Salicyl Alcohol Oxidase of the Chemical Defense Secretion of Two Chrysomelid Leaf Beetles

Abstract

Salicyl alcohol oxidase is an extracellular enzyme that occurs in glandular reservoirs of chrysomelid leaf beetle larvae and catalyzes the formation of salicylaldehyde, a volatile deterrent used by the larvae against predators. Salicyl alcohol is the hydrolysis product of salicin, a plant-derived precursor taken up by the beetle larvae from the leaves of willow and poplar trees. The cDNA encoding salicyl alcohol oxidase from two related species Chrysomela tremulae and Chrysomela populi has been identified, cloned, and expressed in an active form in Escherichia coli. The open reading frame of 623 amino acids begins in both enzymes with an N-terminal signal peptide of 21 amino acids. Sequence comparison has revealed that salicyl alcohol oxidase belongs to the family of glucose-methanol-choline oxidoreductase-like sequences with mostly unknown function. Enzymes of this family share similar overall structure with an essentially identical FAD-binding site but possess different catalytic activities. The data suggest that salicyl alcohol oxidase, essential for the activation of the plant-derived precursor salicin, was originally recruited from an oxidase involved in the autogenous biosynthesis of iridoid monoterpenes and found in related chrysomelid leaf beetle species.

Precise RNAi-mediated silencing of metabolically active proteins in the defence secretions of juvenile leaf beetles

“…Targeting the defensive glands of juvenile poplar leaf beetles by RNA interference Recently, a 1872-bp CpopSAO cDNA (Genbank/ HQ245154.1) encoding a 69 kDa protein for conversion of 2 into 3 was identified from the larval defensive glands of C. populi [13,14] (figure 2a). It belongs to the glucose -methanol -choline (GMC) family of oxidoreductases [38].…”

“…Enzymatic reactions in the defensive secretions of juvenile C. populi and P. cochleariae adapted from Michalski et al [14]. Glc, Glucose.…”

“…The hydrophobic aldehyde forms an organic layer, accounting for 15 per cent of the total discharge volume, while the aqueous phase constitutes 85 per cent [12]. The latter contains the precursor salicyl alcohol (2) and a flavine-dependent salicyl alcohol oxidase (SAO); the SAO uses molecular oxygen as an electron acceptor for alcohol oxidation, yielding the aldehyde and hydrogen peroxide [12][13][14] (figure 1). Salicyl aldehyde is considered as a potent repellent against generalist predators [11,15] and as an antimicrobial agent [16].…”

“…Despite the many current genome-and transcriptomesequencing projects, up to now it has only been shown for SAO sequences to be entangled in allomone production in the defensive secretions of the leaf beetle species Chrysomela tremulae, Chrysomela populi, Chrysomela lapponica and Phratora vitellinae [13,14,26]. To demonstrate the in vivo relevance of a target sequence, gene silencing by RNA interference (RNAi) is a suitable method.…”

To be or not to be convergent in salicin-based defence in chrysomeline leaf beetle larvae: evidence from Phratora vitellinae salicyl alcohol oxidase

“…Therefore, to clarify the origin of salicylaldehyde-based chemical defence, the identification, characterization and comparison of the enzymes involved in its biosynthesis are required. In case of several Chrysomela ssp., salicyl alcohol oxidase (SAO), which catalyses the final step in salicylaldehyde formation, has been elucidated and previous studies indicate a similar enzymatic activity and the presence of an SAO-related protein in the glandular secretion of P. vitellinae [17][18][19][20]. However, the enzyme itself is not known.…”