Swollenin, a protein first characterized in the saprophytic fungus Trichoderma 
reesei, contains an N-terminal carbohydrate-binding module family 1 domain (CBD) 
with cellulose-binding function and a C-terminal expansin-like domain. This 
protein was identified by liquid chromatography-mass spectrometry among many 
other cellulolytic proteins secreted in the coculture hydroponics medium of 
cucumber (Cucumis sativus) seedlings and Trichoderma asperellum, a well-known 
biocontrol agent and inducer of plant defense responses. The swollenin gene was 
isolated and its coding region was overexpressed in the same strain under the 
control of the constitutive pki1 promoter. Trichoderma transformants showed a 
remarkably increased ability to colonize cucumber roots within 6 h after 
inoculation. On the other hand, overexpressors of a truncated swollenin sequence 
bearing a 36-amino acid deletion of the CBD did not differ from the wild type, 
showing in vivo that this domain is necessary for full protein activity. Root 
colonization rates were reduced in transformants silenced in swollenin gene 
expression. A synthetic 36-mer swollenin CBD peptide was shown to be capable of 
stimulating local defense responses in cucumber roots and leaves and to afford 
local protection toward Botrytis cinerea and Pseudomonas syringae pv lachrymans 
infection. This indicates that the CBD domain might be recognized by the plant 
as a microbe-associated molecular pattern in the Trichoderma-plant interaction.