Apoplasmic barriers and their significance in the exodermis and sheath of Eucalyptus pilularis–Pisolithus tinctorius ectomycorrhizas

Abstract

The apoplasmic permeability of ectomycorrhizal roots of intact Eucalyptus pilularis seedlings infected with Pisolithus tinctorius on aseptic agar plates was examined using the nonbinding fluorochrome 8-hydroxypyrene-1,3,6-trisulphonate and lanthanum ions in conjunction with anhydrous freeze substitution and dry sectioning. Most mycorrhizas formed in the air above the agar surface, and in these the sheath rapidly became nonwettable and impermeable to the fluorochrome but was nevertheless permeable to lanthanum ions. In a few mycorrhizas which developed in contact with the agar the sheath remained permeable to both tracers when fully developed. This increased hydrophobicity of the sheath in mycorrhizas in the air above the agar surface might be explained by deposition of hydrophobins, but nevertheless it still allows an apoplasmic pathway for radial movement of ions. Regardless of their sheath permeation both apoplasmic tracers were always found throughout the Hartig net and were arrested at the Casparian bands and suberin lamellae of the exodermis. It is concluded that the fluorochrome must have moved longitudinally along the Hartig net which is a region of higher permeability than the sheath. Casparian bands in the exodermis of ectomycorrhizal roots have similar properties to those in nonmycorrhizal roots in excluding solutes and their exclusion of lanthanum ions indicates that they are not permeable to ions. The data do not support the concept of a totally sealed apoplasmic exchange compartment, but the differential permeability suggests that the sheath might allow radial transfer of ions but block loss of sugars and organic molecules of similar size.

Motile tubular vacuoles in extramatrical mycelium and sheath hyphae of ectomycorrhizal systems

“…This is consistent with our previous findings with fluorescent apoplasmic tracers that the cell walls and cementing material of the sheath of Eucalyptus pilularis-Pisolithus tinctorius ectomycorrhizas are not permeable to fluorochromes (Vesk et al 2000). This clearly applies to young active mycorrhizas as well as older ones.…”

“…Australian isolates of PisoIithus tinctorius are known to express hydrophobin genes during ectomycorrhizal sheath development (Tagu andMartin 1996, Tagu et al 1998) and other hydrophobic molecules such as repellents (Talbot 1997) may be involved as well. The data here further support the proposal of Vesk et al (2000) that in some properties ectomycorrhizal roots are much more like fruiting bodies and other aerial structures, being water repellent and relatively impermeable, than uptake structures such as roots. The data as a whole support the view that it is the extramatrical mycelium and not mycorrhizal :roots that are the primary uptake structures, a view espoused some time ago by David Read and colleagues (Read 1984(Read , 1991(Read , 1993 …”

“…2H20) (Brundrett et al 1996) at 24 ~ in the dark. As previously described (Ashford et al 1999, Vesk et al 2000, mycorrhizas were synthesized and grown in modified simple system plates on agar containing minerals after Burgess et al (1994) (B. S6derstr6m pets. commun), with Pisolithus tinctorius (Pers.…”

Fungal aquaporins: cellular functions and ecophysiological perspectives

“…In addition to long-distance transport structures, the dense hyphal network (fungal sheath) that is commonly surrounding infected fine roots in ECM associations (see below), was shown to be frequently hydrophobic (Bücking et al 2002;Vesk et al 2000).…”

Transcriptome analysis by cDNA-AFLP of Suillus luteus Cd-tolerant and Cd-sensitive isolates

“…Movement of minerals and trace elements from soil solution towards the root exodermis is certainly affected by the mycobionts although such effects depend on the molecular structure of the compounds and on fungal characteristics (Taylor and Peterson 2005;Vesk et al 2000). S. luteus produces large well differentiated hyphal networks in soils, but relatively few mycorrhizas with thick mantles surrounding the absorption roots of P. sylvestris.…”