Search

The most frequently encountered symbiont on tree roots is the ascomycete Cenococcum geophilum, the only mycorrhizal species within the largest fungal class Dothideomycetes, a class known for devastating plant pathogens. Here we show that the symbiotic genomic idiosyncrasies of ectomycorrhizal basidiomycetes are also present in C. geophilum with symbiosis-induced, taxon-specific genes of unknown function and reduced numbers of plant cell wall-degrading enzymes. C. geophilum still holds a significant set of genes in categories known to be involved in pathogenesis and shows an increased genome size due to transposable elements proliferation. Transcript profiling revealed a striking upregulation of membrane transporters, including aquaporin water channels and sugar transporters, and mycorrhiza-induced small secreted proteins (MiSSPs) in ectomycorrhiza compared with free-living mycelium. The frequency with which this symbiont is found on tree roots and its possible role in water and nutrient transport in symbiosis calls for further studies on mechanisms of host and environmental adaptation.

To elucidate the genetic bases of mycorrhizal lifestyle evolution, we sequenced new fungal genomes, including 13 ectomycorrhizal (ECM), orchid (ORM) and ericoid (ERM) species, and five saprotrophs, which we analyzed along with other fungal genomes. Ectomycorrhizal fungi have a reduced complement of genes encoding plant cell wall–degrading enzymes (PCWDEs), as compared to their ancestral wood decayers. Nevertheless, they have retained a unique array of PCWDEs, thus suggesting that they possess diverse abilities to decompose lignocellulose. Similar functional categories of nonorthologous genes are induced in symbiosis. Of induced genes, 7–38% are orphan genes, including genes that encode secreted effector-like proteins. Convergent evolution of the mycorrhizal habit in fungi occurred via the repeated evolution of a 'symbiosis toolkit', with reduced numbers of PCWDEs and lineage-specific suites of mycorrhiza-induced genes.

In temperate and boreal regions, seasons are characterized by two major factors - temperature and water availability. As water availability is affecting essential processes like nutrition and photosynthesis, it is of central importance for plant physiology. Trees and shrubs of temperate and boreal forest ecosystems are characterized by a tight association of their fine roots with certain soil fungi, forming a new symbiotic organ - the ectomycorrhiza. Here, fine roots are often covered by fungal hyphae (the so-called sheath) isolating them from the surrounding soil. Furthermore, fungal hyphae grow within the apoplast of rhizodermis and root cortex, forming a dense hyphal network (Hartig net), which is thought to function as an interface between fungus and plant for the reciprocal exchange of nutrients and metabolites. Mycorrhizas are connected with other parts of the fungal colony (e.g., soil exploring mycelium) by specialized transport hyphae (see below) and, in contrast to a number of well investigated filamentous model ascomycetes (e.g., Neurospora, Aspergillus), EM fungal colonies perform intense nutrient and metabolite exchange (for reviews, see Smith and Read 1997; Anderson and Cairney 2007).

The actin cytoskeleton (AC) of fungal hyphae is a major determinant of hyphal shape and morphogenesis, implicated in controlling tip structure and secretory vesicle delivery. Hyphal growth of the ectomycorrhizal fungus Amanita muscaria and symbiosis formation with spruce are promoted by the mycorrhiza helper bacterium Streptomyces sp. AcH 505 (AcH 505). To investigate structural requirements of growth promotion, the effect of AcH 505 on A. muscaria hyphal morphology, AC and actin gene expression were studied. Hyphal diameter and mycelial density decreased during dual culture (DC), and indirect immunofluorescence microscopy revealed that the dense and polarised actin cap in hyphal tips of axenic A. muscaria changes to a loosened and dispersed structure in DC. Supplementation of growth medium with cell-free bacterial supernatant confirmed that reduction in hyphal diameter and AC changes occurred at the same stage of growth. Transcript levels of both actin genes isolated from A. muscaria remained unaltered, indicating that AC changes are regulated by reorganisation of the existing actin pool. In conclusion, the AC reorganisation appears to result in altered hyphal morphology and faster apical extension. The thus improved spreading of hyphae and increased probability to encounter plant roots highlights a mechanism behind the mycorrhiza helper effect.

