Trichoderma reesei: A Fungal Enzyme Producer for Cellulosic Biofuels (2011)

Enzymes are proteins that catalyse chemical reactions by lowering the activation energy needed and thus speed up the reaction itself.The advantage of enzymes is that they can be applied under mild reaction conditions and that they exhibit high substrate specificity, stereoselectivity and less side product formation than conventional chemical reactions, making biotechnological processes often more cost-effective than chemical approaches.Microorganisms, such as bacteria and fungi are widely exploited for the industrial production of numerous enzymes.Filamentous fungi (moulds) can grow on a wide range of substrates and efficiently degrade biopolymers and are thus an attractive resource for new enzymes.The decomposition of cellulosic plant biomass to glucose monomers for biofuel production is a typical example for an application that requires an enzyme-based approach in order to specifically cleave the glycosidic bonds between the glucose monomers of the cellulose chain and release single glucose molecules.The main enzymes necessary to degrade cellulosic plant material are cellulases and hemicellulases.The filamentous fungus Trichoderma reesei is today a paradigm for commercial scale production of different cellulases and hemicellulases and is well adapted to fermenter cultivations.Beside well established applications of these enzymes in pulp, paper, food, feed or textile processing industries, these plant cell wall degrading enzymes are nowadays also employed for the saccharification of cellulosic plant biomass to simple sugars for biofuel production (Bouws et al., 2008;Harman and Kubicek, 1998; Kumar et al., 2008).The cellulolytic potential of this pantropical fungus was already recognized during WWII through the deterioration of cotton fabrics of the US Army.Strain QM6a (originally named T. viride) was isolated from the cotton canvas of an army tent from Bougainville Island (Solomon Islands).After identification of the fungus as the cause for the massive destruction, it was put under quarantine in the eponymous Quartermaster collection of the US army at Natick.Strain QM6a was later recognized as an own species and named after its principal investigator in those years Elwyn T. Reese (Reese, 1976).It is an important peculiarity that this T. reesei strain QM6a is the ancestor of all enzyme producing T. reesei strains in commercial use.Later it was found, that, based on DNA-based phylogenetic markers, the asexual fungus T. reesei was indistinguishable from the sexually propagating fungus Hypocrea jecorina, thus indicating that they belong to the same species (Kuhls et al., 1996).More recent investigations demonstrated that even the original isolate T. reesei QM6a, which was for a long time considered to be an asexual clonal line, can be sexually crossed with H. jecorina www.intechopen.comBiofuel Production -Recent Developments and Prospects 310 strains when the requirement for a fungal strain with an opposite mating type is considered (Seidl et al., 2009).Thus, although initially classified as different genera, T. reesei and H. jecorina are manifestations of the same organism and according to the 'holomorph concept' the name of the teleomorph (sexual form) should be used for the species, but due to historical reasons the name of the anamorph (asexual form), T. reesei, was -and still ispredominantly used as species name in publications and patents.The genome sequence of T. reesei QM6a was published in 2008 (Martinez et al., 2008) and currently two further annotated genome sequences of Trichoderma species i.e. the mycoparasitic and biocontrol fungi T. atroviride and T. virens (Kubicek et al., 2011) are available via the Joint Genome Institute (http:// genome.jgi-psf.org/).A number of species characteristics of T. reesei such as its smaller genome size in comparison to T. atroviride and T. virens or its reduced carbon utilization pattern indicate that T. reesei is a rather specialized species and highly adapted to a particular habitat which is, however, still not exactly known (Druzhinina et al., 2010).Beside its long history of safe use in industrial enzyme production which is also recognized b y i t s G R A S s t a t u s ( G e n e r a l l y R e c o g n i z e d a s S a f e ) b y t h e U .S .F o o d a n d D r u g Administration (FDA), T. reesei serves today as a model organism for the regulation and biochemistry of (hemi)cellulose degradation (Aro et al., 2005;Kubicek et al., 2009).For further deployment of enzymes for biofuel production active research efforts in different areas are currently ongoing including consolidated bioprocessing, engineering to improve enzymes by rational design or directed evolution, the generation of even better hyperproducing strains or the expression of enzymes already in the plant itself.Today, T. reesei is already the main industrial source for cellulases and hemicellulases (Merino and Cherry, 2007), but despite the fact that industrial strains produce more than 100 g/l of cellulases (Cherry and Fidantsef, 2003), efforts are needed to reduce costs and maximize yield and efficiency of the produced enzyme mixtures (for recent reviews see e.g.Carroll and Somerville, 2009;Wilson, 2009) .In this chapter we will give an overview on T. reesei as a cellulase producer and focus on recent genomic and genetic advances that will aid in improving its cellulolytic enzyme mix, enzyme production and performance.