P27: The transcriptomic landscape of the anaerobic fungus Orpinomyces sp. strain C1A on different cellulosic biomass substrates

Anaerobic gut fungi reside in the rumen, hindgut, and feces of ruminant and non-ruminant herbivores. We recently sequenced the genome of the anaerobic fungus Orpinomyces sp. strain C1A, and described its extensive lignocellulolytic gene repertoire. Here, we hypothesized that the observed expansion of carbohydrate active enzymes (CAZymes) in strain C1A is a reflection of its capability to metabolize a wide range of plant species. We tested this hypothesis by examining the transcriptional profile of strain C1A when grown on defined substrates (glucose, cellobiose, cellulose, and xylan) and a variety of plant substrates  (switchgrass, sorghum, corn stover, alfalfa, Bermuda grass, and energy cane). Our analyses indicated that: 1. A large number of polymer-degrading GH genes were constitutively expressed in low levels when strain C1A was grown on sugar monomers, but that growth on polysaccharides and plant biomass resulted in a significant increase in the overall transcription levels of relevant CAZymes (e.g. GH5 cellulases and GH9 and GH48 cellobiohydrolases in cellulose grown cultures).  2. The increase in overall transcription levels of a specific GH family was mainly attributable to the upregulation of only a fraction of GH genes belonging to this family. 3. When grown on plant substrates, the overall transcription profiles at the CAZyme family level did not change signficantly, although pronounced variations in the expression levels of specific CAZyme genes were identified. The results demonstrate the overall versatility of strain C1A plant metabolism and demonstrate the relative importance of specific GH families in cellulose and hemicellulose degradation in anaerobic fungi.