RNAi downregulation of three key lignin genes in sugarcane improves glucose release without reduction in sugar production

Abstract

BackgroundSugarcane is a subtropical crop that produces large amounts of biomass annually. It is a key agricultural crop in many countries for the production of sugar and other products. Residual bagasse following sucrose extraction is currently underutilized and it has potential as a carbohydrate source for the production of biofuels. As with all lignocellulosic crops, lignin acts as a barrier to accessing the polysaccharides, and as such, is the focus of transgenic efforts. In this study, we used RNAi to individually reduce the expression of three key genes in the lignin biosynthetic pathway in sugarcane. These genes, caffeoyl-CoA O-methyltransferase (CCoAOMT), ferulate 5-hydroxylase (F5H) and caffeic acid O-methyltransferase (COMT), impact lignin content and/or composition.ResultsFor each RNAi construct, we selected three events for further analysis based on qRT-PCR results. For the CCoAOMT lines, there were no lines with a reduction in lignin content and only one line showed improved glucose release. For F5H, no lines had reduced lignin, but one line had a significant increase in glucose release. For COMT, one line had reduced lignin content, and this line and another released higher levels of glucose during enzymatic hydrolysis. Two of the lines with improved glucose release (F5H-2 and COMT-2) also had reduced S:G ratios.ConclusionsAlong with improvements in bagasse quality for the production of lignocellulosic-based fuels, there was only one line with reduction in juice sucrose extraction, and three lines with significantly improved sucrose production, providing evidence that the alteration of sugarcane for improved lignocellulosic ethanol production can be achieved without negatively impacting sugar production and perhaps even enhancing it.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0683-y) contains supplementary material, which is available to authorized users.

Peer Review #2 of "Cell wall composition and lignin biosynthetic gene expression along a developmental gradient in an Australian sugarcane cultivar (v0.1)"

“…We have previously published work describing the downregulation of both the F5H and CCoAOMT in sugarcane, with the result being increased glucose release by enzymatic hydrolysis but with no decrease in lignin. In the F5H lines this was attributed to a change in the lignin monomer ratio (Bewg et al 2016) The final gene assessed in our study, C3H, had the highest expression level in Section B, immediately below the most juvenile Section A. C3H catalyzes the second aromatic hydroxylation reaction in the lignin biosynthesis pathway and is an important hub in controlling metabolic flux into G and S lignin monomer synthesis (Barriere et al 2004;Weng & Chapple 2010). CCoAOMT, along with C3H, are hypothesized to be important control points for cell wall lignification by acting as part of the ferulate production pathway (Barriere et al 2004).…”

Cell wall composition and lignin biosynthetic gene expression along a developmental gradient in an Australian sugarcane cultivar

“…The increased expression of COMT in young tissue in this research may be to ensure S monomer production during the availability of the initial metabolic flux. Previous work has shown that the RNAi suppression of COMT in sugarcane resulted in decreased lignin and altered S:G ratio ( Bewg et al, 2016 ).…”

Sugarcane biomass conversion influenced by lignin

“…For the COMT gene, one line presented lower lignin content and, along with another line, elevated levels of glucose were observed during enzymatic hydrolysis. Two of the improved glucose release lines (F5H‐2 and COMT‐2) also showed reduced S : G ratios …”

“…Two of the improved glucose release lines (F5H-2 and COMT-2) also showed reduced S : G ratios. 43 Research has shown that it is possible to reduce sugarcane lignification successfully through RNA interference, despite its highly complex and polyploid genome. Suppression of COMT expression by 67% to 97% reduced the lignin content by 3.9% to 13.7%, respectively.…”