Effect of an auxin transport inhibitor on aggregation and attachment processes during ectomycorrhiza formation between Laccaria bicolor S238N and Picea abies

Transmission electron microscopy observations performed with cytochemical s tains to detect polysaccharides and cysteine-rich proteins have been done t o study the effect of an auxin transport inhibitor (2,3,5-triiodobenzoic ac id, TIBA) on Laccaria bicolor (Marie) Orton. hypha attachment and aggregati on during mycorrhiza formation in Picea abies (L.) Karst. roots. When the t wo partners were growing separately without any exchange of information, TI BA did not affect the cell wall's polysaccharide or protein structures, whi ch could play a role in the aggregation or attachment process. The presence of the host strongly increased the production of fungal polysaccharide fib rils, allowing hypha aggregation and attachment with the roots. TIBA inhibi ted this host effect. Thus, we can hypothesize that TIBA, by preventing fun gal indole-3-acetic acid (IAA) transport towards the root, inhibited the pr oduction or the efflux of host elicitors responsible for the increase of fu ngal polysaccharide fibril production. However, we cannot exclude that TIBA had other effects than inhibiting fungal IAA transport.