© 2000 Heron Publishing—Victoria, Canada
Dynamics of symbiotic establishment between an IAA-overproducing mutant of the ectomycorrhizal fungus Hebeloma cylindrosporum and Pinus pinaster
H. Tranvan (1), Y. Habricot (1), E. Jeannette (1), G. Gay (2) and B. Sotta (1, 3)
1. Université P. et M. Curie PARIS VI, UMR CNRS 7632, Laboratoire de Physiologie du Développement des Plantes, 4 place Jussieu,
T 53 E5, casier 156, 75252 Paris cedex 05, France / 2. Université Claude-Bernard LYON 1, UMR CNRS 5557, Laboratoire d'Ecologie Microbienne du Sol, 43 Boulevard du 11 Novembre 1918,
69622 Villeurbanne cedex, France / 3. Author to whom correspondence should be addressed (brs@ccr.jussieu.fr) / Received November 26, 1998
Summary
To clarify the early steps of symbiotic establishment, we studied the dynamics of Pinus pinaster (Ait.) Sol. tap root colonization and mycorrhiza formation by an IAA-overproducing mutant of the ectomycorrhizal fungus Hebeloma cylindrosporum Romagnesi and by the corresponding wild type strain. Differences between wild type and mutant strains were quantitative rather
than qualitative and were detected two days after inoculation. Both fungal strains established a typical Hartig net when they
colonized the tap roots. Consequently, colonized tap roots exhibited features of a true mycorrhiza and fungal colonization
enhanced plant growth. Fungal colonization and Hartig net formation were more rapid with the mutant than with the wild type.
Colonization, especially with the mutant strain, increased rhizogenesis and the production of mycorrhizas. The mutant formed
a hypertrophic Hartig net in tap roots and mycorrhizal short roots and we obtained evidence that the process of short root
transformation into mycorrhiza started before their emergence from the tap root. Hyphae of the Hartig net in the tap root
penetrated the cortex of young lateral roots at the beginning of their elongation, after the endodermis layer broke under
the pressure of the elongating lateral root. Colonization was inhibited when triiodobenzoic acid was added to the culture
medium, providing circumstantial evidence that auxin is involved in mycorrhiza formation.
Keywords:
auxin overproducer mutant, growth, Hartig net, mycorrhiza, rhizogenesis, triiodobenzoic acid.
