New Genes Protect Trees From Insects, Disease

Using "doctored" genes to impart desired characteristics into fungi that live symbiotically with tree roots can ultimately help trees grow faster and live healthier lives, according to Project Leader Dr. Gopi Podila, a molecular biologist in Michigan Tech's Department of Biological Sciences.

"Ectomycorrhizal (on top of the root) fungi provide a major network in the soil for making nutrients available to tree root systems," explains Podila. " By breaking down minerals locked up by acidic soils caused by the decomposition of needles, leaves, and other forest litter, fungi facilitate the passage of those nutrients from the soil to the tree. The fungus, in turn, draws food from the tree, enhancing its own survival. Another benefit of this symbiotic relationship is that the fungus can grow wider and deeper into the soil--and this greatly increases the outreach of the tree in its search for essential minerals."

Podila says that normally when you try to put a fungus on a plant, the two try to kill one another, but fortunately this mutually beneficial relationship has evolved in nature over the course of millions of years. "One of the things we want to determine," he says, "is how they communicate with one another--how do they each let the other know that they are friendly." He says the process is a long one.

"It's almost like a courtship. The fungus gradually approaches the tree root system and this can take as long as three or four months. And even then, the fungus is 'turned on' only to specific host signals. The fungus prepares itself to form the association with the host plant by expressing specific symbiosis-related genes. These genes are turned on by the right plant signals, and this process is controlled by DNA sequences called 'promoters.' These promoters are responding to plant signals and determine the symbiosis-related expression of genes. If we can combine a specific gene with the right promoter, we can introduce a variety of genes into the fungus, where they will express their specific qualities to enhance the life of the tree." Podila says the fungus provides a protective "coating" around the tree's roots, protecting it from drought and attack by various microbial pathogens.

"Reforestation techniques use herbicides to prevent grass and weeds from covering up the reforested area and forming competition for tree seedlings," he says. "Insect pests, such as the white grub, which normally feed on grass roots, must then turn to tree roots for food. White grubs are voracious eaters that can completely chew away the roots of a young tree, leaving it unable to draw nutrients from the soil. We can prevent this from happening by altering the mycorrhizal fungus so it will produce insecticidal proteins that are unpalatable to the grubs, causing them to avoid the tree roots. The protein produced by the altered gene is biodegradable and completely harmless to humans." If these genes are put under the control of symbiosis-related gene promoters, then their expression will be limited to mycorrhizal roots and thus will not cause any unintended expression of these genes.

Podila says it is also possible to improve the ectomycorrhizal fungi to reduce the uptake of soluble metals by tree roots. He says this will be very useful in alleviating conditions caused by acid pollution and aluminum toxicity.

"The use of genetically-improved mycorrhizal fungi is a novel and feasible approach for increasing forest biomass," says Podila. "This system, when fully developed, also has the potential to be adapted to handle a variety of problems associated with plant health, and could lead to further research that will use mycorrhizal fungi to help the plant host in many beneficial physiological and ecological ways."