Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/12/2008
Publication Date: 1/13/2009
Citation: Schmelz, E.A., Engelberth, J., Alborn, H.T., Tumlinson, J.H., Teal, P.E. 2009. Phytohormone-based activity mapping of insect herbivore-produced elicitors. Proceedings of the National Academy of Sciences. 106(2):653-657. Interpretive Summary: Many plants respond to insect herbivory by increasing the production of protein and chemical defenses to protect against future damage. Specific biochemicals present in insect gut, salivary and oviposition fluids are recognized by plants during attack and trigger complex signaling cascades leading to these inducible responses. Over the past decade, multiple classes of plant defense elicitors have been identified in insects. Despite these advances, very little is know about the breath of elicitor activity across a diverse array of plants or if different elicitor classes trigger similar signaling pathways in plants. By testing multiple elicitor classes a diverse array of plants, scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that rapid changes in phytohormone signals represent highly conserved plant responses indicative of insect elicitor recognition. In contrast, the presence or absence of elicitor activity is highly idiosyncratic between even closely related plants. Together, these results strongly support the existence of specific plant receptors that detect unique classes of insect elicitors and initiate induced plant defense responses. Knowledge of insect elicitors and their corresponding plant receptors will provide essential tools for engineering new pest resistance mechanisms into crop plants.
Technical Abstract: In response to insect attack many plants exhibit dynamic biochemical changes resulting in the induced production of direct and indirect defenses. Elicitors present in herbivore oral secretions are believed to positively regulate many inducible plant defenses; however, little is known about the specificity of elicitor recognition in plants. To investigate the phylogenic distribution of elicitor activity, we tested representatives from 3 different elicitor classes on the time course of defense-related phytohormone production; including, ethylene, jasmonic acid, and salicylic acid, in range of plant species spanning angiosperm diversity. All families examined responded to at least one elicitor class with significant increases in ethylene and jasmonic acid production within 1 to 2 hours after treatment; yet, elicitation activity among species was highly idiosyncratic. The fatty-acid amino acid conjugate volicitin exhibited the widest range of phytohormone and volatile inducing activity which spanned maize (Zea mays), soybean (Glycine max) and eggplant (Solanum melongena). In contrast, the activity of inceptin-related peptides, originally described in cowpea (Vigna unguiculata), was limited even within the Fabaceae. Similarly, caeliferin A16:0, a disulfooxy fatty acid from grasshoppers, was the only elicitor with demonstrable activity in Arabidopsis thaliana. While precise mechanisms remain unknown, the unpredictable nature of elicitor activity between plant species supports the existence of specific receptor-ligand interactions mediating recognition. Despite the lack of an ideal plant model for studying the action of numerous elicitors, ethylene and jasmonic acid exist as highly conserved and readily quantifiable markers for future discoveries in this field.