The development of an efficient root system has been a major achievement in the success of vascular plants. Mycorrhizal associations facilitated their growth in a new, nutrient poor, and dry environment (Malloch et al. 1980; Simon et al. 1993). The term "mycorrhiza", literally "fungus root", describes an intimate, mutualistic relationship between fungi and plant roots. It was introduced by Frank (1885) to describe the long-lived association between plant roots and fungal mycelium. Mycorrhizas are mutually beneficial symbioses and the two partners form a dual organ with recognizable morphology. Mycorrhizas serve as the main organs for nutrient uptake in terrestial ecosystems (Smith and Read 1997).
The majority of vascular plants are obligately or facultatively mycorrhizal in nature. The hosts comprise most species of angiosperms, all gymnosperms, pteridophytes and some bryophytes (Newman and Reddell 1987). All the major taxonomic groups of fungi (Ascomycotina, Basidiomycotina, Zygomycotina) include mycorrhizal fungi. Some of these are obligate symbionts that cannot survive without the host plant. Mycorrhizal fungi have wide host ranges and usually do not show strict symbiotic relationships. The species composition of mycorrhizal fungi is dependent on host plant age and environmental conditions (Wilcox 1996).
The various different forms of mycorrhizas have been classified in several types. The classification is based on fungal associates and structural characteristics of mycorrhizas at maturity (Isaac 1992). The most ancient, widespread, and studied mycorrhizal class is arbuscular mycorrhiza (Smith and Read 1997). Ectomycorrhiza is a common form of symbiosis in forest trees. Other forms of mycorrhizas are arbutoid, monotropoid, ericoid, and orchid mycorrhizas (Peterson and Farquhar 1994). Both the fungus and the plant may affect the type of mycorrhiza formed. Thus the mycorrhizal classification has to be considered mainly descriptive (Smith and Read 1997). Besides, many fungi are able to form different types of mycorrhizas depending on the host species, and the species of the genera Salix, Prunus and Acacia form both ectomycorrhizas and arbuscular mycorrhizas.
In boreal and temperate forests ectomycorrhizas (ECMs) are common symbiotic associations in trees and shrubs. The ECMs occur often at sites poor in soluble nutrients and where accumulated litter occur on soil surface. Plant hosts include members of Pinaceae, Fagaceae, Betulaceae, Myrtaceae, and also include some monocotyledons and ferns (Wilcox 1996). ECMs are particularly important for tree growth in regions with low nutrient or water status. ECM usually forms between fine roots and dikaryotic mycelia of different Basidiomycete genera. Some Ascomycetes are also involved, e.g. members of Tuberales, and two members from Zygomycetes (Isaac 1992). Both the ECM plants and fungi are generally able to form symbiosis with different species.
The ectomycorrhizal roots are characteristic in appearance (Fig. 1). Fungal mantle with varying depth covers them, containing aggregated, branched, and swollen hyphal cells. The outer mantle hyphae are connected to extramatrical mycelium that takes care of the mineral nutrition and water uptake of the symbiotic tissues. The inner mantle hyphae form a network of finger-like branches, which extend between the epidermal (angiosperms) and cortical (gymnosperms) cells of the host plant (Kottke and Oberwinkler 1986; Barker et al. 1998). This fungus-plant interface is called the Hartig net and it represents the site of fungal nutrient transfer in exchange for plant photosynthates. In contrast to other mycorrhizal types, the ectomycorrhizal hyphae do not form intracellular structures (Peterson and Farquhar 1994).
Fig.1. Diagram of ectomycorrhiza from Pinus sylvestris. A. Lateral root tip region with unbranched, dichotomously branched, and coralloid ectomycorrhizal short roots. B. Short root cross-section with (M) hyphal mantle, (Hn) intercellular hyphae of the fully developed Hartig net, (E) endodermis, and (S) central stele. C. Magnification of the lobed Hartig net hyphae. Based on Kottke and Oberwinkler (1987) and Isaac (1992).
Plant and fungal communities are in a constant process of competition. Access to nutrients is for great importance in competitive surroundings and thus via improving nutrient uptake the species and strain composition of mycorrhizal fungi can greatly determine plant diversity (Van der Heijden et al. 1998). The improved nutrient uptake capacity of ECMs is caused by an increase in the nutrient absorbing surface area of the root system by an extensive extramatrical mycelium (Rousseau et al. 1994). The small hyphal diameter increases the area of the absorbing surface that can explore a large soil volume and the extramatrical mycelium provides an effective high affinity nutrient uptake system. It also facilitates solubilization of soil nutrients by the release of proteases, acid phosphatases, and oxidases (Bending and Read 1995).
