SummaryThis study demonstrates that wild tobacco Nicotiana attenuata plants accumulate the phytoalexin scopoletin to defend against the necrotrophic fungus Alternaria alternata in a JA signalling-dependent manner.

Land plants protect themselves from ultraviolet-B (UV-B) by accumulating UV-absorbing metabolites, which may also function as anti-insect toxins. Previous studies have shown that UV-B enhances the resistance of different plant species to pierce-sucking pests; however, whether and how UV-B influences plant defense against chewing caterpillars are not well understood. Here we show that UV-B treatment increased Spodoptera litura herbivory-induced jasmonic acid (JA) production in Arabidopsis and thereby Arabidopsis exhibited elevated resistance to S. litura. Using mutants impaired in the biosynthesis of JA and the defensive metabolites glucosinolates (GSs), we show that the UV-B-induced resistance to S. litura is dependent on the JA-regulated GSs and an unidentified anti-insect metabolite(s). Similarly, UV-B treatment also enhanced the levels of JA-isoleucine conjugate and defense-related secondary metabolites in tobacco, rice, and maize after these plants were treated with simulated herbivory of lepidopteran insects; consistently, these plants showed elevated resistance to insect larvae. Using transgenic plants impaired in JA biosynthesis or signaling, we further demonstrate that the UV-B-enhanced defense responses also require the JA pathway in tobacco and rice. Our findings reveal a likely conserved JA-dependent mechanism by which UV-B enhances plant defense against lepidopteran insects.

The phytohormone jasmonoyl-L-isoleucine (JA-Ile) is well-known as the key signaling molecule that elicits plant defense responses after insect herbivory. Oxidation, which is catalyzed by the cytochrome P450s of the CYP94 family, is thought to be one of the main catabolic pathways of JA-Ile. In this study, we identified four CYP94B3 homologues in the wild tobacco plant Nicotiana attenuata. Individually silencing the four homologues revealed that NaCYP94B3 like-1 and NaCYP94B3 like-2, but not NaCYP94B3 like-3 and NaCYP94B3 like-4, are involved in the C-12-hydroxylation of JA-Ile. Simultaneously silencing three of the NaCYP94B3 like genes, NaCYP94B3 like-1, -2, and -4, in the VIGS-NaCYP94B3s plants doubled herbivory-induced JA-Ile levels and greatly enhanced plant resistance to the generalist insect herbivore, Spodoptera litura. The poor larval performance was strongly correlated with the high concentrations of several JA-Ile-dependent direct defense metabolites in VIGS-NaCYP94B3s plants. Furthermore, we show that the abundance of 12-hydroxy-JA-Ile was dependent on JA-Ile levels as well as COI1, the receptor of JA-Ile. COI1 appeared to transcriptionally control NaCYP94B3 like-1 and -2 and thus regulates the catabolism of its own ligand molecule, JA-Ile. These results highlight the important role of JA-Ile degradation in jasmonate homeostasis and provide new insight into the feedback regulation of JA-Ile catabolism. Given that silencing these CYP94 genes did not detectably alter plant growth and highly increased plant defense levels, we propose that CYP94B3 genes can be potential targets for genetic improvement of herbivore-resistant crops.