The Airborne Herbivore-Induced Plant Volatile Indole is Converted to Benzoxazinoid Defense Compounds in Maize Plants.

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 a bx2 (benzoxazinone synthesis2)-dependent manner. The bx2 mutant plants showed elevated green leaf volatile (GLV) production in response to wounding and Spodoptera frugiperda regurgitant 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.