A low nitrogen (N) supply is often limiting to plant growth in boreal and temperate forest ecosystems where the rate of organic matter turnover often limits plant growth (Read 1991). Ectomycorrhizal fungi pass nitrogenous compounds to plants (Melin and Nilsson 1952). They improve N mobilisation to plants both by facilitating access to organic N sources and by increasing the uptake of N via extramatrical hyphae (Read 1991; J. Perez-Moreno et al. 2000). The hyphae mobilise organic N by secreted enzymes such as proteases and take it up by high affinity amino acid transporters (Bending and Read 1995; Wallenda and Read 1999). The fungal PEP-carboxylase cycle is important integrator of C and N metabolism, since it supplies C for amino acid skeletons (Wingler et al. 1996; Martin et al. 1998). The predominant forms of N transferred to the host are glutamine, glutamate, and asparagine (Smith and Read 1997).
Phosphorus (P) is often present in the soil as insoluble inorganic or organic forms. The plants can only absorb soluble forms of P and their uptake rapidly causes a P depletion zone around the roots. ECM formation increases plant phosphorus content and the ability of the plant to gain phosphorus (Finlay and Read 1986; Perez-Moreno and Read 2000). ECM fungal hyphae form polyphosphates from part of the imported P. Polyphosphates are an important P storage compound and together with orthophosphate short chain polyphosphates are the predominant form for P transported towards the host plant (Smith and Read 1997).
The fungus gains carbon (C) from the plant creating a fungal sink. The extramatrical mycelium puts a high C demand on the host plant, which can be measured as a negative correlation between plant growth and fungal biomass (Nylund and Wallander 1989; Colpaert et al. 1992). Fungal carbohydrate metabolism dominates in symbiotic tissues (Martin et al. 1998). However, since photosynthesis is under constant feedback control the novel fungal sink may stimulate the rate of C assimilation leading to low "cost" of mycorrhizal symbiosis for the host plant (Loewe et al. 2000).
Ectomycorrhizal symbiosis alters the pattern of carbon allocation in the plant (Hampp et al. 1999). The ectomycorrhizal hyphae prefer hexoses as C sources and glucose resulting from sucrose catabolism is thought to be the primary source of carbon in ectomycorrhizas (Chen et Hampp 1993; Hampp et al. 1999). The ECM fungi probably have no transport system for the primary transport sugar of the plant, sucrose, and the cell wall-bound invertase activity of the host plant has been suggested to be responsible for sucrose hydrolysis prior to fungal monosaccharide uptake (Salzer and Hager 1991). In accordance to this, the expression of the only studied monosaccharide transporter from ectomycorrhizal fungi, AmMST1 of Amanita muscaria (Nehls et al. 1998) is specifically enhanced by increased monosaccharide concentrations. Other sugar-dependently-regulated genes have also been characterized from A. muscaria indicating that the supply of glucose broadly affects fungal gene expression (Nehls et al. 1999; Hampp et al. 1999).
Free-living mycelia convert glucose mainly to mannitol and trehalose and incorporate it to free amino acids indicating an important role for anaplerotic CO 2 fixation via pyruvate carboxylase. The flux through PEP carboxylase, likely sustaining the synthesis of glutamine, is also important in the non-ECM roots where most of the C is incorporated to sucrose (Martin et al. 1998). Both in the host roots and in the symbiotic hyphae ECM development affects C metabolism. The root sucrose levels drop significantly and the amount and nature of fungal sugar synthesis is changed leading to the synthesis of short chain polyols, trehalose and free amino acids (Schaeffer et al. 1995, Hampp et al. 1999). Light independent fixation by both fungal and plant carboxylases fulfils a substantial part of C demand, particularly for amino acid biosynthesis (Martin et al. 1998).
Ectomycorrhizal trees often show better growth and improved resistance in unfavorable conditions than non-mycorrhizal trees. The extramatrical mycelium has potential for water transport and may improve the water relations of the host plant (Guehl et al. 1992). The trees are protected against heavy metal toxicity by ectomycorrhizal fungi, which reduce translocation of heavy metals to the host plant (Jentschke and Godbold 2000).
The soil surrounding ECM roots, mycorrhizosphere, has a rich microbe flora. The microbial diversity depends on the plant and fungal partners of the ECM association. Many types and species of microbes inhabit the area around the roots in soil, containing organisms that are useful, neutral and harmful for the symbiotic partners (Fitter & Garbaye 1994). The ECM associated bacteria affect mycorrhizal functioning in several ways including the regulation of fungal growth, host root-symbiotic fungus recognition events, nutrient mineralization, and protection against pathogens (Duponnois et al. 1993; Fitter and Garbaye 1994).
