alpha-Expansins are extracellular proteins that increase plant cell-wall 
extensibility. We analysed their pattern of expression in cucumber roots in the 
presence and in the absence of the mycorrhizal fungus, Glomus versiforme. The 
distribution of alpha-expansins was investigated by use of two polyclonal 
antibodies (anti-EXPA1 and anti-EXPA2, prepared against two different cucumber 
alpha-expansins) in immunoblotting, immunofluorescence, and immunogold 
experiments. Immunoblot results indicate the presence of a 30-kDa band specific 
for mycorrhizal roots. The two antibodies identify antigens with a different 
distribution in mycorrhizal roots: anti-EXPA1 labels the interface zone, but the 
plant cell walls only weakly. By contrast, the anti-EXPA2 labels only the plant 
cell walls. In order to understand the potential role of alpha-expansins during 
the accommodation of the fungus inside root cells, we prepared semi-thin 
sections to measure the size of cortical cells and the thickness of cortical 
cell walls in mycorrhizal and non-mycorrhizal root. Mycorrhizal cortical cells 
were significantly larger than non-mycorrhizal cells and had thicker cell walls. 
In double-labelling experiments with cellobiohydrolase-gold complex, we observed 
that cellulose was co-localized with alpha-expansins. Taken together, the 
results demonstrate that alpha-expansins are more abundant in the cucumber cell 
walls upon mycorrhizal infection; we propose that these wall-loosening proteins 
are directly involved in the accommodation of the fungus by infected cortical 
cells.