Understanding the effects of solar UV-B radiation on plant defenses: Role of the jasmonate signaling pathway

Background/Question/Methods In terrestrial ecosystems, one of the most important effects of solar ultraviolet-B radiation is on the interactions between plants and herbivorous insects. Exposure to solar UV-B induces plants to express increased levels of secondary compounds. Some of these compounds protect the plants against UV-B-induced damage and, at the same time, may function as anti-herbivore defenses. In fact, certain phenolic compounds induced by UV-B radiation are also produced by plants in response to insect attack. Since the production of chemical defenses triggered by insect herbivory is controlled primarily by hormones of the jasmonate family, we wanted to investigate whether the effects of UV-B on the accumulation of defense-related compounds are also dependent on jasmonate production. To address this question, we tested the effects of UV-B on wild-type (WT) Nicotiana attenuata plants and in lines silenced in their lipoxygease gene, which encodes an enzyme that is essential for jasmonate biosynthesis (as-lox).

Results/Conclusions In a field experiment carried out in Buenos Aires (Argentina), we found that plants of both genotypes accumulated similar quantities of flavonoids in response to solar UV-B, indicating that jasmonates are not involved in this response. In contrast, other phenylpropanoid derivatives that were induced by UV-B in the WT genotype were missing in as-lox plants. In response to insect herbivory, WT expressed increased levels of proteinase inhibitors (PIN), a direct anti-herbivore defense, and UV-B radiation increased this PIN response. The PIN response was missing in as-lox plants, which is consistent with the idea that PIN expression is activated by jasmonates. In a UV-B exposure experiment, as-lox plants were more susceptible to high UV-B than WT plants, suggesting that the phenylpropanoid derivatives that are not produced in these jasmonate-deficient plants are important for UV-B protection. In a herbivory trial under field conditions, as-lox plants supported more insect damage than WT plants, confirming the critical role of jasmonates in defense activation. More importantly, the effects of UV-B reducing insect growth, which were highly significant in WT plants, were not detected in the as-lox line. We conclude that some effects of UV-B on plant defense in N. attenuata require jasmonate biosynthesis, while others do not. Furthermore, the growth and defense phenotypes of as-lox plants under UV-B suggest that those effects of UV-B radiation that do require jasmonate production play a critical role in both UV-B protection and UV-B-induced anti-herbivore defense.