Several functional differences occur between fungal species and strains able to form ECM, which include differences in symbiotic capabilities (Bonfante et al. 1998), ability to proliferate (Rousseau et al. 1994), and to tolerate drought and heavy metals (Guehl et al. 1992; Colpaert and van Assche 1992). Substancial differences in nutrient uptake capacity between both ECM fungal species and strains have also been revealed (Finlay et al. 1992). In general, the ECM fungal species have very different physiologies and morphologies and the total benefit of symbiosis for the host plant depends on the infection pattern of its total root system and the extent of infection by individual varieties of ECM fungal species. ECM fungi are sensitive to variation in soil nutrient status. Additions of abundant fertilizer amounts have significant effects on the ECM fungal diversity and some genotypes may eventually be lost (Read 1991). The reduced mycorrhiza formation of the roots makes them probably more vulnerable to environmental stress and pathogens (Hampp et al. 1999).
The organization of seedling root is determined by organised cell divisions in the embryonic axis (Scheres et al. 1995; Laux and Jürgens 1997). Mutations in the genes regulating the organization of embryonic root lead to severe defects in primary and secondary root cell division and patterning (Cheng et al. 1995; Scheres et al. 1995; Di Laurenzio et al. 1996; Helariutta et al. 2000). Root meristem is activated after germination, and the development of the root is a continuous process, in which the cell types are formed from actively dividing stem cells called initials at the base of differentiated cell layers. The information for correct cell fate in roots is directed through more mature cell layer towards the tip initial cells, which do not themselves act as generators of cell pattern. Differentiated cells can therefore be thought of as a template on which less mature cells are induced to follow particular fate (Van den Berg et al. 1995, 1997; Sabatini et al. 1999). The cell pattern of roots does not depend on a higher order morphogenetic program but on the proper coordination of correct polarity during cell patterning and the ordered cell divisions and cell shape changes directed by the position of each cell (Torrez-Ruis and Jürgens 1994). The root system can be divided in the following zones: 1) meristematic zone for active cell division, 2) elongation zone for cell expansion, and 3) specialization zone for cell differentiation. Although the zones overlap to some extent, their existence emphasises the spatial separation of root cells at different stages of specialization (Schiefelbein and Benfey 1991).
Root systems increase in size by branching via lateral root formation. The initiation of lateral root meristem occurs at some distance from the main root tip in the differentiation zone of the root and the exact positioning of lateral root primordium depends on the underlying vascular tissue and environmental conditions (Zhang et al. 1999). Lateral roots develop from the cells of the pericycle, sometimes with a contribution from dividing endodermal cells. Coordinated cell divisions in the single cell layer of the pericycle or in the endodermis and pericycle result in the formation of a cluster of mitotically active cells, lateral root primordium (LRP; Blakely and Evans 1979). The LRP first appears as a ball like structure of undifferentiated cells within the primary root, then the cell number increases, and finally the primodium emerges through the epidermis of the primary root by cell expansion. The specialization of root cells starts already at the primordium stage (Malamy and Benfey 1997), but the formation of the LR apical meristem starts after its emergence from the primary root (Cheng et al. 1995, Celenza et al. 1995). The formed meristem is similar to the meristem of primary root, and again, cells comparable to initial cells can be distinguished (Malamy and Benfey 1997).
Pine seedlings have three different root types: main root, lateral roots and short roots (Robertson 1954). Short roots constitute a major percentage of the ultimate root branches in pines. Main and lateral roots are similar in structure (Wilcox 1968a). As in main and lateral roots, the short root primordia forms from the pericycle and gives rise to a new apical meristem, which shows specific morphological characteristics. The short root apex is less pointed than that of lateral root (Wilcox 1968b), it has a thin root cap or none and an extremely reduced meristem and elongation zone. Due to low numer of cells produced for the elongation zone differentiation of both stele and cortex occur close to the meristem (Niini and Raudaskoski 1998). Sometimes the lateral root-like primordia may cease in growth soon after emergence and give rise to short roots-like organs and, in contrary, short roots can be transformed to typical lateral roots indicating a dynamic control of root identity (Wilcox 1968b).
The homobasidiomycete hyphae are 5-6 Ám wide tubular structures. They extend by polarized tip growth, which is promoted by locally focused expansion of cell surface. Deposition of new plasma membrane and cell wall precursors occur via exocytosis of secretory vesicles at the growing tip (Wessels 1993). Girbardt (1965) characterized a dense vesicle aggregate in the tips of Basidiomycotina hyphae, and he named it Spitzenkörper. The Spitzenkörper acts as a site for vesicle collection and as a distributor (vesicle supply center) of vesicles to the cell wall of the growing hyphal tip. The positioning of Spitzenkörper to release vesicles largely determines the direction of growth (Lopez-Franco et al. 1994; Reynaga-Pena et al. 1997).
