Abstract
BackgroundLignin deposited in plant cell walls negatively affects biomass conversion into advanced bioproducts. There is therefore a strong interest in developing bioenergy crops with reduced lignin content or altered lignin structures. Another desired trait for bioenergy crops is the ability to accumulate novel bioproducts, which would enhance the development of economically sustainable biorefineries. As previously demonstrated in the model plant Arabidopsis, expression of a 3-dehydroshikimate dehydratase in plants offers the potential for decreasing lignin content and overproducing a value-added metabolic coproduct (i.e., protocatechuate) suitable for biological upgrading.ResultsThe 3-dehydroshikimate dehydratase QsuB from Corynebacterium glutamicum was expressed in the bioenergy crop switchgrass (Panicum virgatum L.) using the stem-specific promoter of an O-methyltransferase gene (pShOMT) from sugarcane. The activity of pShOMT was validated in switchgrass after observation in-situ of beta-glucuronidase (GUS) activity in stem nodes of plants carrying a pShOMT::GUS fusion construct. Under controlled growth conditions, engineered switchgrass lines containing a pShOMT::QsuB construct showed reductions of lignin content, improvements of biomass saccharification efficiency, and accumulated higher amount of protocatechuate compared to control plants. Attempts to generate transgenic switchgrass lines carrying the QsuB gene under the control of the constitutive promoter pZmUbi-1 were unsuccessful, suggesting possible toxicity issues associated with ectopic QsuB expression during the plant regeneration process.ConclusionThis study validates the transfer of the QsuB engineering approach from a model plant to switchgrass. We have demonstrated altered expression of two important traits: lignin content and accumulation of a co-product. We found that the choice of promoter to drive QsuB expression should be carefully considered when deploying this strategy to other bioenergy crops. Field-testing of engineered QsuB switchgrass are in progress to assess the performance of the introduced traits and agronomic performances of the transgenic plants.
Highlights
Lignin deposited in plant cell walls negatively affects biomass conversion into advanced bioproducts
This study validates the transfer of the QsuB engineering approach from a model plant to switchgrass
We found that the choice of promoter to drive QsuB expression should be carefully considered when deploying this strategy to other bioenergy crops
Summary
Lignin deposited in plant cell walls negatively affects biomass conversion into advanced bioproducts. There is a strong interest in developing bioenergy crops with reduced lignin content or altered lignin structures. Other desired traits for bioenergy crops include high biomass yields, stress resilience, reduced recalcitrance to conversion into biofuels and bioproducts, and the accumulation of valuable co-products [1, 2]. The heterologous expression of a bacterial 3-dehydroshikimate dehydratase (QsuB) targeted to plastids resulted in strong lignin reductions (up to 50%) in Arabidopsis [8]. One explanation for this observation is the possible reduction of the cytosolic shikimate pool needed for the synthesis of pcoumaroyl-shikimate catalyzed by hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT) during lignin biosynthesis (Fig. 1)
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