NASA Scientists Disprove 'Lost Gene' Theory in Symbiotic Fungi
In Yellowstone National Park, east of the Virginia Cascade cutoff, NASA scientists found a site composed solely of pine trees and, for the next two years, proceeded to gently twist off one half of the pine needles at the base. By artificially defoliating the trees, the scientists wanted to examine the wood-degrading capability of a fungus that has the potential to digest toxins found in the soil.
Scientists are working to isolate the gene that causes Ectomycorrhizal (EM) fungi to digest wood and its toxins, such as phenolics, which are contaminants similar to those found in soil and groundwater.
A study led by Ken Cullings and Galina Ishkhanova of NASA Ames Research Center, Moffett Field, Calif. and Joan Henson of Montana State University, Bozeman, Mont. found evidence that the gene in the dominant EM fungal species (Suillus granulatus) produced wood-degrading enzymes, and that enzymatic activity increased significantly during defoliation. The study appeared in the July 2008 issue of Oceologia.
For a long time, scientists knew that a group of EM fungi was related to a wood-rotting fungus. Over time, EM fungi evolved into organisms that are dependent on their host plants for food, and no longer use their capability to break down wood for food. Scientists suspected that these fungi lost the genes necessary to convert wood into food.
However, they found that was not the case.
"Our study showed that not only do mycorrhizal fungi still have the gene, but the gene can be highly stimulated by removing some of the needles from the host tree, which restricts photosynthesis and the production of sugar at its root. It disproves the 'lost gene' theory," said Cullings.
Scientists now believe that EM fungi have the potential to play a significant role in carbon, as well as nitrogen and phosphate cycling in the Earth's forested ecosystem. In addition, many above-ground factors that reduce photosynthetic potential, or divert fixed carbon from roots, may have wide-reaching effects on the ecosystem. One such effect applies to global climate change. In Culling's experiment, the loss of needles is comparable to plant diseases, such as dwarf mistletoes and pine beetles, which are killing record numbers of pines in the west. These diseases are predicted to not only increase in range and scope due to global climate change, but also exacerbate the global climate change.
As the pine trees become blighted with disease and lose their needles, EM fungi starts to produce the enzyme that decomposes wood for its food. When the wood breaks down, it releases carbon dioxide, which raises the atmospheric carbon dioxide levels and increases the speed and intensity of global climate change. Cullings is currently undertaking experiments that will allow these phenomena to be added to global climate change models for the first time.
Because these enzymes also break down toxins, such as diesel, this study has application to the broad field of toxic clean-up.
"I'm working to clone out the mycorrhizal toxin-degrading genes to get a purified enzyme of high activity use. For instance, it can be used as a toxic spill clean-up strategy," said Cullings.
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Ruth Dasso Marlaire
Ames Research Center, Moffett Field, Calif.