Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/5/2007
Publication Date: 6/1/2007
Citation: Schmelz, E.A., Leclere, S.L., Carroll, M.J., Alborn, H.T., Teal, P.E. 2007. Cowpea chloroplastic ATP synthase is the source of multiple plant defense elicitors during insect herbivory. Plant Physiology. 144(2):793-805. Interpretive Summary: In response to insect attack some crop plants exhibit dynamic induced responses that include increases in biochemical defenses that inhibit insect growth and also volatiles that serve as long range attractants for natural enemies of the pests. In many systems, mechanisms that initiate the expression and specificity of these natural defenses is unknown. Scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, have discovered that caterpillar digestion of leaf chloroplastic ATP synthase proteins results in the production of 3 related peptide elicitors that stimulate plant defenses in legumes. Using expressed recombinant proteins as diet substrates, caterpillar feeding studies demonstrated the rapid and efficient production of these peptides. The smallest active elicitor isolated, an 11 amino acid peptide termed inceptin, also displayed the greatest residence time in the insect gut. Using different plants and tissues as insect diets, caterpillar oral secretion (OS) activity consistently correlated with the presence of these peptides. Similar to endogenous peptide hormones and bacterial peptide elicitors, inceptins induced three defense related phytohormones namely, jasmonic acid, ethylene and salicylic acid. Inceptins also trigger the production of plant volatiles associated with insect herbivory. Amino acid substitution studies of the peptide demonstrate the critical importance of the aspartic acid residues and conservation of the peptide C-terminal. Similar to peptide hormones and bacterial elicitors, structural characteristics and femtomolar activity levels support the hypothesis that inceptin-induced plant defenses are mediated by plant receptors.
Technical Abstract: Plant responses to damage vary dependant upon the nature of the biotic and abiotic stresses. We recently described an elicitor, from Fall armyworm (Spodoptera frugiperda) oral secretions (OS) termed inceptin, derived from chloroplastic ATP synthase '-subunit (cATPC) proteins that activate phytohormone and volatile production in cowpea (Vigna unguiculata). In the current study, three additional peptides similar to cowpea inceptin [Vu-In; +ICDINGVCVDA-], namely Vu-GE+In [+GEICDINGVCVDA-], Vu-E+In [+EICDINGVCVDA-], and Vu-In-A [+ICDINGVCVD-] were identified in larvae OS. Cowpea assays for induced-ethylene (E) production demonstrated similar EC50 values of 68, 45, and 87 fmol leaf-1 for Vu-E+In, Vu-In, and Vu-GE+In, respectively; however, Vu-In-A proved inactive. To examine production of these plant-derived peptides, a recombinant protein containing 8 repeated sequences spanning the soybean (Glycine max) inceptin core was constructed, expressed in E. coli and incorporated into diets. A time course of elicitor production in larvae OS revealed similar concentrations of the active elicitors shortly following ingestion and 80% recovery of potential inceptins. Rapidly shifting ratios were consistent with continued proteolysis and the preferential stability of inceptin. Spinach (Spinacia oleracea) fed larvae rapidly lost all OS-based E-inducing activity coincident with a trypsin cleavage site within So-In. Larvae OS containing 800 fmol 'l-1 of active inceptins elicited a sequential induction of defense-related phytohormones jasmonic acid (JA), E, and salicylic acid (SA) in cowpea leaves at 30, 120, and 240 min, respectively. Similar to Vu-In-A, a sequential Ala substitution of Vu-In demonstrated an essential role for the penultimate C-terminal Asp for the induction of all investigated analytes. Insect ingestion of plant proteins affords a unique system to explore the dynamics, activity, and role of peptides in mediating plant-biotic interactions.