The most important factors behind fungal tip growth are the vesicles that include the enzymes for cell wall synthesis, the cytoskeleton that directs their transportation to the growing tip, and the machinery that regulates the correct deposition of cell wall polymers (Gow 1995). Cell wall biosynthetic enzymes and novel membrane for expansion are deposited on the plasma membrane by vesicle transport in a highly organized manner. The regulation of hyphal extension involves cAMP dependent and independent signalling (Yarden et al. 1992; Bruno et al. 1996; Alex et al. 1996; Bussink and Osmani 1999). Tip growth is connected to functional cytoskeleton (Salo et al. 1989; Heath 1990; Raudaskoski et al. 1994; Rupes et al. 1995; Torralba et al. 1998). The hierarchy of regulation of cell wall component deposition is not well understood although a large part of regulation appears to be achieved by cytoskeleton based targeting of cell wall biosynthesis enzymes (Harold 1997). Microtubule- and microfilament-associated motor proteins are necessary for vesicle transport towards the Spitzenkörper (McGoldrick et al. 1995; Wu et al. 1998; Seiler et al. 1999). Members of rho-family of small GTPases regulate polarized hyphal growth via the actin cytoskeleton (Wendland and Philippsen 2000; Raudaskoski et al. 2000), which is most abundant in the apical tips of basidiomycete hyphae, and at the site of septa formation (Runeberg et al. 1986; Salo et al. 1989). The general cytoskeletal organization at the hyphal tip may differ between asco- and basidiomycetes. In some basidiomycetes the microtubules do not extend to the extreme apex, whereas in ascomycetes both microtubules and microfilaments are present (Rupes et al. 1995; Harold 1997).
The main structural elements of fungal cell wall are chitin and glucans. Mutations in genes encoding cell wall biosynthetic enzymes or vesicle proteins cause branching and defective hyphal extension (Borgia et al. 1996; Whittaker et al. 1999). The fungal cell wall is more plastic at the extreme end and becomes more rigid at the base of the apical dome due to chitin-glucan crosslinking (Vermeulen and Wessels 1986). The hyphae contain relatively high concentrations of solutes, and the elastic primary cell wall of the apex is expanded due to high turgor pressure built up by vacuoles in the older segments of hyphae (Wessels 1993). Also ion currents are important both for polarized tip growth and introduction of branches (Gadd 1995).
The homobasidiomycete hyphae are compartmentalized in cytoplasmically connected cells by perforated crosswalls called dolipore septa (Girbardt 1965). Septum formation starts with actin ring formation (Runeberg et al. 1986; Salo et al. 1989; Raudaskoski et al. 1991) followed by localized deposition of cell wall material. Signalling for septal formation involves transcription factors and actin cytoskeletal regulators (Harris and Hamer 1995; Harris et al. 1997; Krüger and Fischer 1998). In basidiomycetes septal formation is tightly linked to nuclear division. The homobasidiomycete hyphae that are able to form ECM are dikaryotic. In dikaryotic hyphae, the nuclear divisions of the two nuclei are organized by cytoskeleton and formation of clamp connections, and there is a strong dependence between the site of nuclear division and septum formation. The role of microtubule cytoskeleton is crucial, since the disruption of microtubules affects both the site of the dividing nuclei and the location and structure of dolipore septa (Raudaskoski 1980).
Hyphae form branches at regular intervals. Development, reproduction, and host penetration largely depend on the extent of hyphal branching. In basidiomycetes under optimal growth conditions a new hyphal apex forms just behind the septum of the apical cell, but the regulators for branch site positioning are not yet known. Phosphokinase inhibitors and actin depolymerising drugs induce hyphal branching (Magae and Magae 1993; Rupes et al. 1995; Niini, 1998) indicating an important role for protein phosphorylation and cytoskeletal organization. The rigid cell wall has to be loosened prior to cell wall deposition, presumably by the effect of cell wall lysing enzymes. Vesicles aggregate at the point of outgrowth similarly as in the case of tip growth. During the aging of fungal colony the growth branch angle between "mother" and branched hyphae reduces and branched hyphae show different morphological and growth characteristics than mother hyphae (Carlile 1995). Different physiological and environmental factors affect significantly hyphal growth pattern (Sone and Griffits 1999). Tip growth and septation mutants often show increased branching, indicating a tight connection between the regulation of branch formation and hyphal extension (Yarden et al. 1992; Bruno et al. 1996). Addition of choline affects specifically branch initiation and maintenance of growth is not disturbed, indicating partially different regulatory networks for the establishment and maintenance of growth (Wiebe et al. 1992).
Ectomycorrhiza formation involves changes in the growth pattern of both partners. The development of this "symbiotic organ" facilitates efficient exchange of nutrients in the Hartig net region.
