Why are plants immune to most of the diseases surrounding them in the environment? Lee Hadwiger, Washington State University professor of plant pathology, has been wrestling with this question most of his career.
Were it not for a commonplace but mysterious trait called non-host resistance (NHR), plants would be constantly attacked by fungi, bacteria, and other pathogens swarming in the air, soil, and bodies. For the most part, plants are immune to those challenges because NHR gives them their robust and durable immunity to the myriad pathogens challenging them.
In the January issue of Phytopathology, Hadwiger and his colleague, USDA Agricultural Research Service plant pathologist James Polashock, offer new insight into the mechanism triggering the NHR response in plants.
“Innate immunity has to be triggered by something,” Hadwiger says, “but we are only now gaining some insight on how signaling occurs at the molecular level.”
Hadwiger and Polashock show that fungal DNase enzymes trigger the NHR response in a variety of plant species. They further theorize that these fungal DNase genes appear to provide an unlimited source of components for developing transgenic resistance in all transformable plants.
DNase is the generic term for a wide variety of enzymes that catalyze changes in DNA molecules. Hadwiger explains that DNases from fungal mitochondria have a small peptide molecule that enables them to move through plant cell membranes and thus induce expression of NHR in the plant. Hadwiger and Polashock demonstrated that when a plant encounters a fungal DNase purified in the lab, the NHR response is triggered.
Hadwiger and Polashock used baker’s yeast, a relatively innocuous fungus not known to cause disease, to trigger the NHR response in a pea plant. Hadwiger and students in his laboratory had previously induced this defense response by transferring a fungal DNase gene to tobacco. The tobacco plants then expressed the NHR response to a known tobacco pathogen.
“The potential positive impact of this for agriculture would be a reduction in the use of fungicides,” Hadwiger says. Currently, disease resistance genes are typically introduced in commercially important plants through conventional breeding techniques. But, Hadwiger says, conventional breeding targets races of specific diseases and the introduced immunity may last only about seven years before the fungus evolves and overcomes the plant’s resistance.
“The natural NHR resistance would be preferable,” Hadwiger says. Toward that end, Hadwiger says he will remain vigilant about how best to transfer this natural process to plants that succumb to their specific diseases. He is optimistic that non-genetic engineering techniques may be devised to enhance the activity of the DNases transferred in the fungal-plant interactions.