XVI International Botanical Congess

Eucalyptus pilularis/Pisolithus tinctorius mycorrhizas tend to form in air, are not wettable and are impermeable to apoplasmic tracers. PTS and lanthanum ions are arrested at the exodermal Casparian bands suggesting that they and the sheath are apoplasmic barriers. In contrast the Hartig net apoplast is permeable to both tracers. The data support the theory that the symbiont interface is a sealed apoplasmic compartment. A few mycorrhizas form on the wet agar surface and their sheath is permeable. This difference suggests a role for hydrophobins in modifying sheath permeability and provides an explanation for the up-regulation of hydrophobin genes in eucalypt mycorrhizas during sheath development. The data support the view that eucalypt mycorrhizas are not primarily organs of uptake but have more in common with aerial fungal storage or fruiting structures than roots.