The saprophytic fungus Trichoderma reesei has long been used
as a model to study microbial degradation of lignocellulosic
biomass. The major cellulolytic enzymes of T. reesei are the
cellobiohydrolases CBH1 and CBH2, which constitute more
than 70% of total proteins secreted by the fungus. However,
their physiological functions and effects on enzymatic hydrolysis
of cellulose substrates are not sufficiently elucidated.
Here, the cellobiohydrolase-encoding genes cbh1 and cbh2
were deleted, individually or combinatively, by using an auxotrophic
marker-recycling technique in T. reesei. When cultured
on media with different soluble carbon sources, all three
deletion strains (Δcbh1, Δcbh2, and Δcbh1Δcbh2) exhibited
no dramatic variation in morphological phenotypes, but their
growth rates increased apparently when cultured on soluble
cellulase-inducing carbon sources. In addition, Δcbh1 showed
dramatically reduced growth and Δcbh1Δcbh2 could hardly
grew on microcrystalline cellulose (MCC), whereas all strains
grew equally on sodium carboxymethyl cellulose (CMC-Na),
suggesting that the influence of the CBHs on growth was carbon
source-dependent. Moreover, five representative cellulose
substrates were used to analyse the influence of the absence
of CBHs on saccharification efficiency. CBH1 deficiency
significantly affected the enzymatic hydrolysis rates of various
cellulose substrates, where acid pre-treated corn stover
(PCS) was influenced the least. CBH2 deficiency reduced the
hydrolysis of MCC, PCS, and acid pre-treated and delignified
corncob but improved the hydrolysis ability of filter paper.
These results demonstrate the specific contributions of
CBHs to the hydrolysis of different types of biomass, which
could facilitate the development of tailor-made strains with
highly efficient hydrolysis enzymes for certain biomass types
in the biofuel industry.
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