Jasmonoyl-l-isoleucine is required for the production of a flavonoid phytoalexin but not diterpenoid phytoalexins in ultraviolet-irradiated rice leaves

Rice produces low-molecular-weight antimicrobial compounds known as phytoalexins, in response to not only pathogen attack but also abiotic stresses including ultraviolet (UV) irradiation. Rice phytoalexins are composed of diterpenoids and a flavonoid. Recent studies have indicated that endogenous jasmonyl-l-isoleucine (JA-Ile) is not necessarily required for the production of diterpenoid phytoalexins in blast-infected or CuCl₂-treated rice leaves. However, JA-Ile is required for the accumulation of the flavonoid phytoalexin, sakuranetin. Here, we investigated the roles of JA-Ile in UV-induced phytoalexin production. We showed that UV-irradiation induces the biosynthesis of JA-Ile and its precursor jasmonic acid. We also showed that rice jasmonate biosynthesis mutants produced diterpenoid phytoalexins but not sakuranetin in response to UV, indicating that JA-Ile is required for the production of sakuranetin but not diterpenoid phytoalexins in UV-irradiated rice leaves. Jasmonoyl-l-isoleucine is required for the production of sakuranetin, but not necessarily diterpenoid phytoalexins in UV-irradiated rice leaves.

水稻不仅会在受到病原菌侵染时，还会在紫外线（ultraviolet, UV）照射等非生物胁迫下，产生一类被称为植保素（phytoalexins）的低分子量抗菌化合物。水稻植保素由二萜类（diterpenoids）化合物与一种黄酮类（flavonoid）物质组成。近期研究表明，在稻瘟病菌侵染或氯化铜（CuCl₂）处理的水稻叶片中，内源性茉莉酰-L-异亮氨酸（jasmonyl-l-isoleucine, JA-Ile）并非二萜类植保素合成的必需条件。然而，黄酮类植保素樱黄素（sakuranetin）的积累则依赖于JA-Ile。本研究针对JA-Ile在紫外线诱导的植保素合成中的作用展开探究。结果显示，紫外线照射可诱导JA-Ile及其前体茉莉酸（jasmonic acid）的生物合成。同时，本研究发现，水稻茉莉酸生物合成突变体在响应紫外线胁迫时，可合成二萜类植保素，但无法产生樱黄素，这表明在紫外线照射的水稻叶片中，JA-Ile是樱黄素合成的必需条件，而非二萜类植保素合成的必要条件。茉莉酰-L-异亮氨酸是樱黄素合成所必需的，但对紫外线照射的水稻叶片中二萜类植保素的合成并非必需。