Possible Signals in the ECM

Early morphological changes during ectomycorrhizal development have been identified (Kottke and Oberwinkler 1987; Horan et al. 1988). Based on current knowledge of the molecules released in other plant-microbe interactions, the early plant host signals secreted into the rhizosphere can include flavonoids, diterpenes, hormones and various nutrients (Martin et al. 2001) , Several plant metabolites have been shown to induce striking modifications in hyphal morphology. Rutin, a phenol compound found in eucalyptus root exudates, may be a signal in ectomycorrhizal symbiosis, as it stimulates the hyphal growth of certain Pisolithus tinctorius strains at picomolar concentrations (Lagrange et al. 2001) , On the other hand, the tryp-tophan derivative hypaphorine is secreted by P. tinctorius and can arrest root hair elongation and stimulate the formation of short roots in the plant host, possibly acting as an antagonist of the plant hormone auxin (Martin et al. 2001). On the fungal side, root exudates have been shown to stimulate an enhanced accumulation of fungal molecules such as hypaphorine, the betaine of tryptophan (Beguiristain and Lapeyrie 1997), that can induce morphological changes in root hairs of seedlings. Hypaphorine is produced in larger amounts by P. tinctorius during mycor-rhizal development (Beguiristain and Lapeyrie 1997). Ditengou and Lapeyrie (2000) report an antagonistic effect of hypaphorine on indole-3-acetic acid (IAA).

The production of hormones, including auxins, cytokinins, abscisic acid and ethylene, by ectomycorrhizal fungi was first reported in the early 1990s (Gogala 1991) , Many studies indicate that changes in auxin balance are a prerequisite for mycorrhiza organogenesis (Rupp et al. 1989; Gay et al. 1994; Karabaghli-Degron et al. 1998; Kaska et al. 1999) (e.g., short root development). The presence of plant-derived molecules in the rhizosphere could be sufficient to enhance the biosynthesis of hormones by ectomycorrhizal fungi (Rupp et al. 1989), which induce morphological changes leading to symbiosis development. For example, an IAA-upregulated cDNA, referred to as Pp-C61, was isolated by the differential screening of a cDNA library constructed from auxin-treated roots of the ECM host tree Pinus pinaster

(Reddy et al. 2003). Pp-C61 is present as a single copy in the P. pinaster genome, and homologous genes were detected in other gymnosperm and angiosperm trees. The fact that Pp-C61 is transcriptionally regulated by auxin suggests that Pp-C61 activation corresponds to a reaction in response to fungal colonization.

Hydrophobins, a class of fungal cell wall proteins involved in establishing cell-cell or cell-surface contact, are also probably involved in fungus-plant communication in ECM. A class I hydrophobin (HYD1) was purified from the culture supernatant of TrichoJoma terreum (Mankel et al. 2002). The coding gene (hyd1) expression pattern suggests that hydrophobins might be involved in host recognition and in the host tree specificity of the fungus.

Mitogen-activated protein kinase (MAPK) signal transduction cascades are used by fungi to modulate their cellular responses to environmental conditions, in mating, and for cell-wall integrity. The yeast extracellular signal-regulated kinase (YERK1) is the most thoroughly investigated MAPK subfamily involved in mating response (Fus3) and nitrogen starvation (Kss1). The first MAPK from an ectomy-corrhizal fungus was cloned from Tuber borchii (TBMK) (Menotta et al. 2006). It belongs to the YERK1 (yeast extracellular regulated kinase subfamily). TBMK is present as a single copy in the genome, and the codified protein was phosphorylated during the interaction with the host plant, Tilia americana. TBMK partially restores the invasive growth of Fusarium oxysporum that lack the fmkl gene. This suggests that protein kinase activity may play an important role during the interaction of T borchii with its host plant by modulating the genes needed to establish symbiosis, leading to the synthesis of functional ectomycorrhizae.

3.2 Cytoskeleton and Signal Transduction

The integration of signals received by a cell and their transduction to targets are essential actions for all cellular responses. The cytoskeleton has been identified as being a major target of signaling cascades in animal, plant and yeast cells (Alberts et al. 2002). The cytoskeleton, which is unique to eukaryotic cells, is a dynamic three-dimensional (3D) structure that fills the cytoplasm with an extensive system of protein filaments enabling eukaryotic cells to organize their interiors and to perform various directed functions. The most abundant components of the cytoskeleton in mammalian cells are microfilaments (MFs), microtubules (MTs), and intermediate filaments. The latter structures, however, are yet to be identified in plant and fungal cells. The structures of MTs and MFs and the expression of their structural subunits, actin and tubulins, have been investigated in ectomycorrhizal fungi, non-mycorrhizal roots of Scots pine, and symbiotic ectomycorrhizal roots (Salo et al. 1989; Niini and Raudaskoski 1993; Timonen et al. 1993; Niini et al. 1996; Raudaskoski et al. 2001, 2004). Scots pine roots have three a- and three b-tubulin isoforms, whilst two additional isoforms of a-tubulin are detected in ectomycorrhizal roots, which suggests there are cellular level changes involving MTs when the two partners come into contact (Niini et al. 1996) . Three a- and two b-tubulins that remain unchanged, even during the symbiosis, have been similarly identified in the ectomycorrhizal fungus S. bovinus. The presence of two and four actin isoforms in P. sylvestris lateral root tips and short roots, respectively, and two actin isoforms in S. bovinus has also been reported (Niini et al. 1996). The fungal tubulins (Niini and Raudaskoski 1998) and actins (Tarkka et al. 2000) are constitutively expressed at the mRNA and protein levels, suggesting that the reorganization of the cytoskeleton during ectomycorrhizal formation of S. bovinus with the P. sylvestris short roots is not mediated via differential expression of these genes. Ectomycorrhizal association, however, leads to major changes in the growth patterns of both plant and fungal partners (Niini 1998; Barlow and Baluska 2000; Raudaskoski et al. 2001). On the basis of the visualization of the MTs and MFs in vegetative hyphae of S. bovinus and in ectomycorrhiza (Timonen et al. 1993; Raudaskoski et al. 2001, 2004), it has been deduced that the cytoskeleton plays a role in fungal morphogenesis during the formation of ectomycorrhiza.

The small GTPases Cdc42 and Rac1, the regulators of the actin cytoskeleton in eukaryotes, have been isolated from the ectomycorrhizal fungus Suillus bovinus (Hanif 2004). IIF microscopic analysis suggests that the small GTPases Cdc42 may play a significant role in the polarized growth of S. bovinus hyphae and may regulate fungal morphogenesis during ectomycorrhizal formation by reorganizing the actin cytoskeleton. A small GTPase (TbRhoGDI) was more recently isolated from the ectomycorrhizal fungus T. borchii (Menotta et al. 2008). The specificity of the actions of TbRhoGDI was underscored by its inability to elicit a growth defect in Saccharomyces cerevisiae or to compensate for the loss of a Dictyostelium discoi-deum RhoGDI.

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