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Summary Herbivore‐induced plant volatiles act as danger signals to prime defense responses in neighboring plants, yet in many cases the mechanism behind this priming is not known. Volatile signals may be recognized directly by receptors and/or converted into other active compounds. Here we investigate the metabolic fate of volatile indole, a known priming signal in maize (Zea mays), to determine if its conversion to other compounds could play a role in its priming of defenses.We identified benzoxazinoids as major products from volatile indole using heavy isotope‐labeled volatile indole and Pathway of Origin Determination in Untargeted Metabolomics (PODIUM) analysis. We then used benzoxazinoid biosynthesis maize mutants to investigate their role in indole‐mediated priming.Labeled volatile indole was converted into DIMBOA‐glucoside in abx2(benzoxazinone synthesis2)‐dependent manner. Thebx2mutant plants showed elevated green leaf volatile (GLV) production in response to wounding andSpodoptera frugiperdaregurgitant irrespective of indole exposure.Thus, volatile indole is converted into benzoxazinoids, and part of its priming mechanism may be due to the enhanced production of these phytoanticipins. However, indole‐mediated enhanced GLV production does not rely on the conversion of indole to benzoxazinoids, so indole also has other signaling functions. 

Abstract Prenylated quinones are membrane-associated metabolites that serve as vital electron carriers for respiration and photosynthesis. The UbiE (EC 2.1.1.201)/MenG (EC 2.1.1.163) C-methyltransferases catalyze pivotal ring methylations in the biosynthetic pathways of many of these quinones. In a puzzling evolutionary pattern, prokaryotic and eukaryotic UbiE/MenG homologs segregate into 2 clades. Clade 1 members occur universally in prokaryotes and eukaryotes, excluding cyanobacteria, and include mitochondrial COQ5 enzymes required for ubiquinone biosynthesis; Clade 2 members are specific to cyanobacteria and plastids. Functional complementation of an Escherichia coli ubiE/menG mutant indicated that Clade 1 members display activity with both demethylbenzoquinols and demethylnaphthoquinols, independently of the quinone profile of their original taxa, while Clade 2 members have evolved strict substrate specificity for demethylnaphthoquinols. Expression of the gene-encoding bifunctional Arabidopsis (Arabidopsis thaliana) COQ5 in the cyanobacterium Synechocystis or its retargeting to Arabidopsis plastids resulted in synthesis of a methylated variant of plastoquinone-9 that does not occur in nature. Accumulation of methylplastoquinone-9 was acutely cytotoxic, leading to the emergence of suppressor mutations in Synechocystis and seedling lethality in Arabidopsis. These data demonstrate that in cyanobacteria and plastids, co-occurrence of phylloquinone and plastoquinone-9 has driven the evolution of monofunctional demethylnaphthoquinol methyltransferases and explains why plants cannot capture the intrinsic bifunctionality of UbiE/MenG to simultaneously synthesize their respiratory and photosynthetic quinones.