In a mature root system newly formed roots are colonized by the Hartig net hyphae of the mother root (Wilcox 1968b). At germination or in planted seedlings, fungal growth towards host roots may involve chemical signalling which induces growth of hyphae in the direction of plant root (Melin 1954; Horan and Chilvers 1990), but the specificity of signalling or the nature of substances involved are not known. Fungal cell wall proteins and cell surface polysaccharides have been identified as important molecules in the establishment of symbiosis. The adhesion on the root tips or distal to the root apical meristem (Kottke 1997; Smith and Read 1997) may involve hydrophobic interactions (Martin and Tagu 1995) and interactions between the plant and fungal polysaccharides and glycoproteins (Lei et al. 1991; Giollant et al. 1993; Martin et al. 1999). After adhesion at root surface, the hyphae make a firm contact on the host cell wall. After the contact with the root surface the fungal cell wall structure loosens (Bonfante et al. 1998). The hyphae start to swell and branch (Brunner and Scheidegger 1992). This fungal morphogenesis is associated with changes in cytoskeletal organization and regulation of fungal cell wall proteins, and it can be induced by plant flavonoids (Timonen et al. 1993; Laurent et al. 1999; Martin et al. 1999).
A network of branched hyphae, the hyphal mantle, forms on top of the root surface. The mantle varies in thickness but it usually consists of layers with differing structure and density of hyphae (Brunner and Scheidegger 1992). The region of the mantle closest to root epidermis is called pseudoparenchyma due to appearance of branched and fused hyphae that store lipids, trehalose and polysaccharides (Brunner and Scheidegger 1992; Peterson and Farquhar 1994). The pseudoparenchymatous hyphae are glued tightly together with extracellular material that contains polysaccharides and glycoproteins. The mantle separates the host root from soil, and the host plant may depend in large part on the supply of water and nutrients from its symbiotic fungus. Lateral root growth is slowed down by the fungal colonisation, and due to the hyphal production of an auxin-betaine, hyphaphorine, root hair formation is prevented (Peterson 1991; Beguiristan and Lapeyrie 1997).
Penetration between root apical cells is mostly mechanical in nature but also involves production of ECM fungal lysing enzymes for digestion of host cell walls (Ramstedt and Söderhåll, 1983; Dahm et al. 1987; Cairney and Burke 1994). The host cell walls are separated by fungal intrusion and they become swollen and less compact (Peterson and Farquhar 1994). The damage to plant cell walls causes a transient production of plant defense substances and proteins. According to studies in cell cultures this defense-response is partly halted by plant enzymes (Salzer et al. 1997). In planta , the eliciting activity of the fungus depends on the extent of compatibility between the symbiotic partners (Burgess et al. 1995; Bonfante et al. 1998).
The development of the symbiotic interface is very similar in different host-mycobiont interactions. The finger-like hyphae mostly penetrate the cortex to form a complex, highly ordered web of tightly packed hyphae between the epidermal and cortical cells, the Hartig net. Fungal and plant cell walls merge and form a novel type of interface which contains a complex matrix of constituents from both fungal and plant origin (Bonfante et al. 1998; Niini 1998). The formation of a functional Hartig net concludes the development of a structure, which can be referred to as a symbiotic organ.
Hyphae extend from the mantle to facilitate nutrient solubilization and transport. Part of the transport takes place in the symplast of living hyphae. Transport in the symplast occurs by motile tubular vacuoles that can move material across long intracellular distances (Shepherd et al. 1993). Most of the basidiomycete ECM fungi, like Suillus bovinus, can also form rhizomorphs, linear aggregates of fungal hyphae containing large central "vessel" hyphae that may represent significant extensions to the root system (Duddrigde et al. 1980; Foster 1981; Rousseau et al. 1994). At the onset of rhizomorph formation the leading hyphae grow in parallel approaching each other, they form linear aggregates, and allow the formation of branches and intercellular bridges (Cairney 1992). After the tight tubular aggregate of hyphae is formed, cellular contents of the central hyphae disappear and septal cross-walls break down, leading to vessel hypha formation (Agerer 1992). The vessel hyphae have been implicated for acropetal C transport and the living cortical hyphae for symplastic transport of P and other nutrients (Cairney 1992), but this has not yet been proven.
The age of the infected roots varies considerably, but mostly ECM fungal infection prolongs the age of fine roots (Wilcox 1968b). In aged ECMs Hartig net host cell walls disintegrate and cannot be distinguished from the plant-fungal cell wall matrix, which probably leads to a decrease in nutrient transport (Kottke and Oberwinkler 1986). Some ECM fungi may survive the death of the cortical cells and live parasitically between the root cells. Others die simultaneously with the collapse of the Hartig net (Wilcox 1996).
