In situ translocation of volicitin by beet armyworm larvae to maize and systemic immobility of the herbivore elicitor in planta

Abstract

Volicitin (N-[17-hydroxylinolenoyl]-L glutamine) present in the regurgitant of beet armyworm (Spodoptera exigua) activates the emissions of volatile organic compounds (VOCs) when in contact with damaged corn (Zea mays L.) leaves. VOC emission in turn serves as a signaling defense for the plant by attracting female parasitic wasps that prey on herbivore larvae. Chemical tracking of volicitin within plants has yet to be reported. Here we present biochemical data that beet armyworm regurgitant serves as a vector for the introduction of volicitin to the site of leaf damage under natural feeding conditions. Corn seedlings were 14CO2-labeled in situ, and beet armyworm larvae were allowed to feed on the labeled leaves. Herbivore oral secretions collected from late-third-instar larvae contained approximately 120 pmol volicitin (0.05 nCi pmol(-1)) per larva. When radiochemically labeled larvae were placed on unlabeled leaves, the amount of volicitin introduced to the damaged site was approximately 5.0 nCi (calc. 100 pmol/larvae). The mobility of volicitin in leaves was examined by allowing radiolabeled beet armyworms to feed on unlabeled plants. In such tracking experiments, radioactivity was not detected in the upper leaves; however, the exogenous application of 5 nCi of [U-14C]sucrose to the lower leaf did result in subsequent radioactivity being detected in the upper portion of the plant. The detection of labeled sucrose with the same radioactivity as that of administered volicitin indicated that volicitin was not readily transported to undamaged leaves and that volicitin may not directly serve as a mobile messenger in triggering the emissions of VOCs systemically.

Absolute Configuration of Volicitin from the Regurgitant of Lepidopteran Caterpillars and Biological Activity of Volicitin-Related Compounds

“…More interestingly, the substitution of Lleucine, L-phenylalanine, L-proline, and L-threonine for L-glutamine resulted in a loss of the elicitor activity ( 24) and also that volicitin may not directly serve as a mobile messenger in triggering the release of volatile compounds systemically. 25) Considering the existence of a volicitin-binding protein, the results obtained in the present study suggest that the Lglutamine moiety of volicitin played an important role in binding. At this point, we do not know how a volicitin-binding protein functions as an activator of second messenger(s) to trigger the release of volatile organic compounds at undamaged sites in plants.…”

Active role of fatty acid amino acid conjugates in nitrogen metabolism in Spodoptera litura larvae

“…A substance(s) other than volicitin was suggested to serve as a mobile signal within the plant for systemically induced VOC emissions (15). Furthermore, application of volicitin to mechanically wounded leaves specifically induces the transcription of indole-3-glycerol phosphate lyase (Igl) and a specific sesquiterpene cyclase (stc1) in wounded as well as undamaged leaves on the same plant (16,17).…”

“…The metabolism of FACs was exploited by tracing 15 N-, 13 C-, and 14 C-labeling components. This report offers both specifics and perspective on FAC function(s) for caterpillar physiology.…”

“…These results clearly revealed that a 15 N-labeled nitrogen atom was incorporated into volicitin and the other FACs. However, 15 N NMR analyses of FACs, partially purified from the gut contents to remove other possible 15 N-labeled contamination such as amino acids, gave only one signal at 106.7 ppm ( To demonstrate the role of linolenic acid on nitrogen assimilation, S. litura larvae were fed a diet enriched with glutamic acid and ammonium chloride, with or without the addition of linolenic acid. Comparison of the amount of nitrogen in the diet with that excreted in the frass by the larvae showed that larvae on the nitrogen-enriched diet were able to assimilate 37.7 Ϯ 1.2% (n ϭ 3) of its nitrogen contents.…”

“…However, adding linolenic acid to the diet increased the nitrogen assimilation efficiency to 60.6 Ϯ 0.8% (n ϭ 5). The experiments were repeated with 15 N labeling on ammonium chloride or glutamic acid to see in detail how linolenic acid worked for nitrogen assimilation. In both labeling cases, linolenic acid in the diet approximately doubled the total amount of labeled glutamine in the hemolymph (Fig.…”