Abstract
The association of cabbage white butterflies (Pieris spec., Lepidoptera: Pieridae) with their glucosinolate-containing host plants represents a well-investigated example of the sequential evolution of plant defenses and insect herbivore counteradaptations. The defensive potential of glucosinolates, a group of amino acid-derived thioglucosides present in plants of the Brassicales order, arises mainly from their rapid breakdown upon tissue disruption resulting in formation of toxic isothiocyanates. Larvae of P. rapae are able to feed exclusively on glucosinolate-containing plants due to expression of a nitrile-specifier protein in their gut which redirects glucosinolate breakdown to the formation of nitriles. The release of equimolar amounts of cyanide upon further metabolism of the benzylglucosinolate-derived nitrile suggests that the larvae are also equipped with efficient means of cyanide detoxification such as β-cyanoalanine synthases or rhodaneses. While insect β-cyanoalanine synthases have recently been identified at the molecular level, no sequence information was available of characterized insect rhodaneses. Here, we identify and characterize two single-domain rhodaneses from P. rapae, PrTST1 and PrTST2. The enzymes differ in their kinetic properties, predicted subcellular localization and expression in P. rapae indicating different physiological roles. Phylogenetic analysis together with putative lepidopteran rhodanese sequences indicates an expansion of the rhodanese family in Pieridae.
Highlights
Plants have evolved various defense mechanisms against herbivore attacks
As efficient cyanide detoxification mechanisms might have likely been a prerequisite for the host plant shift from Fabales to Brassicales, we were especially interested in the catalytic properties of rhodaneses in comparison to β-cyanoalanine synthases from P. rapae as well as rhodanese diversification within Lepidoptera
Two entries annotated as thiosulfate sulfurtransferase were retrieved from a P. rapae RNAseq transcriptome database and designated as P. rapae thiosulfate sulfurtransferase 1 (PrTST1) and P. rapae thiosulfate sulfurtransferase 2 (PrTST2)
Summary
Plants have evolved various defense mechanisms against herbivore attacks. In turn, insects have developed strategies to overcome these defenses. The presence of isotopically labelled β-cyanoalanine and thiocyanate (SCN−) in P. rapae larvae after fumigation with [15N]HCN indicated that larvae detoxify cyanide by both β-cyanoalanine synthases and rhodaneses[18] Both enzyme activities were detected in larval gut tissue[26,27]. The ubiquitous distribution of rhodanese activity in organisms that are normally not exposed to high levels of cyanide and the low physiological concentration of the substrate thiosulfate make the case for alternative physiological functions[23,33] These include a role in sulfur and selenium metabolism, the synthesis of iron-sulfur proteins, and redox regulation[33,34]). As efficient cyanide detoxification mechanisms might have likely been a prerequisite for the host plant shift from Fabales to Brassicales, we were especially interested in the catalytic properties of rhodaneses in comparison to β-cyanoalanine synthases from P. rapae as well as rhodanese diversification within Lepidoptera
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