Coumarins also have a C6-C3 skeleton, but they possess an oxygen heterocycle as part of the C3-unit. There are numerous coumarins, many of which play a role in disease and pest resistance, as well as UV-tolerance. The coumarin umbelliferone (1.21) is popular in enzyme assays. Umbelliferone esters can be used as a substrate for non-specific esterase enzyme assays and in fluorescent immunoassays (Jacks and Kircher, 1967). In order to quantify the enzyme activity of the popular reporter gene P-glucuronidase (GUS), plant extracts can be incubated with 4-methylumbelliferyl P-D-glucuronide (4-MUG; 1.22), which upon hydrolysis
by GUS produces the fluorescent compound 4-methylumbelliferone (4-MU), along with glucuronic acid (Gallagher et al., 1992).
Isocoumarins, such as bergenin (1.23) have a structure similar to coumarins, but the position of the oxygen and carbonyl groups within the oxygen heterocycle are reversed. Isocoumarins also play a role in defense responses. For example, bergenin has been shown to inhibit the growth of the powdery mildew on pea (Prithiviraj et al., 1997).
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