plants under elevated atmospheric CO2 initially results in higher photosynthetic rates, but eventually is followed by a down-regulation of photosynthetic activity presumably due to the negative feedback resulting from inherently limited sink capacity [9,10,11] Based on these observations it has been hypothesized that maintenance of high photosynthetic rate is dependent on the rate of carbon utilization and/or capacity for carbon storage of sink tissues. The cross-talk between the source and sink is facilitated in part by the regulation of phloem transport from source to sink, we still do not fully understand the mechanisms underlying phloem transport In part, this has been due to limited availability of technologies to measure phloem transport dynamics (e.g., photoassimilate transport velocity). Improving our understanding of the mechanisms that drive phloem transport may lead to new approaches for manipulating photoassimilate allocation

Methods

Results

Conclusion