Tracking and receptor identification of the herbivore elicitor volicitin in plants

Volicitin (17-hydroxylinoleno)l-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) 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 from the herbivore larva to the damaged leaf and interaction of volicitin within the plant has yet to be reported.; First, we present biochemical data that beet armyworm feeding 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. The radiochemically labeled larvae were then placed on unlabeled leaves, and the introduction of volicitin was observed at the site of damage. The mobility of volicitin was examined in the upper leaves and radioactivity was not detected. However, the exogenous application of [14C-U]-sucrose to the lower leaf did result in radioactivity detection in the upper leaves.; To examine the interaction of volicitin at the cellular level, a tritiated form of volicitin was synthesized and shown to induce volatiles in the same fashion as the biological form. [3H]-L-volicitin rapidly, reversibly, and saturably bound to plasma membrane fractions isolated from maize leaves with an apparent K_d of 1.3 nM and a Hill coefficient of 1.07. Analog studies showed that the L-glutamine and hydroxy moieties of volicitin play an important role in binding. Treatment of plants with methyl jasmonate increased the total binding of [3H]-L-volicitin to the enriched plasma membrane fraction more than three-fold, suggesting that methyl jasmonate activates transcription of the gene encoding the binding protein. Beet armyworm feeding also increased total binding four-fold. Cycloheximide pre-treatment of plants significantly decreased binding of radiolableled volicitin. These data provide the first experimental evidence that volicitin is readily transported to undamaged leaves with herbivore feeding and can bind to a plasma membrane receptor protein that participates in the emission of VOCs.