2.7.4
INTERACTION BETWEEN THE BIOCONTROL AGENT TRICHODERMA HARZIANUM AND THE ARBUSCULAR MYCORRHIZAL FUNGUS GLOMUS INTRARADICES IN ROOT-FREE SOIL

H GREEN1, J LARSEN2, P AXEL OLSSON3, DF JENSEN1, and I JAKOBSEN2

1Department of Plant Biology, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark; 2Department of Environmental and Technological Sciences, Risø National Laboratory, DK-4000 Roskilde, Denmark; 3Department of Microbial Ecology, Lund University, S-223 62 Lund, Sweden

Background and objectives
Trichoderma harzianum Rifai is an effective biocontrol agent against several fungal soil-borne plant pathogens. However, possible adverse effects of this fungi on plant growth-promoting microorganisms such as arbuscular mycorrhizal (AM) fungi might be a drawback in the use of this agent in plant protection. AM fungi are obligate biotrophic endosymbionts in roots of most herbaceous plants. The fungus grows from the root out into the surrounding soil, forming an external hyphal network which transports P and other inorganic nutrients to the plant, enhancing the nutritional status of the plant.

The objective of the present work was to examine the interactions between T. ;harzianum and the arbuscular mycorrhizal fungus Glomus intraradices Schenck & Smith in root-free soil.

Materials and methods
The use of a compartmented growth system [1] with root-free soil compartments (RFSC) allowed us to study fungal interactions without interfering effects from roots. The interactions were studied in terms of fungal growth and activity. Fungal growth was measured by the use of specific phospholipid fatty acid (PLFA) signatures [2] in combination with hyphal length [3] or population size measurements [4]. Hyphal P transport and enzyme activity (GUS) [4] were used to monitor activity of G. ;intraradices and T.nbsp;harzianum, respectively.

Results and conclusion
The density of external mycelium of G. ;intraradices was reduced by the presence of the metabolically active T. ;harzianum. However, judging from the AM-specific PLFA signature of 16:1w5, T. ;harzianum did not have any effect on the biomass of G. ;intraradices. Hyphal P transport appeared to be unaffected. This suggests that T. ;harzianum is able to exploit dead mycelium but not the living biomass of G. ;intraradices. Results also showed that G. ;intraradices can profit by organic amendments (in this case wheatbran) in root-free soil even in competition with the biocontrol agent T. ;harzianum. In terms of T. ;harzianum the population development (cfu/g soil) and the GUS-activity (nmole MU/min) was significantly reduced by the presence of G. ;intraradices.

In conclusion, it seems that the biocontrol agent T. ;harzianum can be used without adverse effects on plant growth-promoting microorganisms such as G. ;intraradices.

References
1. Larsen J, Jakobsen I, 1996. Mycorrhiza 6, 259-264.
2. Olsson PA, Bååth E, Jakobsen I, Söderström B, 1995. Mycological Research 99, 623-629.
3. Jakobsen I, Abbott LK, Robson AD, 1992. New Phytologist 120, 371-380.
4. Green H, Jensen DF, 1995. Phytopathology 85, 1436-1440.