Norway spruce (Picea abies (L.) Karst.) seedlings were
inoculated with the ectomycorrhizal fungus Laccaria bicolor
((Marie) Orton), strain S238 N, in axenic conditions. The presence of the
fungus slowed tap–root elongation by
26% during the first 15 d after inoculation and then stimulated it by 136%.
In addition, it multiplied in vitro
lateral root formation by 4.3, the epicotyl growth of the seedlings by
8.4 and the number of needles by 2. These
effects were maintained when the fungus was separated from the roots by
a cellophane membrane preventing
symbiosis establishment, thus suggesting that the fungus acted by non-nutritional
effects. We tested the
hypothesis that IAA produced by L. bicolor S238 N would be responsible
for the stimulation of fungal induced
rhizogenesis. We showed in previous work that L. bicolor S238
N can synthesize IAA in pure culture. Exogenous
IAA supplies (100 and 500 μm) reproduced the stimulating effect
of the fungus on root branching but inhibited
root elongation. The presence of 2,3,5-triiodobenzoic acid (TIBA) in the
culture medium significantly depressed
lateral root formation of inoculated seedlings. As TIBA had no significant
effect on IAA released in the medium
by L. bicolor S238 N, but counteracted the stimulation of lateral
rhizogenesis induced by an exogenous supply of
IAA, we suggest that TIBA inhibited the transport of fungal IAA in the
root. Furthermore TIBA blocked the
colonization of the main root cortex by L. bicolor S238 N and
the formation of the Hartig net. These results
specified the role of fungal IAA in the stimulation of lateral rhizogenesis
and in ectomycorrhizal symbiosis
establishment.