title: Identity, regulation, and activity of inducible diterpenoid phytoalexins in maize.
abstract: 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.
authors.affiliation: Chemistry Research Unit, Center of Medical, Agricultural, and Veterinary Entomology, US Department of Agriculture, Agricultural Research Service, Gainesville, FL 32608, USA. eric.schmelz@ars.usda.gov
authors.forename: Eric A
authors.initials: EA
authors.lastname: Schmelz
authors.forename: Fatma
authors.initials: F
authors.lastname: Kaplan
authors.forename: Alisa
authors.initials: A
authors.lastname: Huffaker
authors.forename: Nicole J
authors.initials: NJ
authors.lastname: Dafoe
authors.forename: Martha M
authors.initials: MM
authors.lastname: Vaughan
authors.forename: Xinzhi
authors.initials: X
authors.lastname: Ni
authors.forename: James R
authors.initials: JR
authors.lastname: Rocca
authors.forename: Hans T
authors.initials: HT
authors.lastname: Alborn
authors.forename: Peter E
authors.initials: PE
authors.lastname: Teal
mesh_terms: D000818:Animals; D000935:Antifungal Agents / Q000737:chemistry / Q000378:metabolism; D020231:Colletotrichum / Q000472:pathogenicity; D004224:Diterpenes / Q000737:chemistry / Q000378:metabolism*; D007313:Insecta / Q000472:pathogenicity; D007306:Insecticides / Q000737:chemistry / Q000378:metabolism; D015394:Molecular Structure; D009091:Mucormycosis / Q000382:microbiology; D010935:Plant Diseases / Q000382:microbiology; D010937:Plant Growth Regulators / Q000378:metabolism; D012233:Rhizopus / Q000472:pathogenicity; D012717:Sesquiterpenes / Q000737:chemistry / Q000378:metabolism*; D003313:Zea mays / Q000737:chemistry* / Q000382:microbiology / Q000469:parasitology / Q000502:physiology*; D000094162:Phytoalexins
chemical_list: D000935:Antifungal Agents; D004224:Diterpenes; D007306:Insecticides; D010937:Plant Growth Regulators; D012717:Sesquiterpenes; D000094162:Phytoalexins