Glucosinolates, a class of secondary metabolites from cruciferous plants, are derived from amino acids and have diverse biological activities, such as in biotic defense, depending on their side chain modification. The first structural modification step in the synthesis of aliphatic (methionine-derived) glucosinolates—S-oxygenation of methylthioalkyl glucosinolates to methylsulfinylalkyl glucosinolates—was found to be catalyzed by five flavin-containing monooxygenases (FMOs), FMOGS-OX1-5. Here, we report two additional FMOGS-OX enzymes, FMOGS-OX6, and FMOGS-OX7, encoded by At1g12130 and At1g12160, respectively. The overexpression of both FMOGS-OX6 and FMOGS-OX7 decreased the ratio of methylthioalkyl glucosinolates to the sum of methylthioalkyl and methylsulfinylalkyl glucosinolates, suggesting that the introduction of the two genes converted methylthioalkyl glucosinolates into methylsulfinylalkyl glucosinolates. Analysis of expression pattern revealed that the spatial expression of the two genes is quite similar and partially overlapped with the other FMOGS-OX genes, which are primarily expressed in vascular tissue. We further analyzed the responsive expression pattern of all the seven FMOGS-OX genes to exogenous treatment with abscisic acid, 1-aminocyclopropane-1-carboxylic acid (ACC), jasmonic acid (JA), salicylic acid, indole-3-acetic acid (IAA), and low and high temperatures. Although these genes showed same tendency toward the changing stimulus, the sensitivity of each gene was quite different. The variety in spatial expression among the FMOGS-OX genes while responding to environmental stimulus indicated a complex and finely tuned regulation of glucosinolates modifications. Identification of these two novel FMOGS-OX enzymes will enhance the understanding of glucosinolates modifications and the importance of evolution of these duplicated genes.

Glucosinolates, a class of specialized metabolites specific to the order Brassicales, have diverse bioactivities that are largely dependent on the structures of their side chains. Flavin‐containing monooxygenases (FMOs) encoded by the FMOGS‐OX genes have been found to catalyze side‐chain modifications during the synthesis of methionine‐derived aliphatic glucosinolates. Seven FMOGS‐OX genes have been identified in Arabidopsis Heynh., but the evolution of these genes in the Brassicaceae, a family including many economically important vegetables, is poorly understood. In this study, the phylogenetic and syntenic relationships of the FMOGS‐OX genes belonging to 12 sequenced Brassicaceae species were analyzed. Our results showed that the FMOGS‐OX genes included two tandem arrays, the FMOGS‐OX2‐4 group (group A) and the FMOGS‐OX5‐7 group (group B). The evolutionary histories of the FMOGS‐OX groups A and B were similar across the Brassicaceae, but two lineage‐specific evolutionary routes developed after these two separate species lineages diverged from Aethionema arabicum (L.) Andrz. ex DC. In the lineage I route, FMOGS‐OX gene copies tended to increase due to frequent tandem duplication events in most species and a whole genome triplication in Camelina sativa (L.) Crantz. In the lineage II route, gene copies decreased due to deletion events. Combining these results with those of previous studies, we speculated that the FMOGS‐OX genes were derived from an ancestral gene with a broad expression distribution and a broad range of substrates, which then underwent subfunctionalization to generate progeny limited in either spatial expression or substrate structure. Furthermore, the absence of FMOGS‐OX5 substrates in some FMOGS‐OX5‐containing species may suggest neofunctionalization of these genes.

Plants are constantly defending themselves against an array of assaults by pathogenic organisms. This has led to the evolution of precise and elaborate chemical defense systems involving glucosinolates (GSLs) in cruciferous plants. These GSLs and their hydrolysis products are biologically active and are implicated as enabling formidable plant defense processes in certain economically important members of Brassicaceae like broccoli, cabbage and mustard seed. This review provides a comprehensive report of how indole and aliphatic GSLs mitigate incidents of plant pathogenesis. By evaluating the roles of GSLs in plant-pathogen interaction of some brassica plants, this review highlights the associated mechanism that culminates in disease suppression. Moreover, seven economically important brassica pathogens were reviewed in terms of their ability to disrupt proper plant functioning as well as the mechanisms by which GSLs and their hydrolysis products in Brassica lower the susceptibility to them. Future perspectives of the application of GSLs in plant pathogen resistance using advanced molecular techniques are also discussed.