During the symbiotic interaction between ectomycorrhizal fungi and their hosts the straight, tubular hyphae swell and branch on the surface of plant roots to produce finger-like hyphae. The transition in growth pattern is necessary for the penetration into the intercellular space of the plant root and for the formation of the Hartig net (Kottke and Oberwinkler 1985; Smith and Read 1997; Barker et al. 1998). The signal transduction pathways which lead to these fundamental changes in growth pattern are yet unknown. Specific plant flavonoids (Martin et al. 1999) have been suggested to trigger symbiotic growth. Treatment with protein kinase inhibitors and drugs that disrupt actin cytoskeleton (Niini 1998) can mimic the hyphal morphogenesis that takes place in ECM. These observations suggest that the morphogenetic signalling pathways may involve ligand-receptor-interactions and signalling via protein kinase cascades, which lead to reorganisation of actin cytoskeleton (Niini 1998; Martin et al. 1999; Raudaskoski et al. 2000).
The growth pattern of pathogenic fungi also changes in compatible interactions with the plant host. Fungal pathogens perceive and respond to molecules from the plant and on the plant cell wall, which trigger pathogenic development. The haploid yeast-like form of the basidiomycete corn smut fungus Ustilago maydis is non-pathogenic and the infection process is associated with sexual development. The formation of dikaryon is necessary for filamentous growth and only the filamentous dikaryon is infectous (Kahmann et al. 1999). Both pheromone-receptor interaction and cAMP signalling are needed for pathogenic development in U. maydis, and for a successful infection the fungus also needs an intimate contact with the host plant (Hartmann et al. 1996; Dürrenberger et al. 1998; Basse et al. 2000). Crosstalk between the pheromone-receptor and cAMP signalling pathways is probably mediated by a G-protein G a -subunit Gpa3, which is presumed to activate the enzyme adenylate cyclase that catalyses cAMP production (Regenfelder et al. 1997; Krüger et al. 1998; Kahmann et al. 1999).
Specialised infection structures, appressoria, are formed for the penetration of plant cells by many plant pathogenic fungi (Hamer and Talbot 1998). Appressoria are dome-shaped cells with specific, strong cell walls, which facilitate the turgor-driven penetration into the host plant (Thines et al. 2000). Their formation has been recently studied in the ascomycete pathogen of rice, Magnaporthe grisea , where cAMP-linked signalling cascades regulate the formation of appressorium (Xu and Hamer 1996; Liu and Dean 1997; Adachi and Hamer 1998). In the ascomycete Cryphonectria parasitica , which causes chestnut blight, the best-characterised signalling component for fungal virulence is a G-protein G a -subunit Cpg1 (Choi et al. 1995). In contrast to the situation in U. maydis and M. grisea, Cpg1 in the C. parasitica disease signalling is assumed to be a negative regulator of adenylate cyclase activity (Gao and Nuss 1996).
Signalling networks that are conserved in pathogenic development (Bölker 1998; Hamer and Talbot 1999; Kahmann et al. 1999) may also regulate hyphal adhesion on plant surface, morphogenesis, and penetration of host tissues during symbiosis. To isolate possible regulators of symbiotic growth, homology-based PCR approach has led to the identification of one putative G a cDNA and two ras cDNAs from Suillus bovinus cDNA library (Raudaskoski et al. 2000). All of these genes are expressed in symbiotic hyphae.
In Scandinavian forests, Scots pine ECM with Suillus bovinus represents a common type of ectomycorrhiza. These partners are well adapted to dry soils poor in nutrients and they are sensitive to surplus N levels (Ingestad 1979). S. bovinus host range includes only Pinus species, but already in Finland, 100 different species of ECM fungi for Scots pine have been identified (Treu and Miller 1996; Väre and Ohtonen 1996).
The development and functioning of pine root system is highly affected by ECM fungi. In general, mycorrhizal fungi invade both lateral roots and short roots, but in Pinus sylvestris predominantly the short roots are colonized (Robertson 1954; Wilcox 1968b; Piche and Fortin 1982). Ectomycorrhiza formation causes differences in short root structure (Fig. 1). Instead of normal branching from the side, the infected short roots branch dichotomously at their tips. The short roots of some pine species, such as Pinus pinaster or Pinus taeda dichotomize spontaneously, but others including Scots pine dichotomize rarely without the presence of ectomycorrhizal fungi (Robertson 1954; Wilcox 1968b; Faye et al. 1980; Piche and Fortin 1982; Kaska et al. 1999).
Before the formation of branch primordia some of the central apical meristem cells vacuolate. Lateral meristem cells are activated to proliferate on opposite sides of the stele. The previous root apex flattens and continuous vacuolization of central meristem cells separates the two newly formed meristems. In most cases, after some growth the two root tips branch again dichotomously. Dichotomy may continue until even 30-branch root forms, but large variation in the extent of dichotomous branching occurs between the individuals of same species (Wilcox 1968b; Faye et al. 1980).
