Rapid biosynthesis of N-linolenoyl-L-glutamine, an elicitor of plant volatiles, by membrane-associated enzyme(s) in Manduca sexta

Abstract

In response to elicitors in the oral secretions of caterpillars, plants produce and release volatile chemicals that attract predators and parasitoids of the caterpillar while it feeds. The most prevalent elicitors are fatty acid amides consisting of 18-carbon polyunsaturated fatty acids coupled with l-glutamine. We demonstrate rapid CoA- and ATP-independent in vitro biosynthesis of the fatty acid amide elicitor, N-linolenoyl-l-glutamine, by microsomal fractions of several alimentary tissues in Manduca sexta. N-linolenoyl-l-glutamine is a structural analog of several other elicitors including volicitin, the first fatty acid amide elicitor identified in caterpillars. The enzyme(s) that catalyzed biosynthesis of N-linolenoyl-l-glutamine was localized within the integral membrane protein fraction extracted from microsomes by Triton X-114 detergent phase partitioning and had maximum activity at alkaline pH. We found no evidence suggesting microbial or tissue-independent biosynthesis of N-linolenoyl-l-glutamine in M. sexta. The in vitro biosynthesis of N-linolenoyl-l-glutamine by membrane-associated enzyme(s) in M. sexta represents direct evidence of fatty acid amide synthesis by caterpillar tissues.

Fig. 1.

HPLC chromatograms of linolenic acid and related FAAs (structures shown) detected by UV analysis (200 nm) in M. sexta oral secretions (regurgitant) (A), M. sexta anterior midgut microsomes after in vitro incubation for 8hat4°C with sodium linolenate and either glutamine (B) or glutamic acid (C), and microbial suspensions derived from M. sexta regurgitant after in vitro incubation for8hat4°C with sodium linolenate and glutamine (D). Numbers represent the following: 1, N-linolenoyl-l-glutamine; 2, N-linolenoyl-l-glutamic acid; 3, linolenic acid. mAU, milliabsorbance units.

Fig. 2.

Time-dependent in vitro biosynthesis of N-linolenoyl-l-glutamine by Triton X-114-partitioned M. sexta midgut microsomes at pH 7.5. Integral, peripheral, and extremely hydrophobic protein fractions were each incubated with glutamine and sodium linolenate at 4°C. Data points represent mean ± SEM (n = 7).