Sulfur‐Containing Secondary Metabolites from Arabidopsis thaliana and other Brassicaceae with Function in Plant Immunity

Abstract

Biosynthesis of antimicrobial secondary metabolites in response to microbial infection is one of the features of the plant immune system. Particular classes of plant secondary metabolites involved in plant defence are often produced only by species belonging to certain phylogenetic clades. Brassicaceae plants have evolved the ability to synthesise a wide range of sulfur-containing secondary metabolites, including glucosinolates and indole-type phytoalexins. A subset of these compounds is produced by the model plant Arabidopsis thaliana. Genetic tools available for this species enabled verification of immune functions of glucosinolates and camalexin (A. thaliana phytoalexin), as well as characterisation of their respective biosynthetic pathways. Current knowledge of the biosynthesis of Brassicaceae sulfur-containing metabolites suggests that the key event in the evolution of these compounds is the acquisition of biochemical mechanisms originating from detoxification pathways into secondary metabolite biosynthesis. Moreover, it is likely that glucosinolates and Brassicaceae phytoalexins, traditionally considered as separate groups of compounds, have a common evolutionary origin and are interconnected on the biosynthetic level. This suggests that the diversity of Brassicaceae sulfur-containing phytochemicals reflect phylogenetic clade-specific branches of an ancient biosynthetic pathway.

Two cytochromes P450 catalyze S-heterocyclizations in cabbage phytoalexin biosynthesis

“…1 ). 1 , 2 These compounds arise from the indole glucosinolate pathway 3 ( Fig. 1 ) and have been linked to the health benefits of these vegetables; e.g.…”

“…The link between glucosinolates and nutrition has motivated the commercialization of Beneforté broccoli 11 – a line bred for enhanced levels of glucoraphanin. In the context of plant health, glucosinolates 12 , 13 and camalexin, a phytoalexin made through a distinct biosynthetic pathway in Arabidopsis, 2 has exposed the role of small molecules in plant innate immunity. The availability of plant genome sequences offers an unprecedented opportunity to accelerate the discovery of plant biosynthetic pathways; however, there is an urgent need to translate functional genomics tools from model organisms to crops and medicinal plants.…”

Minimum Set of Cytochromes P450 for Reconstituting the Biosynthesis of Camalexin, a Major Arabidopsis Antibiotic

“…Subsequent coupling of this electrophile with glutathione could be facilitated in vivo by a dedicated or broadly acting GST followed by γ-glutamyl peptidase GGP1 or GGP3 and a yet-unidentified carboxypeptidase to yield Cys-IAN. [17] The sequential action of a P450 and GST is reminiscent of a conserved detoxification mechanism utilized here for natural product biosynthesis. P450s have acquired a stunning diversity of catalytic capacities through evolution, and protein-engineering efforts have revealed how this plasticity can have industrial importance as biocatalysts.…”

Both Biosynthesis and Transport Are Involved in Glucosinolate Accumulation During Root-Herbivory in Brassica rapa

“…Although camalexin is not present in B. rapa, there are other phytoalexins that the species produces in response to biotic stressors. A prominent phytoalexin in several Brassica species is brassinin (Klein and Sattely, 2017), which is synthesized from the unstable ITCs that are formed during hydrolysis of glucobrassicin (indol-3ylmethylglucosinolate) (Pedras et al, 2009;Bednarek, 2012). Glucobrassicin is one of the indole GSL that was induced by D. radicum feeding in this study.…”