Evolution of specifier proteins in glucosinolate-containing plants.

BACKGROUND: The glucosinolate-myrosinase system is an activated chemical defense system found in plants of the Brassicales order. Glucosinolates are stored separately from their hydrolytic enzymes, the myrosinases, in plant tissues. Upon tissue damage, e.g. by herbivory, glucosinolates and myrosinases get mixed and glucosinolates are broken down to an array of biologically active compounds of which isothiocyanates are toxic to a wide range of organisms. Specifier proteins occur in some, but not all glucosinolate-containing plants and promote the formation of biologically active non-isothiocyanate products upon myrosinase-catalyzed glucosinolate breakdown. RESULTS: Based on a phytochemical screening among representatives of the Brassicales order, we selected candidate species for identification of specifier protein cDNAs. We identified ten specifier proteins from a range of species of the Brassicaceae and assigned each of them to one of the three specifier protein types (NSP, nitrile-specifier protein, ESP, epithiospecifier protein, TFP, thiocyanate-forming protein) after heterologous expression in Escherichia coli. Together with nine known specifier proteins and three putative specifier proteins found in databases, we subjected the newly identified specifier proteins to phylogenetic analyses. Specifier proteins formed three major clusters, named AtNSP5-cluster, AtNSP1-cluster, and ESP/TFP cluster. Within the ESP/TFP cluster, specifier proteins grouped according to the Brassicaceae lineage they were identified from. Non-synonymous vs. synonymous substitution rate ratios suggested purifying selection to act on specifier protein genes. CONCLUSIONS: Among specifier proteins, NSPs represent the ancestral activity. The data support a monophyletic origin of ESPs from NSPs. The split between NSPs and ESPs/TFPs happened before the radiation of the core Brassicaceae. Future analyses have to show if TFP activity evolved from ESPs at least twice independently in different Brassicaceae lineages as suggested by the phylogeny. The ability to form non-isothiocyanate products by specifier protein activity may provide plants with a selective advantage. The evolution of specifier proteins in the Brassicaceae demonstrates the plasticity of secondary metabolism within an activated plant defense system.

A thiocyanate-forming protein generates multiple products upon allylglucosinolate breakdown in Thlaspi arvense.

Glucosinolates, amino acid-derived thioglycosides found in plants of the Brassicales order, are one of the best studied classes of plant secondary metabolites. Together with myrosinases and supplementary proteins known as specifier proteins, they form the glucosinolate-myrosinase system that upon tissue damage gives rise to a number of biologically active glucosinolate breakdown products such as isothiocyanates, epithionitriles and organic thiocyanates involved in plant defense. While isothiocyanates are products of the spontaneous rearrangement of the glucosinolate aglycones released by myrosinase, the formation of epithionitriles and organic thiocyanates depends on both myrosinases and specifier proteins. Hydrolysis product profiles of many glucosinolate-containing plant species indicate the presence of specifier proteins, but only few have been identified and characterized biochemically. Here, we report on cDNA cloning, heterologous expression and characterization of TaTFP, a thiocyanate-forming protein (TFP) from Thlaspi arvense L. (Brassicaceae), that is expressed in all plant organs and can be purified in active form after heterologous expression in Escherichia coli. As a special feature, this protein promotes the formation of allylthiocyanate as well as the corresponding epithionitrile upon myrosinase-catalyzed hydrolysis of allylglucosinolate, the major glucosinolate of T. arvense. All other glucosinolates tested are converted to their simple nitriles when hydrolyzed in the presence of TaTFP. Despite its ability to promote allylthiocyanate formation, TaTFP has a higher amino acid sequence similarity to known epithiospecifier proteins (ESPs) than to Lepidium sativum TFP. However, unlike Arabidopsis thaliana ESP, its activity in vitro is not strictly dependent on Fe²âº addition to the assay mixtures. The availability of TaTFP in purified form enables future studies to be aimed at elucidating the structural bases of specifier protein specificities and mechanisms. Furthermore, identification of TaTFP shows that product specificities of specifier proteins can not be predicted based on amino acid sequence similarity and raises interesting questions about specifier protein evolution.