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- Under the Microscope

Biochemical Barriers: Investigating Anthracnose Fruit Rot Resistance in Tomatoes ***ARS researchers are breeding new lines of tomatoes to counter the damaging anthracnose disease*** Imagine going to the grocery store and finding lots of tomatoes that look fresh. You bring one home, but within a couple of days, you notice dark bruises all over the tomato. It doesn't seem to be so fresh after all. Is it rotten? Is it infected? The diagnosis is often anthracnose, a disease caused by a fruit rot pathogen that causes tomatoes to bruise and rot. At the Genetic Improvement for Fruits and Vegetables Laboratory, located at the Beltsville Agricultural Researcher Center in Beltsville, MD, ARS plant geneticist John Stommel, research plant pathologists Richard Jones and Christopher Clarke, and post-doctoral fellow Matthew Fabian are focusing on mitigating this damaging fruit disease.  "Anthracnose is a disease caused by Colletotrichum, a type of fungal pathogen that causes decay of ripe tomato fruit," Stommel said. "It impacts everything from large scale commercial production right down to the local farmers market," Jones added. But fruits of some types of tomato are naturally resistant to anthracnose, a trait that would be beneficial to tomato farmers who encounter anthracnose in the field. To learn more about how anthracnose resistance works, the researchers conducted experiments using an anthracnose-resistant tomato and another tomato that is susceptible to anthracnose. By crossing these two parent lines, they developed offspring with various levels of anthracnose resistance. After analyzing the biochemical composition of these different tomato lines, they found a connection between anthracnose resistance and molecules that accumulate in tomato fruit known as steroidal glycoalkaloids, which are produced at high levels in tomatoes. Four of these steroidal glycoalkaloids, including a compound known as alpha-tomatine, can accumulate at high levels in ripe tomato fruit, and this biochemical profile is associated with enhanced anthracnose resistance.  "When an insect pest, or a fungal or bacterial pathogen attacks, it's an active process, and the plant responds," said Stommel. "It's defensive and it shuts down the pathogen, but in this case it appeared that certain tomatoes had compounds that were already there, which inhibited the growth of the fungus." Next, the team decided to conduct another experiment, to see whether they could increase the levels of these beneficial compounds in tomatoes that are normally susceptible to anthracnose. After analyzing genes that regulate the production of steroidal glycoalkaloids, they altered tomatoes to have lower expression of these genes. "We found that alpha-tomatine and certain derivatives, like hydroxytomatine, contribute to anthracnose resistance in ripe red fruit," Fabian said. "Therefore, we used targeted genetics, in the gene pathway that regulates the production of these compounds, to generate tomatoes that have higher levels of anthracnose resistance." Fabian and the team believe their research can improve marketable yield with production of tomatoes that do not develop anthracnose lesions. "We can also reduce the need for pesticide inputs to control disease and those are the two primary benefits to exploit this research," Stommel said. "So, when a consumer goes to the grocery store, the tomatoes will not be infected with anthracnose, and that juicy tomato will stay fresh and ripe as promised." — *By Fiona Hernandez, ARS's Office of Communications*


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