Chitinase Expression Due to Reduction in Fusaric Acid Level in an Antagonistic Trichoderma harzianum S17TH

To study the effect of reduction in phytotoxin level on fungal chitinases, antagonistic Trichoderma spp. were screened for their ability to reduce the level of fusaric acid (FA), the phytotoxin produced by Fusarium spp. A T. harzianum isolate S17TH was able to tolerate high levels of FA (up to 500 ppm) but was unable to reduce the toxin to a significant level (non-toxic) added to minimal synthetic broth (MSB). However, the isolate was able to reduce 400 ppm FA in the liquid medium after 7 days to a non-toxic level and displayed similar level of antagonism over the control (without FA). In studies of the effect of the reduction in FA (400 ppm) level on chitinase gene expression in PCR assays, nag1 was significantly repressed but ech42 expression was only slightly repressed. Chitinase activity was either reduced or absent in the extracellular proteins of MSB supplemented with 400 ppm FA, which could be attributed to the effect of residual FA or its breakdown products through unknown mechanisms. Selection of S17TH as a toxin insensitive isolate that could commensurate the negative effect on chitinase activity makes it a potential antagonist against Fusarium spp.