Cheating spectroscopy

REDOR, a new form of NMR has been used by researchers in the US to figure out why the cheatgrass weed out-strips soy crops, particularly in higher carbon dioxide. Their results have serious implications for agriculture in the face of climate change.

Quite ironically, cheatgrass (Bromus tectorum) is spreading like wildfire in the American West, displacing native sagebrush it is partly responsible for fuelling the increasingly frequent natural fires that occur in this part of the world. It is known as cheatgrass because it often fools farmers into believing their winter wheat is coming along well, when in fact it is simply the weed that is thriving.

Chemist Jacob Schaefer, of Washington University in St. Louis, Missouri, was intrigued to learn of this problem. He and his colleagues research the response of soybeans to stressful growing conditions and had found that this crop species does not cope nearly so well as a weed like cheatgrass. Its spread and survivability suggested that there is something very different about the metabolism in this plant species.

Cheatgrass gains a foot hold where other plants fail by growing early in the season and rapidly, depleting soil moisture before other plants break dormancy. It then sets seed and dries completely in early summer, creating dense mats rich in lignin and aromatic compounds that are readily ignited and burn well with a lightning strike or other spark. Once cheatgrass invades an area, fires are wont to start earlier, when native plants are more susceptible to injury. Repeated burning eventually kills all the native plants, and cheatgrass competition prevents their re-establishment.

Schaefer has spent the best part of two decades developing the instrumentation and methods that would explain the metabolic profile of cheatgrass that allows it to thrive even where other plants cannot. His technique, magic-angle spinning REDOR (rotational-echo double-resonance) solid-state NMR can analyse proteins, starches and sucrose intact leaves.

Schaefer wanted to determine whether differences in photosynthesis at the molecular level gave cheatgrass its competitive edge. Like soybean, cheatgrass is a C3 plant. It fixes carbon dioxide in such a ways that ribulose bisphosphate (a 5-carbon sugar) is converted into two units of 3-phosphoglycerate. C3 plants account for 95% of the Earth's plant biomass. But, C3 plants are paradoxical, they undergo photorespiration when it is hot and their stomata on the undersides of their leaves close conserve water. With the stomata closed, carbon dioxide levels within the leaf rise and require metabolism that uses energy rather than storing it as sugars.

The received wisdom suggests that the enzyme responsible evolved when there was little or no oxygen in the Earth's atmosphere. Today, oxygen levels are much higher, which means the oxygen-based photorespiration reactions get in the way of the carbon-based reactions. Scientists at Monsanto in the 1960s and 1970s, including Schaefer, assumed that photorespiration was simply wasteful and if it could be inhibited plants would be more productive.

Monsanto had a big research effort, with hundreds of scientists looking for a photorespiration inhibitor and at the time, Schaefer, was using NMR to characterize the process. Unfortunately, at the time NMR had not evolved to allow the necessary insights to be made.

With REDOR in hand, Schaefer and his colleagues carried out nitrogen and carbon labelling experiments with plants at CO2 levels of 200 parts per million, and 600 ppm (a concentration predicted to be reached by 2050). The cheatgrass spectra were then compared with the results of labelling experiments performed on with soybean in 2006. Neither plant, it seems, behaves as it should in the low carbon dioxide atmosphere.

Both plants produced the amino acid glycine, an intermediate of photorespiration, and the glycine then feeds back into the carbon fixation process to form structural proteins that help counter the effects of dehydration, something that in the past has coincided with low carbon dioxide levels.

At high concentration of carbon dioxide, however, the soybean responded by routing most of its glycine back into carbon fixation in classic energy-wasting photorespiration, while the cheatgrass continued to make glycine-rich protein. Soybeans seem to have a switch that responds to changing CO2 levels, whereas cheatgrass does not.

In other words, cheatgrass biochemistry seems better suited to elevated carbon dioxide concentrations than soybean biochemistry. The research adds to a growing body of evidence challenging the idea that all plants will benefit from rising carbon dioxide levels. Some plants will be helped, but others will be harmed.

Unfortunately, the climate of the future is likely to be more like the one to which cheatgrass is adapted. "We believe," says Schaefer, "that as carbon dioxide rises and we try to farm more marginal land to feed a burgeoning population, plants like cheatgrass will have a large advantage over plants like soybeans."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.