Identity, regulation, and activity of inducible diterpenoid phytoalexins in maize

Abstract

Phytoalexins constitute a broad category of pathogen-and insectinducible 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 entkaur-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 entcopalyl 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.

Discovery, Biosynthesis and Stress-Related Accumulation of Dolabradiene-Derived Defenses in Maize

“…These results suggest distinct protective functions and biological roles for individual dolabralexins. The inducible accumulation of dolabralexins in response to oxidative and pathogen stress is consistent with dual functionality in abiotic and biotic stress responses, similar to the kauralexin and zealexin pathways Schmelz et al, 2011;Vaughan et al, 2015). Recent findings enabled by mutant analyses highlight an important yet thus far rarely proven role of diterpenoids in protecting roots against environmental perturbations.…”

“…ZmAN2 was first described in a differential display analysis of highly elicited transcripts following F. graminearum challenge of maize silks (Harris et al, 2005). Following the first description of ZmAN2, pathogen-elicited transcripts have been observed and reported in a large number of maize studies with diverse microbes Schmelz et al, 2011Schmelz et al, , 2014van der Linde et al, 2011;Vaughan et al, 2014;Christie et al, 2017). The nearly ubiquitous presence of ZmAN2 as a pathogen defense transcript marker is consistent with the essential biosynthetic role in the modular formation of multiple specialized diterpenoid metabolites, including not only A-and B-series kauralexins but also dolabralexins.…”

“…Maize deploys functionally diverse class II diTPSs (blue), class I diTPSs (yellow), and P450 enzymes (orange) to form an array of biologically active terpenoid metabolites that function as GA phytohormones and in the response to both biotic and abiotic stress. The zealexin sesquiterpenoids and kauralexin diterpenoids were identified previously as components of the maize defense system Schmelz et al, 2011). The stress-inducible dolabralexins represent an additional family of defense-related maize diterpenoids.…”

“…Inducible zealexin and kauralexin production contributes to defense, both aboveground and belowground, against pathogenic fungi, including Fusarium, Aspergillus, and Colletotrichum spp. Schmelz et al, 2011;Vaughan et al, 2015). Kauralexins also function as insect feeding deterrents and accumulate in response to salt and drought stress (Vaughan et al, 2015).…”

“…1), are part of a complex repertoire of defense-related specialized metabolites, such as phenylpropanoids, oxylipins, benzoxazinoids, and volatile terpenoids (Schnee et al, 2006;Köllner et al, 2008a;Santiago and Malvar, 2010;Ahmad et al, 2011;Huffaker et al, 2011;Schmelz et al, 2011;Christensen et al, 2015;Richter et al, 2016;Wouters et al, 2016). Inducible zealexin and kauralexin production contributes to defense, both aboveground and belowground, against pathogenic fungi, including Fusarium, Aspergillus, and Colletotrichum spp.…”