Fungal auxin overproduction affects the anatomy of Hebeloma cylindrosporum-Pinus pinaster ectomycorrhizas

We studied the effect of fungal auxin overproduction on the growth polarity of cortical cells in pine mycorrhizas by comparing the anatomy of Pinus pi naster (Ait.) Sol. mycorrhizas formed by an IAA-overproducing mutant of Heb eloma cylindrosporum Romagnesi or by the corresponding wild type with non-m ycorrhizal short roots. Both wildtype and mutant strains induced an increas e in root diameter that was mostly a result of the influence of the fungus on root cortical development. Both strains affected growth polarity of P. p inaster cortical cells and induced a change in their shape. The main modifi cations were a large reduction in axial diameter and an increase in the rad ial diameter of the cortical cells. The modifications were more marked with the mutant than with the wild type. The mutant induced a 43% reduction in cortical cell elongation and a 35% increase in radial diameter, whereas the corresponding changes induced by the wild type were 30 and 10%, respective ly. The volume of cortical cells in mature mycorrhizas was generally lower than in uninoculated short roots indicating that wild-type and mutant strai ns induced a reorientation of cortical cell growth but did not induce an in crease in turgor pressure of the cells. Immunolocalization allowed visualiz ation of a-tubulin in root cortical cells, but no obvious modification in a -tubulin distribution was detected as a consequence of symbiosis establishm ent. Likewise, cytochemical localization of polysaccharides in cortical cel l walls did not show significant modification following symbiosis establish ment and Hartig net formation. The only noticeable modification was a reduc tion in cortical cell wall thickness in mycorrhizas compared with uninocula ted short roots. The possible involvement of fungal auxin in the observed m odifications is discussed.