Identification of Homogentisate Dioxygenase as a Target for Vitamin E Biofortification in Oilseeds

Abstract

Soybean (Glycine max) is a major plant source of protein and oil and produces important secondary metabolites beneficial for human health. As a tool for gene function discovery and improvement of this important crop, a mutant population was generated using fast neutron irradiation. Visual screening of mutagenized seeds identified a mutant line, designated MO12, which produced brown seeds as opposed to the yellow seeds produced by the unmodified Williams 82 parental cultivar. Using forward genetic methods combined with comparative genome hybridization analysis, we were able to establish that deletion of the GmHGO1 gene is the genetic basis of the brown seeded phenotype exhibited by the MO12 mutant line. GmHGO1 encodes a homogentisate dioxygenase (HGO), which catalyzes the committed enzymatic step in homogentisate catabolism. This report describes to our knowledge the first functional characterization of a plant HGO gene, defects of which are linked to the human genetic disease alkaptonuria. We show that reduced homogentisate catabolism in a soybean HGO mutant is an effective strategy for enhancing the production of lipid-soluble antioxidants such as vitamin E, as well as tolerance to herbicides that target pathways associated with homogentisate metabolism. Furthermore, this work demonstrates the utility of fast neutron mutagenesis in identifying novel genes that contribute to soybean agronomic traits.

WRINKLED1 and ACYL‐COA:DIACYLGLYCEROL ACYLTRANSFERASE1 regulate tocochromanol metabolism in Arabidopsis

“…Interestingly, in contrast to other tocochromanol mutants or transgenic plants in which tocochromanol metabolism has been enhanced, wri1 and dgat1 mutants did not accumulate any tocotrienols (Fig. a,b; Karunanandaa et al ., ; Zhang et al ., ; Stacey et al ., ). This suggests that, although HGA content is probably enhanced in wri1 and dgat1 to support increased tocochromanol synthesis, the prenyl pyrophosphate metabolism producing the phytyl side chain is very probably enhanced too, thus preventing the synthesis of tocotrienols.…”

“…Previous studies have concluded that the availability of tocochromanol biosynthetic precursors is an important regulatory mechanism controlling vitamin E synthesis in plants (reviewed by M ene-Saffran e & Pellaud, 2017). To date, only the HOMOGENTISATE DIOXYGENASE (HGO1) gene controlling HGA availability has been shown to alter tocotrienol biosynthesis in soybean (Glycine max) (Stacey et al, 2016). By contrast, tocopherol metabolism was not modified in the soybean hgo1 mutant, and hence tocopherol regulatory genes remained unknown.…”

Current strategies for vitamin E biofortification of crops

“…Since free HGA levels partially control tocochromanol biosynthesis, its degradation by HGA dioxygenase might potentially limit vitamin E biosynthesis. Indeed, MO12 mutant seeds contained 30 times higher HGA amounts and twice as much tocochromanols [51 ]. As for high-HGA transgenic plants, tocotrienol content was strongly enhanced in MO12 mutant seeds, while tocopherols were unchanged.…”

“…A novel mechanism limiting HGA availability and tocochromanol metabolism has been recently identified in the soybean MO12 mutant [51 ]. This deletion mutant notably lacks HGO1, a gene encoding an HGA dioxygenase that catalyzes the degradation of HGA into 4-maleylacetoacetate (Figure 2a).…”

Medicine is not health care, food is health care: plant metabolic engineering, diet and human health

“…Protection strategies may also prove effective in selecting for varieties of oil crops with enhanced vitamin E activity. A mutant of soya bean affected in the seed‐specific activity of homogentisate dioxygenase (which breaks down homogentisate) has two‐fold elevated concentrations of γ‐ and β‐tocopherols, four‐fold higher concentrations of α‐tocopherol and 27‐fold higher concentrations of tocotrienols compared to near isogenic controls (Stacey et al ., ). This analysis showed that tocotrienol synthesis is more dependent on homogentisate concentrations than tocopherol synthesis in oil seeds.…”