Integration of C₁ and C₂ Metabolism in Trees.

Kolby Jardine,Vinicius Fernandes de Souza,Jeffrey Chambers,Niro Higuchi,Ülo Niinemets,Markus Bill,Patty Oikawa,Rachel Porras

doi:10.3390/ijms18102045

Abstract

C1 metabolism in plants is known to be involved in photorespiration, nitrogen and amino acid metabolism, as well as methylation and biosynthesis of metabolites and biopolymers. 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 pathway. In this study, we evaluated the C1 pathway and its integration with the 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. Delivery of [13C]methanol and [13C]formaldehyde rapidly stimulated leaf emissions of [13C]methanol, [13C]formaldehyde, [13C]formic acid, and 13CO2, confirming the existence of the C1 pathway and rapid interconversion between methanol and formaldehyde. However, while [13C]formate solutions stimulated emissions of 13CO2, emissions of [13C]methanol or [13C]formaldehyde were not detected, suggesting that once oxidation to formate occurs it is rapidly oxidized to CO2 within chloroplasts. 13C-labeling of isoprene, a known photosynthetic product, was linearly related to 13CO2 across C1 and C2 ([13C2]acetate and [2-13C]glycine) substrates, consistent with reassimilation of C1, respiratory, and photorespiratory CO2. Moreover, [13C]methanol and [13C]formaldehyde induced a quantitative labeling of both carbon atoms of acetic acid emissions, possibly through the rapid turnover of the chloroplastic acetyl-CoA pool via glycolate oxidation. The results support a role of the C1 pathway to provide an alternative carbon source for glycine methylation in photorespiration, enhance CO2 concentrations within chloroplasts, and produce key C2 intermediates (e.g., acetyl-CoA) central to anabolic and catabolic metabolism.

Highlights

Methanol is the most abundant non-methane volatile organic compound (VOC) in the atmosphere with an array of surface sources dominated by emissions from terrestrial ecosystems [1,2]
Using dynamic 13C-pulse chase experiments, we evaluated the potential existence of the complete C1 pathway and its integration with C2 metabolism in individual branches of a tropical pioneer species using aqueous solutions of 13C-labeled C1 and C2 intermediates delivered via the transpiration stream
We employed the dynamic 13C-pulse chase technique under photorespiratory CO2 concentrations (100 ppm) to evaluate the existence of the C1 pathway and its integration with C2 metabolism in individual branches of a tropical tree species using aqueous solutions of 13C-labeled C1 and C2 intermediates delivered via the transpiration stream. [13C]methanol labeling resulted in the rapid production of each major volatile intermediate of the oxidative C1 pathway including [13C]formaldehyde, [13C]formic acid, and 13CO2, confirming that methanol initiates the complete C1 pathway in I. edulis (Figure 1)

Summary

Introduction

Methanol is the most abundant non-methane volatile organic compound (VOC) in the atmosphere with an array of surface sources dominated by emissions from terrestrial ecosystems [1,2]. Owing to its high solubility in water (Henry’s law constant H = 0.461 Pa·m3·mol−1 at 25 ◦C), previous experimental and physicochemical emission modeling studies have suggested that changes in stomatal conductance can uncouple instantaneous methanol production rates from emission rates due to an increase in aqueous phase concentrations [13,14]. These physicochemical mechanisms do not include active metabolism of methanol [13,15]. Feeding leaves with 10% methanol solutions strongly altered the expression of hundreds of genes involved in energy metabolism, cell communication and transduction processes, and cell division and growth [19], further confirming that methanol can be taken up by leaves and profoundly alter metabolism

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Discussion

Conclusion