Critical early-defense trigger in plants found

The gene for an enzyme that is key to natural disease resistance in plants has been discovered by biologists at the Boyce Thompson Institute for Plant Research (BTI) and at Cornell University. The researchers say that by enhancing the activity of the enzyme they might be able to boost natural disease resistance in crop plants without resorting to pesticides or the introduction of non-plant genes.

The research, reported in the latest (May 16) issue of the journal Cell , describes the discovery of the gene that codes for an enzyme (a protein that carries out a chemical reaction) that is activated when a plant senses it is being attacked by a pathogen. When activated, the enzyme produces nitric oxide (NO), a hormone that tells the plant to turn on its defense arsenal.

According to plant pathologist Daniel F. Klessig, lead author of the Cell paper and president of BTI, located on the Cornell campus, the discovery provides a new understanding of the biochemical and genetic pathways in plants that enable them to protect themselves from disease.

“It’s known that the hormone nitric oxide plays an important role in immunity in plants as well as in humans and other animals,” says Klessig. “But the enzyme responsible for its production in plants was unknown until now. With this discovery, we may be able to modify plants so that they produce nitric oxide more quickly, or in larger amounts, when they are attacked by a disease-causing pathogen, enabling them to better protect themselves from invaders.”

Authors of the Cell paper, “The Pathogen-Inducible Nitric Oxide Synthase (iNOS) in Plants is a Variant of the P Protein of the Glycine Decarboxylase Complex,” also include Meena Chandok, a BTI senior research associate; Anders Jimmy Ytterberg, Cornell doctoral candidate in plant biology; and Klaas J. van Wijk, Cornell assistant professor of plant biology.

“This discovery really is a surprise because the plant enzyme looks very different from mammalian nitric oxide-synthesizing enzymes,”said Brian Crane, Cornell assistant professor of chemistry and chemical biology. Crane now is working with Klessig and Chandok to determine the three-dimensional structure of the protein that will lead biologists to understand its chemical mechanism.

The discovery is significant, the researchers note, because NO is a critical early-warning signal to the plant that it needs to activate its immune response. The difficulty inherent in the research, according to Klessig, was that the plant’s NO-producing enzyme has a completely different sequence than enzymes with similar activity found in all animals. The new research suggests, he says, that the chemistry the plant and animal enzymes use to produce NO also is different.

These differences, Klessig says, could provide clues concerning the way the animal enzyme works, which, in turn, could lead to improved treatment of human diseases by enhancing the activity of the enzyme.

“Part of the success of the green revolution depends on the use of chemical-based fungicides and other pesticides to protect crops against microbial pathogens and insects,” says Klessig. “An alternative strategy to protect crops utilizes a plant’s own natural defenses. An approach in which plant molecular biologists have overproduced plant proteins with antimicrobial activity, such as PR proteins or defensin, has met with only limited success to date, perhaps because only a small portion of the defense arsenal is involved.

“Our discovery of the enzyme that produces the critical early-defense signal, NO, means that we now may be able to regulate the production of this signal.

The turning up of this signal should lead to the turning on of a large portion of the defense arsenal. The end result could be crop plants that can better ward off disease without the use of potentially harmful fungicides and other pesticides, or the introduction of non-plant genes.”

Van Wijk, whose research group identified the protein by tandem mass spectrometry, stresses that without the availability of the very sensitive mass spectrometry instruments and the plant genome information “we would not have been able to find this.”

The Boyce Thompson Institute was opened in 1924 and is an independent, not-for-profit plant research organization. BTI funding for the Cell research was provided, in part, by a Plants and Human Health Grant from the Triad Foundation.