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

Handrick, Vinzenz; Robert, Christelle A. M.; Ahern, Kevin R; Zhou, Shaoqun; Machado, Ricardo A. R.; Maag, Daniel; Glauser, Gaétan; Fernandez-Penny, Felix E.; Chandran, Jima N.; Rodgers-Melnick, Eli; Schneider, Bernd; Buckler, Edward S.; Boland, Wilhelm; Gershenzon, Jonathan; Jander, Georg; Erb, Matthias; Köllner, Tobias G (2016). Biosynthesis of 8-O-methylated benzoxazinoid defense compounds in maize. The Plant Cell, 28(7), pp. 1682-1700. American Society of Plant Biologists 10.1105/tpc.16.00065

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Benzoxazinoids are important defense compounds in grasses. Here, we investigated the biosynthesis and biological roles of the 8-O-methylated benzoxazinoids, DIM2BOA-Glc and HDM2BOA-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 DIM2BOA-Glc by a previously described O-methyltransferase BX7. Furthermore, we identified an O-methyltransferase (BX14) that converts DIM2BOA-Glc to HDM2BOA-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 DIM2BOA-Glc and HDM2BOA-Glc, and Il14H, which has an inactive Bx14 allele and lacks HDM2BOA-Glc in leaves. Experiments with near-isogenic maize lines derived from crosses between B73 and Oh43 revealed that the absence of DIM2BOA-Glc and HDM2BOA-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 DIM2BOA-Glc and/or HDM2BOA-Glc provide specific protection against phloem feeding insects.

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