July 23, 2001 PROVIDENCE, R.I. -- By observing the battle between bacterial speck disease and tomatoes, biologists have discovered how plant cells resist some ailments. Researchers from the Boyce Thompson Institute (BTI) for Plant Research Inc. and Cornell University can now demonstrate how disease-causing organisms deliver destructive agents to plants, and how the plants fight back.
"It's like radar detecting an incoming missile" says Gregory B. Martin, senior scientist at BTI and a Cornell plant pathologist. "Consider it trench warfare at the molecular level." While Cornell and BTI are both located in Ithaca, N.Y., Martin will present this information to a plenary session of the American Society of Plant Biologists on Tuesday, July 24, at 4 p.m. at the Rhode Island Convention Center in Providence. The session is titled "Signal transduction mechanisms in plant defense activation."
One combatant is Pseudomonas syringae , the bacterium responsible for causing bacterial speck disease. Martin and his colleagues have learned that P. syringae attacks healthy tomato plants by attaching itself to the plant cell, inserting a microscopic tube and sending a pathogenic protein -- like ammunition -- into the cell.
Despite the attack, the plant cell is prepared for the invading onslaught. Using a molecular surveillance system behind the cell wall, the plant cell detects alien proteins and mounts a defense.
Although bacterial speck disease has been known since the early 1930s, it did not result in serious losses until the winter tomato crop of 1977-78 in southern Florida. Cool, moist
environmental conditions contributed to the development of the disease, and it has now established itself as a major production problem, according to Thomas A. Zitter, Cornell professor of plant pathology.
The disease produces black lesions, often with a discrete yellow halo that can appear on the plant leaves and cause them to curl. Growers had been instructed to spray a copper-based pesticide to ward off bacterial speck. But, over the years, the disease became resistant to the copper, rendering the pesticide virtually useless. The disease is now controlled by naturally-occurring resistance genes that are bred into certain tomato plants.
A project centered at Cornell is determining the DNA sequence of all the genes of P. syringae . Martin and colleagues are then concentrating on those genes which produce proteins involved in causing plant disease.
Describing the importance of the Pseudomonas gene sequences to the botanical and agricultural world, Martin said it was "analogous to sequencing the human genome or the Arabidopsis genome." He explained that while the human genome has about 25,000 genes, P. syringae has about 6,000. As with the human genome sequence, knowing the entire set of genes encoded by an organism give scientists the entire blueprint for how that organism behaves. Cornell and BTI scientists were able to find nearly 30 genes in the P. syringae bacterium involved in the attack and resistance system. "Research over the past 10 years has explained the array of surveillance proteins produced by plants and we now know the entire set of attacking proteins of this bacterium," says Martin.
This research was made possible thanks to a five-year, $5 million grant from the National Science Foundation, which was awarded last fall. Alan Collmer, Cornell professor of plant pathology, serves as the primary investigator on this grant.
Joining Martin on this research: Cornell graduate students Nai-Chun Lin, Anjali Iyer, Jeffrey Anderson, Hye-Sook Oh, Rob Abramovitch, and Pete Pascuzzi; post-doctoral scientists Young Jin Kim and Jonathan Cohn; and Purdue University graduate student Brendan Riely.
Tomatoes belong to the Solanaceae family, a large group of plants that include potatoes, peppers, eggplants, tobacco and petunias. Economically, the Solanaceae family is one of the most valuable crop families in the United States, says Collmer. In 1999, about 494,000 acres of tomatoes for the fresh market and commercial processing were planted, and about 482,090 acres were harvested in the United States, according to the USDA's Economic Research Service. This accounts for 29.2 billion pounds of tomatoes grown that year, or about 60,600 pounds per acre. For the crop in 1999, this represented a farm value of about $1.8 billion.
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