The plant hormone auxin has been known for a long time to be an important regulator of root development. In higher concentrations, it inhibits the elongation growth of the root, but also induces pericycle cell division at the initiation of lateral root formation. Recently, it has also been implicated as a central regulator of root cell patterning (Sabatini et al. 1999; Scheres et al. 2000). Polarized auxin transport and auxin perception is known to play an important role in the establishment of roots (Hamann et al. 1999; Mattsson et al. 1999; Steinmann et al. 1999). Another plant growth regulator, ethylene, decreases root growth rate and affects epidermal cell patterning. It is preceived by a family of integral membrane receptors, and the ethylene signal transduction involves both negative (Chang et al. 1993; Kieber et al. 1993) and positive signal transducers (Chao et al.1997; Alonso et al. 1999). Ethylene and auxin signal transduction pathways influence each other in roots. Several auxin mutants also show reduced sensitivity to ethylene in roots (Lincoln et al. 1990; Timpte et al. 1995; Lehman et al. 1996), and two auxin mutants show selective resistance to ethylene (Pickett et al. 1990; Roman et al. 1995). Auxins and cytokinin (Abel et al. 1995; Vogel et al. 1998) can activate ethylene synthesis, and a signal transduction pathway for the control of root growth involves ethylene-mediated auxin accumulation (Timpte et al. 1995).
Both the changes in plant endogenous hormones caused by fungal colonization and hormone production by ECM fungi affect the balance of plant hormones in ECM (Gogala 1991). The roles of auxin and ethylene in mycorrhiza formation and short root branching in pine have been extensively studied. Fungal extracts and synthetic auxins induce dichotomous branching of short roots that resembles ectomycorrhizal morphology (Slankis et al. 1949; Rupp and Mudge 1985). An "auxin theory" for ectomycorrhiza formation has been proposed by Slankis indicating that fungal auxins are responsible for root morphogenesis during ECM formation, but more recent studies suggest that the auxin theory represents an oversimplification (Smith and Read 1997). Many ectomycorrhizal strains do not produce auxins, and the IAA content of Scots pine ectomycorrhizal roots was found to be lower than in the controls in the study of Wallander et al. (1992). The existence of correlation between fungal auxin production and numbers of mycorrhizas (Gay et al. 1994; Rudawska and Kieliszewska-Rokicka 1997) was not supported by the study of Nylund et al. (1994) where no such relationship was found. A more detailed in vitro analysis of pine short root branching (Kaska et al. 1999) suggested that auxin has a role in pine root branching via activation of ethylene production. In addition to auxins ectomycorrhizal fungi have also been shown to produce ethylene (Strelczyk et al. 1994). The morphological features of Douglas fir and loblolly pine seedling roots are more significantly correlated with the in vitro ethylene production rates than in vitro IAA producing capacity of ECM fungi (Scagel and Linderman 1998). Ethylene alone is sufficient to cause dichotomous branching and it is probably responsible for both the symbiotic fungus-induced and the spontaneous root tip branching of some of the pine species, such as Pinus taeda and Pinus pinaster (Kaska et al. 1999). Due to the interactions between ethylene and auxin signal transduction pathways (Lincoln et al. 1990; Timpte et al. 1995; Lehman et al. 1996) their possible separate roles cannot be easily addressed. Plant hormones may be considered as only one of the many factors regulating ectomycorrhizal root morphology, since high nutrient levels can inhibit short root branching (Kaska et al. 1999).
Some fungal isolates from pine roots can produce minute amounts of gibberellin-like substances (Strelczyk and Pokojska-Burdziej 1984) and cytokinins (Ng et al. 1982), and treatment with jasmonic acid accelerates the fungal colonization of spruce roots (Regar and Gogala 1996). The roles of these hormones on mycorrhizal development have not been studied in any detail but they may be involved in the regulation of formation and longevity of mycorrhizal roots (Gogala 1991).
Two major events are common to all cell cycles: S-phase, during which chromosomes are replicated, and M-phase, when the replicated chromosomes are segregated into two daughter nuclei. Genetic and biochemical approaches have led to the identification of the basic components of eukaryotic cell cycle machinery (Nurse 1990). In the genetic approach temperature-sensitive budding and fission yeast cell cycle ( cdc ) mutants that are delayed in different phases of cell cycle were characterised and the corresponding genes isolated (Nurse and Thuriaux 1980; Simanis and Nurse 1986; Booher and Beach 1987). In the second, biochemical approach invertebrate and vertebrate oocytes were used to isolate the proteins that induce the oocyte entry into M-phase (Evans et al. 1983; Swenson et al. 1986; Gautier et al. 1988). These studies revealed the existence of an universal regulator of eukaryotic cell cycle, cyclin dependent kinase complex (Cdk) that consists of a catalytic kinase subunit bound to an activating cyclin protein (Nurse 1990). Cdk activity is regulated by interactions with cyclins and different Cdk inhibitors, by modifications in the phosphorylation status, and by ubiquitin-mediated protein degradation (Genschik et al. 1998; Nakayama 1998).
