초록
<P><B>Background</B></P><P>The inherent recalcitrance of lignocellulosic biomass is one of the major economic hurdles for the production of fuels and chemicals from biomass. Additionally, lignin is recognized as having a negative impact on enzymatic hydrolysis of biomass, and as a result much interest has been placed on modifying the lignin pathway to improve bioconversion of lignocellulosic feedstocks.</P><P><B>Results</B></P><P>Down-regulation of the caffeic acid 3-<I>O</I>-methyltransferase (COMT) gene in the lignin pathway yielded switchgrass (<I>Panicum virgatum</I>) that was more susceptible to bioconversion after dilute acid pretreatment. Here we examined the response of these plant lines to milder pretreatment conditions with yeast-based simultaneous saccharification and fermentation and a consolidated bioprocessing approach using <I>Clostridium thermocellum</I>, <I>Caldicellulosiruptor bescii</I> and <I>Caldicellulosiruptor obsidiansis</I>. Unlike the <I>S. cerevisiae</I> SSF conversions, fermentations of pretreated transgenic switchgrass with <I>C. thermocellum</I> showed an apparent inhibition of fermentation not observed in the wild-type switchgrass. This inhibition can be eliminated by hot water extraction of the pretreated biomass, which resulted in superior conversion yield with transgenic versus wild-type switchgrass for <I>C. thermocellum</I>, exceeding the yeast-based SSF yield. Further fermentation evaluation of the transgenic switchgrass indicated differential inhibition for the <I>Caldicellulosiruptor</I> sp. strains, which could not be rectified by additional processing conditions. Gas chromatography–mass spectrometry (GC-MS) metabolite profiling was used to examine the fermentation broth to elucidate the relative abundance of lignin derived aromatic compounds. The types and abundance of fermentation-derived-lignin constituents varied between <I>C. thermocellum</I> and each of the <I>Caldicellulosiruptor</I> sp. strains.</P><P><B>Conclusions</B></P><P>The down-regulation of the COMT gene improves the bioconversion of switchgrass relative to the wild-type regardless of the pretreatment condition or fermentation microorganism. However, bacterial fermentations demonstrated strain-dependent sensitivity to the COMT transgenic biomass, likely due to additional soluble lignin pathway-derived constituents resulting from the COMT gene disruption. Removal of these inhibitory constituents permitted completion of fermentation by <I>C. thermocellum</I>, but not by the <I>Caldicellulosiruptor</I> sp. strains. The reason for this difference in performance is currently unknown.</P>