Munronin O, a potential activator for plant resistance.

A series of limonoids (1-8) were isolated from the whole plant of Munronia henryi and antiviral activities of the compounds were evaluated. The bioassay results demonstrated that Munronin O (1) showed remarkable protective activity and compounds 7 and 8 showed significant inactivating, protective, and curative activities against tobacco mosaic virus (TMV). With a 50% effective concentration (EC) value of 91.5 μg/mL, compound 1 exhibited the best protective activity compared with ningnanmycin (192.3 μg/mL). The potential for these compound of inducing systemic acquired resistance (SAR) was also evaluated, and compound 1 showed excellent induction activities. Furthermore, it was found that potentiation of defense-related enzyme activity and the contents of SA was increased. Compound 1 could also inhibit the expression of TMV CP and up-regulate the expression of defense-related genes. This work revealed that compound 1 can induce resistance and enhance plant tolerance to TMV infection. Hence, compound 1 can be considered as a potential activator for inducing plant resistance.