The plant cyclins are classified by sequence homology (Renaudin et al. 1996), and the classification correlates with the timing of their expression at cell cycle phases. The A-type cyclins are expressed at DNA-synthesis phase, G 2 -phase and at mitosis, and the B-type cyclins during mitosis. Together they are referred to as mitotic cyclins. The expression level of the mitotic cyclins is mostly tissue-specific (Ferreira et al. 1994; Renaudin et al. 1998). Instead the D-type cyclins are expressed during G1/S transition and their synthesis depends more on mitogenic stimuli, which indicates that the different environmental and developmental regulators are linked via D-type cyclins to the cell cycle machinery (Dahl et al. 1995; Riou-Khamlichi et al. 1999, 2000). Cyclin expression is necessary for normal growth rate of the plant. When mitotic and D-type cyclins were overexpressed both an increase in cell size and cell number was observed, suggesting that cell division activity is a crucial factor determining the rate of growth (Doerner et al. 1996; Cockroft et al. 2000).
Several observations about the development of non-mycorrhizal and mycorrhizal short roots of Pinus sylvestris suggest that components regulating cell cycle are involved. First, BrdU-labelling of nuclei with replicating DNA and DAPI-staining of the short root nuclei showed that cell cycle progression was restricted to the short root apex and the first cell layer above the meristem. In contrast, in lateral roots BrdU-labelled nuclei and the dividing nuclei detected by DAPI staining were detected in four to five layers above the actual meristem (Niini 1998; Niini and Raudaskoski 1998). These observations suggest that cell cycle could be repressed in short roots by negative regulators of the cell cycle. Several mutants that are defective in cell proliferation at the root tip have been characterised (Cheng et al. 1995; Willemsen et al. 1998), but genes regulating meristem identity, such as CLAVATA or WUSCHEL genes in the shoot meristem (Schoof et al. 2000) have not yet been isolated from roots. The exit from the cell cycle is mediated by the accumulation of Cdk inhibitors and by inactivation and degradation of Cdk activators. In plants as in other eukaryotes, a Wee1-related kinase has been identified that inactivates the Cdk by negatively phosphorylating the kinase subunit (Sun et al. 1999). A second conserved plant regulator of cell cycle in is CCS52 protein, which may be involved in the proteolytic processing of cyclin proteins. The ccs52 gene family consists of several members, and their expression may be tissue-related (Cebolla et al. 2000). Other types of yet unindentified cell cycle inhibitors may also be involved (Nakayama 1998; Sherr and Roberts 1999). None of these genes have yet been identified in pine.
In mycorrhizal short roots mitotic index increases to the same level than in lateral roots, although the dividing nuclei are in the short root apex covered with fungal mycelium (Niini 1998). First the size of the mycorrhizal short root meristem increases and later the central cells of the apical meristem begin to differentiate (Piche and Fortin 1982; Niini and Raudaskoski 1998). After reaching a certain size the short root tip dichotomises. It has been suggested that the symbiotic fungus induces meristematic activity in mycorrhizal short roots (Niini 1998). This would take place either by the production of phytohormones or some other signalling molecules able to trigger the plant cell cycle.
Positive regulation of cell division activity has been well characterised in legume- Rhizobium symbiosis. As the first step of root nodule formation, bacterial lipo-oligosaccharides induce the formation of nodule primordium by activating the cell cycle in specific cortex cells (Spaink et al. 1991; Yang et al. 1994; Goormachtig et al. 1997). The activated cortex cells guide infection thread into the primordium, and upon infection it forms a nodule meristem and other nodule tissues. Early nodulin gene ENOD40 encodes for a small peptide that positively regulates nodule primordium and meristem formation (Yang et al. 1993; Charon et al. 1997). The gene is positively regulated by the plant hormone cytokinin (Fang and Hirsch 1998). The cortical cell divisions during root nodule development may be regulated by local changes in auxin/cytokinin balance, since nodule-like structures can be induced both by the addition of cytokinins (Cooper and Long 1994) and auxin transport inhibitors (Hirsch et al. 1989). In contrast, ethylene acts as a negative regulator of cortical cell divisions in nodules and it largely determines the positioning of nodule primordia (Heidstra et al. 1997). Dichotomous branching of pine short roots can be activated by ethylene and auxin treatment but not by cytokinins (Kaska et al. 1999). In short roots the existing meristem is triggered to continue dividing in its sides and the cells in the middle of the meristem differentiate. This is in contrast to the situation at nodule (Yang et al. 1994; Goormachtig et al. 1997) formation where the cell cycle of differentiated cortical cells is activated. However, highly localized changes in cytokinin/auxin ratio may also be involved in the control of the short root meristem activity, particularly since ethylene can act as an auxin transport inhibitor (Suttle 1988).