How can knowledge of plant and fungal biology be used to prevent the spread of wheat rust?
Today, Puccinia graminis strain Ug99 continues its spread and threatens wheat crops worldwide. In the Green Revolution of the 1960s to 1980s, wheat plants were bred to have the gene complexes Sr24 and Sr31, R genes that conferred resistance to existing strains of wheat rust fungus. The first strain of Ug99 found in Uganda had Avr genes that form elicitor proteins that do not bind to Sr31 and therefore overcome part of the plant’s resistance. Then, in 2006, a new genetic variant of Ug99 was found in Kenya with additional Avr genes that allow the pathogen to overcome the Sr24 gene resistance as well. Because 90 percent of the wheat grown in the world has no resistance to this new strain of Ug99, an intensive search is under way for additional wheat resistance genes. In the regions where Ug99 infections are now present, occasional wheat plants grow that are resistant. Samples of both wheat plants and fungus are sent to laboratories, where, under high security and sterility, they are tested and examined for Avr and R genes. In addition, seeds from thousands of varieties of wheat, collected from all over the world, are being grown into seedlings and examined for R genes.
Future directions
The roles of the hormones jasmonic acid and salicylic acid in plant immunity are well described. It is now apparent that other hormones are involved as well. This is yet another example of crosstalk between signaling pathways. Auxins affect the architecture of the plant cell wall, leading to wall loosening. But this has a downside, making the plant’s mechanical defenses less effective, and pathogens grow more readily in loose walls. Auxins can also interrupt other hormone signaling pathways, leading to reduced immunity. So there is a trade-