Leaf Developmental Stages Strongly Modulate Indole Emissions in Response to Simulated Insect Herbivory

Maize plants challenged by insect herbivores activate an array of defense measures, all aimed to reduce damage and repel. Among those are the activation of proteins that in-terfere with the digestion of consumed plant material in the herbivore (proteinase in-hibitors), the production of toxic compounds like benzoxazinoids, and the biosynthesis and emission of herbivore-induced plant volatiles (HIPV). Among those HIPVs are mainly a variety of terpenoids, green leaf volatiles (GLVs), and indole. While often serving as attractants for natural enemies of the attacking herbivores, many of those volatiles have also been found to induce defense responses in neighboring plants and/or prime them against future menace. Indole is of particular interest since it can be involved in a variety of biosynthetic pathways including those leading to auxin, benzoxazinoids, and tryp-tophan. Here, we demonstrate that indole emissions in response to simulated insect herbivory strongly depends on the developmental status of the affected leaf. Outgrown leaves emit significantly higher amounts of indole compared to the next younger, still growing leaves, distinguishing indole from other herbivore-induced plant volatiles (HIPVs), which are typically released at higher levels by young leaves. As a central and flexible metabolic intermediate, indole emissions appear to be mediated though variable allocation between growth-related processes and defense-associated outcomes, depending on developmental stage of the damaged leaf. These findings highlight the importance of considering plants as inherently dynamic organisms.