UGT74S1 is the key player in controlling secoisolariciresinol diglucoside (SDG) formation in flax
Citation
Fofana, B., Ghose, K., McCallum, J., You, F.M., Cloutier, S. (2017). UGT74S1 is the key player in controlling secoisolariciresinol diglucoside (SDG) formation in flax. BMC Plant Biology, [online] 17(1), http://dx.doi.org/10.1186/s12870-017-0982-x
Plain language summary
The health benefits of flax seeds are largely attributed to one of its phytochemical called lignan and present in the seed coat. Understanding how lignan is produced in flax seed at genetic and biochemical levels is necessary to produce flax seed that have maximum health benefit. Previous studies in our laboratory identified a flax gene that produces the final form of lignan through two steps. Due to the presence of many similar genes in the flax genome, it was however unknown if these other genes of the family were also functional in the lignan biosynthesis. In the current study, the entire flax genome was screened for all genes present in the family using bioinformatics tools. The study determined the number of genes, the evolutionary process that led to the gene diversification, and identified two genes as being a copy of one another and both were the most closely related to the lignan-making gene previously reported. The two genes were biochemically characterized for their ability to make lignan in vitro and were found unable to accomplish the full lignan production as performed by the control known gene. The results demonstrated that one gene in the flax genome may be the key player in controlling the final step of lignan production in flax seed. The genetic control of the final step in lignan biosynthesis in flax seed ensures the chemical stability of the lignan molecule and reduces its reactivity. Nonetheless, it reduces the bioavailability of the phytochemical to the human body. Knowledge of the biosynthetic genes and biosynthetic processes are essential for the production of healthy and bioavailable metabolites in the seed and/or in vitro (e.g. in a fermenter system) by creating and using different mutant versions of the gene.
Abstract
Background: Flax lignan, commonly known as secoisolariciresinol (SECO) diglucoside (SDG), has recently been reported with health-promoting activities, including its positive impact in metabolic diseases. However, not much was reported on the biosynthesis of SDG and its monoglucoside (SMG) until lately. Flax UGT74S1 was recently reported to sequentially glucosylate SECO into SMG and SDG in vitro. However, whether this gene is the only UGT achieving SECO glucosylation in flax was not known. Results: Flax genome-wide mining for UGTs was performed. Phylogenetic and gene duplication analyses, heterologous gene expression and enzyme assays were conducted to identify family members closely related to UGT74S1 and to establish their roles in SECO glucosylation. A total of 299 different UGTs were identified, of which 241 (81%) were duplicated. Flax UGTs diverged 2.4-153.6 MYA and 71% were found to be under purifying selection pressure. UGT74S1, a single copy gene located on chromosome 7, displayed no evidence of duplication and was deemed to be under positive selection pressure. The phylogenetic analysis identified four main clusters where cluster 4, which included UGT74S1, was the most diverse. The duplicated UGT74S4 and UGT74S3, located on chromosomes 8 and 14, respectively, were the most closely related to UGT74S1 and were differentially expressed in different tissues. Heterologous expression levels of UGT74S1, UGT74S4 and UGT74S3 proteins were similar but UGT74S4 and UGT74S3 glucosylation activity towards SECO was seven fold less than UGT74S1. In addition, they both failed to produce SDG, suggesting neofunctionalization following their divergence from UGT74S1. Conclusions: We showed that UGT74S1 is closely related to two duplicated genes, UGT74S4 and UGT74S3 which, unlike UGT74S1, failed to glucosylate SMG into SDG. The study suggests that UGT74S1 may be the key player in controlling SECO glucosylation into SDG in flax although its closely related genes may also contribute to a minor extent in supplying the SMG precursor to UGT74S1.