Chemistry Research Unit, Center of Medical, Agricultural, and Veterinary Entomology, US Department of Agriculture, Agricultural Research Service, Gainesville, FL 32608, USA.
Proc Natl Acad Sci U S A. 2011 Mar 29;108(13):5455-60. doi: 10.1073/pnas.1014714108. Epub 2011 Mar 14.
Phytoalexins constitute a broad category of pathogen- and insect-inducible biochemicals that locally protect plant tissues. Because of their agronomic significance, maize and rice have been extensively investigated for their terpenoid-based defenses, which include insect-inducible monoterpene and sesquiterpene volatiles. Rice also produces a complex array of pathogen-inducible diterpenoid phytoalexins. Despite the demonstration of fungal-induced ent-kaur-15-ene production in maize over 30 y ago, the identity of functionally analogous maize diterpenoid phytoalexins has remained elusive. In response to stem attack by the European corn borer (Ostrinia nubilalis) and fungi, we observed the induced accumulation of six ent-kaurane-related diterpenoids, collectively termed kauralexins. Isolation and identification of the predominant Rhizopus microsporus-induced metabolites revealed ent-kaur-19-al-17-oic acid and the unique analog ent-kaur-15-en-19-al-17-oic acid, assigned as kauralexins A3 and B3, respectively. Encoding an ent-copalyl diphosphate synthase, fungal-induced An2 transcript accumulation precedes highly localized kauralexin production, which can eventually exceed 100 μg · g(-1) fresh weight. Pharmacological applications of jasmonic acid and ethylene also synergize the induced accumulation of kauralexins. Occurring at elevated levels in the scutella of all inbred lines examined, kauralexins appear ubiquitous in maize. At concentrations as low as 10 μg · mL(-1), kauralexin B3 significantly inhibited the growth of the opportunistic necrotroph R. microsporus and the causal agent of anthracnose stalk rot, Colletotrichum graminicola. Kauralexins also exhibited significant O. nubilalis antifeedant activity. Our work establishes the presence of diterpenoid defenses in maize and enables a more detailed analysis of their biosynthetic pathways, regulation, and crop defense function.
植物抗毒素是一大类由病原体和昆虫诱导的生化物质，可局部保护植物组织。由于其在农业上的重要意义，玉米和水稻被广泛研究其萜类防御物质，包括昆虫诱导的单萜和倍半萜挥发物。水稻还产生一系列复杂的由病原体诱导的二萜植物抗毒素。尽管早在 30 多年前就证明了玉米中真菌诱导的 ent-kaur-15-ene 的产生，但功能类似的玉米二萜植物抗毒素的身份仍然难以捉摸。为了应对欧洲玉米螟（Ostrinia nubilalis）和真菌对茎的攻击，我们观察到六种 ent-kaurane 相关二萜的诱导积累，统称为 kauralexins。主要 Rhizopus microsporus 诱导代谢物的分离和鉴定揭示了 ent-kaur-19-al-17-oic 酸和独特的类似物 ent-kaur-15-en-19-al-17-oic 酸，分别被指定为 kauralexin A3 和 B3。真菌诱导的 An2 转录物的积累，编码 ent-copalyl diphosphate synthase，先于高度本地化的 kauralexin 产生，最终可超过 100μg·g(-1) 鲜重。茉莉酸和乙烯的药理应用也协同诱导 kauralexin 的积累。kauralexin 在所有检测的自交系的 scutella 中含量升高，在玉米中普遍存在。kauralexin B3 的浓度低至 10μg·mL(-1)时，就显著抑制了机会性坏死营养型菌 Rhizopus microsporus 和炭疽病茎腐病的病原体 Colletotrichum graminicola 的生长。kauralexin 还表现出显著的玉米螟拒食活性。我们的工作确立了玉米中二萜防御的存在，并使对其生物合成途径、调控和作物防御功能的更详细分析成为可能。
