Biosynthesis of 8-O-methylated benzoxazinoid defense compounds in maize

Abstract

Benzoxazinoids are important defense compounds in grasses. Here, we investigated the biosynthesis and biological roles of the 8-O-methylated benzoxazinoids, DIM 2 BOA-Glc and HDM 2 BOA-Glc. Using quantitative trait locus mapping and heterologous expression, we identified a 2-oxoglutarate-dependent dioxygenase (BX13) that catalyzes the conversion of DIMBOA-Glc into a new benzoxazinoid intermediate (TRIMBOA-Glc) by an uncommon reaction involving a hydroxylation and a likely ortho-rearrangement of a methoxy group. TRIMBOA-Glc is then converted to DIM 2 BOA-Glc by a previously described O-methyltransferase BX7. Furthermore, we identified an O-methyltransferase (BX14) that converts DIM 2 BOA-Glc to HDM 2 BOA-Glc. The role of these enzymes in vivo was demonstrated by characterizing recombinant inbred lines, including Oh43, which has a point mutation in the start codon of Bx13 and lacks both DIM 2 BOA-Glc and HDM 2 BOA-Glc, and Il14H, which has an inactive Bx14 allele and lacks HDM 2 BOA-Glc in leaves. Experiments with near-isogenic maize lines derived from crosses between B73 and Oh43 revealed that the absence of DIM 2 BOA-Glc and HDM 2 BOA-Glc does not alter the constitutive accumulation or deglucosylation of other benzoxazinoids. The growth of various chewing herbivores was not significantly affected by the absence of BX13-dependent metabolites, while aphid performance increased, suggesting that DIM 2 BOA-Glc and/or HDM 2 BOA-Glc provide specific protection against phloem feeding insects.

Selinene Volatiles Are Essential Precursors for Maize Defense Promoting Fungal Pathogen Resistance

“…Maize biochemicals either demonstrated or predicted to mediate insect and pathogen defense include diverse volatiles (Degenhardt, 2009;Degenhardt et al, 2009a), benzoxazinoids (Frey et al, 2009;Ahmad et al, 2011;Meihls et al, 2013;Handrick et al, 2016), flavonoids and C-linked flavonoid glycosides (Meyer et al, 2007;Balmer et al, 2013;Casas et al, 2016), nonprotein amino acids (Yan et al, 2015), oxylipins Borrego and Kolomiets, 2016), and nonvolatile terpenoids (Schmelz et al, 2014). Among all biosynthetic classes, terpenoids are the most diverse structurally and in demonstrated breadth of ecological interactions mediated (Gershenzon and Dudareva, 2007).…”

“…In maize (Zea mays), seedlings are largely protected from attack by a complex suite of hydroxamic acid-based defenses, termed benzoxazinoids, responsible for resistance to diverse threats spanning fungal pathogens and herbivores, including northern corn leaf blight (Setosphaeria turtica) and the European corn borer (Ostrinia nubilalis; Beck et al, 1957;Couture et al, 1971;McMullen et al, 2009a). Sixty years of research has resulted in a nearly complete metabolic and genetic benzoxazinoid pathway in maize involving over a dozen individual enzymes and metabolites (Frey et al, 2009;Meihls et al, 2013;Handrick et al, 2016). Additionally, diverse terpenoids and underlying terpene synthases (Tps) also have been demonstrated to play important protective roles (Degenhardt, 2009;Schmelz et al, 2014).…”

DIMBOA levels in hexaploid Brazilian wheat are not associated with antibiosis against the cereal aphids Rhopalosiphum padi and Sitobion avenae

“…Hydroxamic acids have been shown to possess negative effects against aphids (Niemeyer 2009;Makowska et al 2015;Handrick et al 2016;Zu¨st and Agrawal 2016). A number of previous reports have shown an association between hydroxamic acids and the wheat resistance against aphids.…”

“…In this way, bioassays performed with aphids collected recently could give different results to those in the past, especially if collected from fields in different countries. Finally, recent studies on aphid resistance in maize have shown that natural variation in aphid resistance is associated with 2,4-dihydroxy-7-methoxy-1,4-benzoxazinone glucoside methyltransferase activity (Meihls et al 2013) and 8-O-methylated benzoxazinoids (Handrick et al 2016), suggesting that DIMBOA may not be the active principle in wheat for secondary-metabolite based aphid resistance.…”

“…Different types of hydroxamic acids (HDMBOA, HMBOA, MBOA, MBOA-N-Glc, BOA and TRIMBOA-Glc) can be found in plants (Niemeyer 2009) and different levels of toxicity are reported for these molecules (Meihls et al 2013). In wheat, DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzox-azin-3-one) is the main hydroxamic acid isolated (Meihls et al 2013;Maag et al 2014;Makowska et al 2015;Handrick et al 2016;Wouters et al 2016). DIMBOA has been shown to have a toxic effect against aphids when applied in artificial diets (Elek et al 2013a) and it has also been shown to induce callose deposition as a plant defense response (Ahmad et al 2011;Meihls et al 2013).…”