The formation of ectomycorrhizas, a tight association between fine roots of trees and certain soil fungi, improves plant nutrition in a nutrient-limited environment and may increase plant survival under water stress conditions. To investigate the impact of mycorrhiza formation on plant water uptake, seven genes coding for putative water channel proteins (aquaporins) were isolated from a poplar ectomycorrhizal cDNA library. Four out of the seven genes were preferentially expressed in roots. Mycorrhiza formation resulted in an increased transcript level for three of these genes, two of which are the most prominently expressed aquaporins in roots. When expressed in Xenopus laevis oocytes, the corresponding proteins of both genes were able to transport water. Together, these data indicate, that the water transport capacity of the plasma membrane of root cells is strongly increased in mycorrhized plants. Measurements of the hydraulic conductance of intact root systems revealed an increased water transport capacity of mycorrhized poplar roots. These data, however, also indicate that changes in the properties of the plasma membrane as well as those of the apoplast are responsible for the increased root hydraulic conductance in ectomycorrhizal symbiosis.

Ectomycorrhizas, the dominating mycorrhizal symbiosis in boreal, temperate and some tropical forests, are formed by 5000–6000 species of the asco- and basidiomycetes. This high diversity of fungal partners allows optimal foraging and mobilisation of various nitrogen and phosphorus forms from organic soil layers. In this review, two approaches to study the functioning of this multitude of symbiotic associations are presented. On selected culture models, physiological and molecular investigations have shown that the supply of hexoses has a key function in controlling the plant–fungus interaction via partner-specific regulation of gene expression. Environmental factors which affect fungal carbon supply, such as increased nitrogen availability, also affect mycorrhiza formation. Based on such laboratory results, the adaptative capability of ectomycorrhizas to changing field conditions is discussed. The second approach consists of analysing the distribution of mycorrhizas in ecosystem compartments and to relate distribution patterns to variations of ecological factors. Recent advances in identification of fungal partners in ectomycorrhizas by analysing the internal transcribed spacer of ribosomal DNA are presented, which can help to resolve sampling problems in field studies. The limits of the laboratory and the field approaches are discussed. Despite some problems, this combined approach is the most promising. Direct investigation of gene expression, which has been introduced for soil bacteria, will be difficult in the case of mycorrhizal fungi which constitute organisms with functionally varying structures.

Ectomycorrhiza development involves the differentiation of structurally specialized fungal tissues (e.g., mantle and Hartig net) and an interface between symbionts. Polypeptides presenting a preferential, up-, or down-regulated synthesis have been characterized in several developing ectomycorrhizal associations. Their spatial and temporal expressions have been characterized by cell fractionation, two-dimensional polyacrylamide gel electrophoresis, and immunochemical assays in the Eucalyptus spp. – Pisolithus tinctorius mycorrhizas. These studies have emphasized the importance of fungal cell wall polypeptides during the early stages of the ectomycorrhizal interaction. The increased synthesis of 30- to 32-kDa acidic polypeptides, together with the decreased accumulation of a prominent 95-kDa mannoprotein provided evidence for major alterations of Pisolithus tinctorius cell walls during mycorrhiza formation. Differential cDNA library screening and shotgun cDNA sequencing were used to clone symbiosis-regulated fungal genes. Several abundant transcripts showed a significant amino acid sequence similarity to a family of secreted morphogenetic fungal proteins, the so-called hydrophobic. In P. tinctorius, the content of hydrophobin transcripts is high in aerial hyphae and during the ectomycorrhizal sheath formation. Alteration of cell walls and the extracellular matrix is therefore a key event in the ectomycorrhiza development. An understanding of the molecular mechanisms that underlies the temporal and spatial control of genes and proteins involved in the development of the symbiotic interface is now within reach, as more sophisticated techniques of molecular and genetic analysis are applied to the mycorrhizal interactions.