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A Tool for Monitoring Trichoderma harzianum: II. The Use of a GUS Transformant for Ecological Studies in the Rhizosphere. Helge Green, Department of Plant Biology, Plant Pathology Section, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark; Dan Funck Jensen, Department of Plant Biology, Plant Pathology Section, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Copenhagen, Denmark. Phytopathology 85:1436-1440. Accepted for publication 28 August 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-1436.

Population development, activity, and stability of a β-glucuronidase (GUS)-transformed strain of Trichoderma harzianum were studied in the rhizosphere of cucumber plants grown in sphagnum peat. When compared to the wild-type isolate, there was no significant difference in terms of population development (CFU/g of sphagnum peat). In a treatment with a combination of the wild-type isolate and the transformant, the proportion of GUS-active colonies on plates was constant throughout the experiment. On that basis, we concluded that the transformant was genetically stable when grown in a natural potting mixture. GUS was extracted from infested rhizosphere peat and assayed spectrophotometrically. The activity curve was inversely related to the population curve. Quantification of activated conidia by fluorescein diacetate staining indicated that GUS activity correlated with the activity of the fungus. Hyphae of T. harzianum stained with 5-bromo-4-chloro-3-indoyl-β-d-glucuronide were easily detected on the roots, especially around small wounds, such as damaged epidermal cells. Colonization of wounds may be important for the antagonistic effect of the organism, because it may block a possible way for pathogens to infect root tissue. Our results demonstrate the suitability of using a GUS transformant for detecting and monitoring a specific strain of T. harzianum deliberately released into the environment.