Plant dynamics: NMR offers cellular clues

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  • Published: Oct 1, 2013
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Plant dynamics: NMR offers cellular clues

A boon for biomass

Sensitivity-enhanced solid-state NMR detection of expansin’s target in plant cell walls Photo Hong et al

Researchers in the USA have used dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy on what they describe as a super-sensitive instrument to locate precisely where a protein binds to plant cell walls, reveal a process that loosens the cell walls and thus explains how plants can grow. The research might ultimately lead to more abundant biomass crop yields for renewable energy.

There are only tiny quantities of the protein involved in cell growth - expansin - in the complex cell walls found in plants explains chemist Mei Hong, a lead researcher at Iowa State University who is also on the faculty at the US Department of Energy's Ames Laboratory. Locating the binding target for expansin has thus remained a particular challenge to structural biologists, but solid state NMR spectroscopy has now come to the rescue. Writing in the Proceedings of the National Academy of Sciences (USA) online in a paper with the self-explanatory title "Sensitivity-enhanced solid-state NMR detection of expansin's target in plant cell walls" Hong and biologist colleague Daniel Cosgrove of Penn State University and their respective teams Tuo Wang (Iowa State and Ames), Linghao Zhong, Yong Bum Park, (Penn State) and Marc Caporini and Melanie Rosay of the Bruker BioSpin Corporation in Billerica, Massachusetts explain the details.

Enhanced sensitivity

In their work, supported by three grants from the DoE, they used the expertise of the Bruker specialists to enhance the sensitivity of their instrumentation based on the technology developed by chemist Robert Griffin of Massachusetts Institute of Technology and coupled that with differential isotopic labelling of expansin and polysaccharides for their investigation.

The team investigated Arabidopsis thaliana, a model plant, analogous to the fruit fly and yeast in animal and microbial studies, respectively. They found that the expansin protein binds to specific regions of the cellulose microfibrils, the long, parallel chains of cellulose that make up plant cell walls. This binding process weakens the network formed by a cell wall's cellulose, hemicellulose and pectins, loosening the cell wall and allowing cell growth. More specifically, the researchers showed that the target site is the part of the cellulose microfibril that is enriched with the hemicellulose xyloglucan, which has a different cellulose structure to the plant's bulk cellulose.

In their investigation, the team transferred the electron polarization of a biradical dopant to the nuclei and exploited DNP to allow them to carry out a selective detection of carbon-13 spin diffusion from trace concentrations of expansin, labelled with both carbon-13 and also nitrogen-15, in the cell wall to nearby polysaccharides. "From the spin diffusion data of wild-type and mutant expansins, we conclude that to loosen the cell wall, expansin binds highly specific cellulose domains enriched in xyloglucan, whereas more abundant binding to pectins is unrelated to activity," the team reports.

No trivial solution

"This result wasn't trivial to get and we are quite happy that the DNP NMR technology is so useful for understanding this plant biochemistry question," Hong says. Knowing where expansin binds to cells walls could have practical benefits. It should be possible to design or engineer novel expansin analogues that are more potent than the native protein and so could be used to loosen the cell wall and stimulate greater plant growth and so boost harvests of biomass crops, for instance, without the need to have bigger farms for such crops.

"Future research plans are best left in grant proposals," Hong told SpectroscopyNOW. "But, the strategy of DNP NMR can certainly be extended to other structural biology questions in plant biochemistry and in biomedically relevant protein sciences," she says. As with many such studies having a particular focus, there is the potential for more general applicability. DNP-enhanced NMR thus provides a powerful approach for investigating protein binding to other complicated biomolecular targets.

Related Links

Proc Natl Acad Sci (USA) 2013, online: "Sensitivity-enhanced solid-state NMR detection of expansin’s target in plant cell walls"

Article by David Bradley

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

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