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Environmental Transformations and Interactions

Molecular Factors are Key for Allocating Carbon toward Poplar Roots

New research describes how demands from plant roots affect source-to-sink carbon relationships mediated by a ubiquitous hormone in poplar trees.


As part of a recent study, scientists examined relationships between carbon source (leaves) and sinks (roots) at the molecular level by applying synthetic auxins to poplar foliage. (iStock | mahiruysal)

The Science 

A key part of tissue growth and biomass productivity in plants is their ability to shuttle carbon to specific parts of a plant, thereby enabling those parts to become a carbon sink (e.g., roots). Even so, the molecules and biomolecular processes that drive plants, and particularly trees, to create sinks for carbon is not well understood. Specifically, the molecular drivers of sink strength in root tissues are not identified. Because auxin—an important plant hormone for growth, reproduction, and lifespan—is central to carbohydrate transport toward carbon sinks in plant tissues, it was used as a tool to study carbon sink strength in root tissues. By using auxin modulators to evaluate how changes in auxin function affected root growth and carbon allocation, new research has shown that several molecular factors likely play vital roles in sink strength. 

The Impact 

Poplar trees are used extensively as a feedstock for biofuels and are the highest biomass-productive tree native to the Northern Hemisphere. They can also separately control what’s in specific parts of their tissues, both above and belowground, making them strong candidates for carbon sequestration. Better understanding the mechanisms behind carbon sink strength in poplars is therefore an important consideration for improving these processes. This study is the first to comprehensively examine relationships between carbon sources (leaves) and sinks (roots) at the molecular level by applying synthetic auxins to poplar foliage, opening doors for additional research on ways to enhance the strength of the carbon sink in poplar roots. 


carbon dioxide labeling to track carbon flow
A recent study is the first to comprehensively examine carbon source–sink relationships in a plant by applying synthetic auxins—a plant hormone—to foliage. The results show that several molecular factors likely play vital roles in carbon sink strength. (Image courtesy of Nathan Johnson and Amir Ahkami | Pacific Northwest National Laboratory)

Sink strength—the ability of specific plant tissues like roots to import carbon—is key to tissue growth and biomass productivity in plants because it drives the movement of carbon from source areas of a plant (e.g., the leaves) to sink areas (e.g., roots). Despite its importance, information is lacking regarding the molecular drivers of sink strength in trees. Auxin, a vital plant phytohormone, is central to moving photoassimilates (biological compounds produced through assimilation via photosynthesis) and specifically carbohydrates to sink tissues in plants. Using an “auxin-stimulated carbon sink” approach, a multi-institutional team investigated the molecular processes of source–sink carbon allocation in poplar trees. The team sprayed the foliage of poplar cuttings with polar auxin transport modulators—auxin enhancers (AE) and auxin inhibitors. They also comprehensively analyzed leaf, stem, and root tissues. Results showed that AE and auxin inhibitors affected the root dry weight and branching pattern, and AE specifically increased the amount of photosynthetically fixed carbon allocation in the leaves and roots. This was confirmed by tracking the allocated carbon from leaves to roots using a stable isotope probing experiment, which resulted in more net fixed carbon exported to roots under AE conditions. Transcriptomic analysis demonstrated that the AE treatment resulted in highly expressed genes in the root tissue that increased the size of the sink and its activity. An overall shift in metabolism was also observed through metabolic analyses. This study suggests that source-sink carbon relationships in poplar are driven by molecular factors, including mobile sugar alcohols, starch metabolism-derived sugars, and intermediates in the Krebs cycle—all important elements of sink strength. 


Vimal Kumar Balasubramanian, EMSL, 

Amir H. Ahkami, EMSL, 


This research was supported by the Environmental Molecular Sciences Laboratory, a Department of Energy, Office of Science user facility sponsored by the Biological and Environmental Research program.  


V.K. Balasubramanian, et al. “Modulation of polar auxin transport identifies the molecular determinants of source–sink carbon relationships and sink strength in poplar.” Tree Physiology, tpad073 (2023). [DOI: 10.1093/treephys/tpad073]