ARS | Publication request: Novel acidic sesquiterpenoids constitute a dominant class of pathogen-induced phytoalexins in maize

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 8, 2011
Publication Date: August 2, 2011
Citation: Huffaker, A., Kaplan, F., Vaughan, M.M., Dafoe, N.J., Ni, X., Rocca, J.R., Alborn, H.T., Teal, P.E., Schmelz, E.A. 2011. Novel acidic sesquiterpenoids constitute a dominant class of pathogen-induced phytoalexins in maize. Plant Physiology. 156:2082-2097.

Interpretive Summary: Maize is a crop of great economic importance, yet its protective mechanisms that guard against pathogen attack remain poorly understood. Maize pathogens of significant concern are fungi, such as Fusarium graminearum and Aspergillus flavus, which produce mycotoxins that are harmful to both human health and animals. While studying maize defense responses induced by the F. graminearum, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL discovered a new and diverse class of terpenoid phytoalexins. These maize defense metabolites, named zealexins, also accumulate to high levels in response to many fungal pathogens including A. flavus and herbivory by the European corn borer (Ostrinia nubilalis). Expression of candidate terpene synthase genes, which are hypothesized to initiate phytoalexin biosynthesis, also rapidly increase following fungal inoculation. Moreover, these expression levels highly correlate with zealexin production. Based on transcript analyses, zealexin accumulation is also co-regulated with a collection of established protein-based defenses. Zealexin production is ubiquitous in maize and co-occurs with elevated levels of diterpene phytoalexins and a subclass of benzoxazinoid hydroxamic acid defenses. Importantly, zealexins significantly inhibit the growth of all fungi examined when tested at physiologically relevant concentrations. This discovery highlights a predominant yet previously unrecognized defense mechanism in maize which protects against important pathogens. Future elucidation of the genetic and molecular regulation of zealexin biosynthesis will enable the development of improved disease resistance in maize.

Technical Abstract: Non-volatile terpenoid phytoalexins occur throughout the plant kingdom, but until recently were not known constituents of chemical defense in maize (Zea mays). We describe a novel family of ubiquitous maize sesquiterpenoid phytoalexins, termed zealexins, which were discovered through characterization of Fusarium graminearum-induced responses. Zealexins accumulate to levels greater than 800 µg g-1 fresh weight in F. graminearum-infected tissue. Their production is also elicited by a wide variety of fungi, Ostrinia nubilalis herbivory and the synergistic action of jasmonic acid and ethylene. Zealexins exhibit antifungal activity against numerous phytopathogenic fungi at physiologically relevant concentrations. Structural elucidation of four members of this complex family revealed that all are acidic sesquiterpenoids containing a hydrocarbon skeleton that resembles ß-macrocarpene. Induced zealexin accumulation is preceded by increased expression of the TPS6 and TPS11 genes which encode terpene synthases that catalyze ß-macrocarpene production. Furthermore, zealexin accumulation displays direct positive relationships with the transcript levels of both genes. Microarray analysis of F. graminearum-infected tissue revealed that TPS6/TPS11 were among the most highly upregulated genes, as was An2, an ent-copalyl diphosphate synthase associated with production of kauralexins. Transcript profiling suggests that zealexins co-occur with a number of antimicrobial proteins, including chitinases and pathogenesis-related proteins. In addition to zealexins, kauralexins and the benzoxazinoid 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one-Glc (HDMBOA-Glc) were produced in fungal-infected tissue. HDMBOA-Glc accumulation occurred in both wild type and benzoxazine-deficient 1 (bx1) mutant lines, indicating that bx1 gene activity is not required for HDMBOA biosynthesis. Together these results indicate an important cooperative role of terpenoid phytoalexins in maize biochemical defense.