3-D images reveal sappy secrets

Caren B. Les, caren.les@photonics.com

If you prick a tree, does it not bleed? Actually, not as much as you might think. And confocal laser scanning microscopy is helping scientists explore wood cell anatomy and chemical composition on the nanoscale to figure out why not.

Using a 4Pi microscope, researchers have found that the tiny cavities called bordered pits in wood-fiber cells, which allow sap to circulate through adjacent cells, contain nanostructures filled with a membrane of nanofibrils, and that these tiny fibers radiate from a solid center called the torus.

They saw for the first time that pectin surrounds the torus and forms ringlike structures at the margin of the torus, said Dr. Barry Goodell, a professor of sustainable materials at Virginia Tech in Blacksburg. These features helped them to better understand how the pit membranes seal off fluid flow and prevent embolism (air bubbles that would cause harm) in the tree, he added. “As part of the pit-sealing process, pectin separates to form an outer fringe around the torus, while the bulk of the pectin gets pulled into the aperture to block it.”

This 3-D micron-scale cross section of pine wood shows three bordered pits in a wood cell. A specialized 4Pi microscope captured the image, revealing the distribution of pectin (red) in ringlike structures at the nanolevel, which helps to explain how trees control sap flow. Cellulose appears in green. Courtesy of Virginia Tech.

They also found a hollow in the center of the torus, which they believe helps to stabilize the pit membrane in its natural unaspirated state.

There are other reasons to better understand the bordered pit system; sealed wood fiber cells make some wood types very difficult to dry, and they also make it difficult to extract natural chemicals from wood to make a range of products including medicinal polymers.

The Virginia Tech researchers collaborated with colleagues from Georg-August University Göttingen in Germany and the Jackson Laboratory in Bar Harbor, Maine. Their study was featured in the American Journal of Botany (doi: 10.3732/ajb.1300004).

Goodell would like to use a similar microscopy system to better understand fungal deconstruction of woody biomass, which he said is critical to our understanding of issues ranging from carbon cycling in the environment to how we can develop better systems for the generation of cellulosic biofuels.

“Viewing things in every dimension possible provides more information,” Goodell said. “This is particularly important, I think, when trying to understand the chemistry of biological samples. Much of the chemistry is dictated by the three-dimensional environment.”