Crop plants are grown as extensive monocultures, which increases the risk of disease epidemics. Major diseases include mildew, rust, Septoria in cereals, and late blight in potato. These diseases limit productivity, and currently necessitate substantial applications of agrochemicals. The cost of this can be measured. For example, a 1995 outbreak of late blight in the Columbia Basin of Washington and Oregon is estimated to have cost $30M in lost yield and increased agrochemical applications.
In the last 10 years, we have learned a great deal about the genes that are important for plants to resist disease. Genes encode proteins and these proteins are small molecules that do specific jobs within the cell. One important job is to detect when invading microorganisms attempt to enter the plant and steal the carbon building blocks the plant needs to maintain health. Plant disease resistance genes (R genes) encode such “antenna” proteins, and confer on plants the capacity to recognize these microorganisms, and upon recognition, to activate defense mechanisms. Pathogens “overcome” R genes by mutating the genes (the “avirulence” genes) encoding the molecules that the plant detects. Durable R genes detect molecules that are indispensable for the pathogen's attack. Upon detection of an invader, the plant unleashes a local battery of defense mechanisms, including reactive oxygen molecules, antibiotics, hydrolytic enzymes, and cell wall strengthening mechanisms. R genes exist as gene families and exhibit tremendous intraspecific variation; disease control in wild species is probably accomplished through frequency dependent selection and balancing polymorphism. The model plant Arabidopsis carries 100–150 R gene families; crop plants may carry 3–10 times that number. Species related to crop plants (this includes other crop plants) carry useful genetic variations for resistance to disease. For example, useful resistance to Xanthomonas disease of tomato has been found in pepper. Introducing this variation into crop varieties by conventional plant breeding (“introgression”) has been effective, but is slow, and is constrained by barriers to interspecific sexual reproduction. Today, we can use homologies to known R genes to identify R genes from one species that may be useful in another, and introduce these R genes by transformation. This will dramatically extend the range of genes that can be used to control disease, and will increase the rate of introgression. This should provide new opportunities and strategies for managing diseases in crops.
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