The Science

An oxidative C₁ pathway is known to exist in plants where intermediates with a single carbon atom beginning with methanol are oxidized to carbon dioxide (CO₂). Although the flux of carbon through the C₁ pathway is thought to be large, its intermediates are difficult to measure and relatively little is known about this potentially ubiquitous and mysterious pathway. In this study, scientists at Lawrence Berkeley National Laboratory (LBNL) evaluated the C₁ pathway and its integration with central metabolism using aqueous solutions of ¹³C-labeled C₁ and C₂ intermediates delivered to branches of the tropical species Inga edulis via the transpiration stream.

The Impact

The team’s results demonstrate that methanol activates the C₁ pathway in plants that provides an alternative carbon source for glycine methylation in photorespiration, enhance CO₂ concentrations within chloroplasts, and produce key C₂ intermediates (e.g., acetyl-CoA) for central metabolism. Their observations are consistent with previous studies that demonstrated formaldehyde integrates into photorespiration in the mitochondria by providing an alternate source of CH₂-THF used for the methylation of serine to glycine. By eliminating the need for a second glycine for the production of CH₂-THF with the subsequent loss of CO₂ and ammonia (NH₃), the integration of C₁ pathway into photorespiration may convert it from a net loss of carbon to a net gain. By also suppressing photorespiration via the production of CO₂ in chloroplasts, the study presents the hypothesis that the integration of C₁ pathway into C_2/3 metabolism may boost carbon use efficiency and therefore represent an important mechanism by trees under photorespiratory conditions (e.g., high-temperature stress). As agricultural crops are known to be high methanol producers, genetic manipulation of the C₁ pathway has the potential to improve yields and tolerance to environmental extremes, thereby providing a new tool to the agriculture, bioenergy, and biomanufacturing industries.

Summary

Methanol is highly abundant in the global atmosphere and is known to be tightly connected to plant growth. However, to date, it is assumed that methanol represents a byproduct of the expansion of cell walls during growth processes. Although evidence for the existence of a C₁ pathway in plants was first collected over 50 years ago, its intermediates are difficult to measure and relatively little is known about this potentially ubiquitous, yet mysterious biochemical pathway. Previous research by one of the founding fathers of photosynthesis research (Dr. Andrew Benson), for whom this paper is dedicated, found evidence for an important role of methanol in boosting plant photosynthesis, biomass, and productivity. However, this topic remains controversial as subsequent researchers were unable to observe these effects, and the biochemical mechanism(s) remain unclear.

In this paper, scientists from LBNL employ the newly developed technique in their lab termed dynamic ¹³C-pulse chase to evaluate the potential existence of the complete C₁ pathway and its integration with C_2/3 metabolism in individual branches of a tropical pioneer species using aqueous solutions of ¹³C-labeled C₁ (methanol, formaldehyde, and formic acid) and C₂ (acetic acid and glycine) intermediates delivered via the transpiration stream. They confirm that methanol initiates the complete C₁ pathway in plants (methanol, formaldehyde, formic acid, and carbon dioxide) by providing the first real-time dynamic ¹³C-labeling data showing their interdependence. The team present novel aspects about the pathway including the rapid interconversion between methanol and formaldehyde, whereas once oxidation to formate occurs, it is quickly oxidized to CO₂ within chloroplasts where it can be re-assimilated by photosynthesis. The scientists show for the first time that reassimilation of C₁, respiratory, and photorespiratory CO₂ is a common mechanism for isoprene biosynthesis; a strong linear dependence of ¹³C-labeling of isoprene on ¹³C-labeling of CO₂ was observed across all C₁ and C₂ ¹³C-labeled substrates. Thus, this analysis presents a new method for studying the reassimilation of internal CO₂ sources in plants. Finally, the LBNL research team show, for the first time, that methanol and formaldehyde delivery to the transpiration stream leads to a rapid and quantitative conversion of carbon pools used in the biosynthesis of central C₂ compounds (acetic acid and acetyl CoA) and therefore represents a new uncharacterized route to the biosynthesis of these key C₂ intermediates widely used in cells as precursors for a diverse suite of anabolic (e.g., fatty acid biosynthesis) and catabolic (e.g., mitochondrial respiration) processes.

These observations are consistent with previous studies that demonstrated formaldehyde integrates into photorespiration in the mitochondrial by providing an alternate source of CH₂-THF used for the methylation of serine to glycine. By eliminating the need for a second glycine for the production of CH₂-THF with the subsequent loss of CO₂ and NH₃, the integration of C₁ pathway into photorespiration may convert it from a net loss of carbon to a net gain. By also suppressing photorespiration via the production of CO₂ in chloroplasts, this study presents the hypothesis that the integration of C₁ pathway into C_2/3 metabolism may boost carbon use efficiency during photorespiratory conditions (e.g., high-temperature stress). As all agricultural crops have been shown to be high methanol producers, genetic manipulation of the C₁ pathway has the potential to improve yields and tolerance to environmental extremes, thereby providing a new tool to the agriculture, bioenergy, and biomanufacturing industries.

Principal Investigator

Kolby Jardine
Lawrence Berkeley National Laboratory
kjjardine@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This material is based on work supported as part of the GOAmazon 2014/5 and the Next-Generation Ecosystem Experiments (NGEE)–Tropics project funded by the Terrestrial Ecosystem Science Program of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science, through Contract No. DE-AC02-05CH11231 to LBNL. Additional funding for this research was provided by the Brazilian Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

References