Carbohydrates are synthesized by photosynthesis in plants and are partitioned in the form of sucrose, via the phloem, to organs and tissues which have a demand for carbon and form a ‘sink’. Ectomycorrhizal fungi, which live in intimate symbiosis with trees, receive up to 30% of the total carbon fixed by the plant host (Finlay & Söderström, 1992) and thus function as an important sink. In exchange, the tree receives mineral nutrients from the fungus. Understanding how the fungus can increase its ‘sink strength’, and hence demand carbon from the host, is clearly important. In this issue of New Phytologist (pp. 389–398), Lopez et al. have worked towards this by investigating carbon partitioning in the poplar (Populus tremula × tremuloides)–Amanita muscaria ectomycorrhizal symbiosis.
‘Thus, while trehalose is not the only fungal carbon sink synthesized upon feeding glucose, it would seem to be particularly important in the Hartig net.’
Knowing that trehalose is used as an intermediate storage pool for carbohydrates and is present in large quantities in A. muscaria, Lopez et al. hypothesized that trehalose may act as an important carbon sink and thus set about determining the compartmentalization of trehalose biosynthesis between the hyphae of the fungal sheath and the Hartig net. Through the use of advanced techniques and much skill, the authors succeeded in physically separating the fungal sheath from the Hartig net. Transcript levels of the genes encoding key enzymes of fungal trehalose biosynthesis were found to be higher in the Hartig net compared to the other tissues; in particular, trehalose phosphate synthase (TPS), trehalose phosphate phosphatase (TPP) and trehalose phosphorylase (TP) were increased. The TPS and TPP enzymes form the classic pathway for trehalose synthesis, whereas TP is thought to function as trehalose synthase when glucose is abundant. Further expression analysis has shown that the Amanita genes (AmTPS, AmTPP and AmTP) are largely unaffected by sugar and nitrogen supply. This indicates that their increased trehalose gene expression observed in the Hartig net is under developmental control. It is of note that global gene expression studies addressing ectomycorrhizal development have not discovered an up-regulation of genes encoding enzymes of trehalose biosynthesis (Duplessis et al., 2005; Wright et al., 2005). Lopez et al. go on to show that both TPS activity and trehalose concentrations are considerably higher in the Hartig net than in the fungal sheath, correlating directly with the transcript abundance and enzyme activity data. The authors propose that in this fungal tissue, both pathways of trehalose synthesis, TPS/TPP and TP, are operating, and that the transformation of two glucose molecules into trehalose is important in maintaining the sink for glucose.
What is known about trehalose and ectomycorrhizal fungi?
In previous work, Martin et al. (1998) showed that the glucose accumulating in Eucalyptus globulus roots was utilized by the ectomycorrhizal fungus Pisolithus tinctorius, and that it was converted to short chain polyols (namely, arabitol and erythritol) and trehalose. At that time it was not known whether the accumulation of these soluble carbohydrates was located in the fungal sheath or in the Hartig net.
A number of studies have suggested that trehalose fulfils multiple functions in ectomycorrhiza. Trehalose (and mannitol) concentrations have been found to relate to fungal vitality (Niederer et al., 1989), and, when exposed to desiccation by frost the concentration of trehalose in excised mycorrhizal roots was shown to double (Niederer et al., 1992). Several Hebeloma strains were able to survive to −10 °C and accumulated arabitol, mannitol and trehalose, apparently for cryoprotection (Tibbett et al., 2002). In a global change study under an atmosphere of elevated CO2, an increase in the uptake of glucose and synthesis of trehalose was found in nutrient-rich but not in nutrient-poor soils; increased trehalose synthesis was also found to correlate with an increase in fungal biomass (Wiemken et al., 2001). Pisolithus tinctorius has been reported to accumulate large amounts of trehalose during growth on media containing glucose. A shift to a carbon-free medium resulted in the consumption of this large trehalose pool, while the arabitol pool decreased by only approx. 50%. During the formation of the ectomycorrhizal symbiosis following contact between axenically grown Pisolithus and pine seedlings, the fungal trehalose pool was consumed in the first 10 days but then refilled after the establishment of symbiosis, especially in the extraradical mycelium (Ineichen & Wiemken, 1992). This demonstrates an important function of trehalose as an easily available source of glucose for energy and carbon.
Is trehalose synthesis necessary for the gain of carbon by the fungus?
Lopez et al. investigated trehalose biosynthesis at the plant–fungus interface and considered trehalose as a relevant carbohydrate sink in symbiosis. However, when glucose is fed to ectomycorrhizal fungi (Cenococcum graniforme, Hebeloma crustuliniforme), it can be transformed into various carbohydrates as well as lipids (Martin et al., 1984a,b; Laczko et al., 2004). Martin et al. (1998), experimenting with Eucalyptus–Pisolithus ectomycorrhizas, detected trehalose, mannitol, arabitol and erythritol in similar amounts after feeding with labelled glucose. Thus, while trehalose is not the only fungal carbon sink synthesized following feeding with glucose, it would seem to be particularly important in the Hartig net, as was revealed by Lopez and colleagues. We are left curious regarding what the reasons for this might be.
What could be the advantages of forming trehalose for the fungus?
2Trehalose is considered as a transport sugar in ectomycorrhizal fungi, in analogy to sucrose in plants (Söderström et al., 1988).
3The storage and transport of carbon in the form of trehalose and the later gain of two molecules of glucose by only one step for degradation is an energetically favourable process at the site of consumption, compared to the conversion of polyols to glucose.
4It was demonstrated that trehalose protects proteins and membranes from heat and cold stress (see, e.g. Crowe, 2007). A large group of ectomycorrhizal fungi form hydrophobic surfaces which allow growth in dry areas, such as litter layers that might be exposed to daily desiccation. In these instances trehalose could act as a protectant for proteins and membranes.
5The fungus might use trehalose to ‘manipulate’ the plant in order to increase by some means the sink for sucrose in the roots. A recent study has shown that trehalose-6-phosphate is implicated in sugar signalling in Arabidopsis (Lunn et al., 2006). Furthermore, Nicotiana tabacum transformed with E. coli trehalose biosynthetic genes had an enhanced photosynthetic capacity, pointing to a role for trehalose (or trehalose phosphate) as a signal in carbon allocation (Pellny et al., 2004). Similarly, photosynthesis is enhanced in mycorrhizal compared to non-mycorrhizal trees (Durall et al., 1994). Therefore, an interaction between the trehalose metabolism of fungal origin with plant signalling processes has to be borne in mind.
One aspect that might be considered in future work is the possibility that trehalose synthesis could occur by several additional pathways which have not yet been investigated in Basidiomycetes (Fig. 1). For example, in Mycobacteria trehalose is synthesized from two molecules of glucose cleaved from the glycogen polymer by a single enzyme (DeSmet et al., 2000) and thus, upon demand, the carbohydrate reserve in the form of glycogen can easily be converted into trehalose.
In conclusion, the work of Lopez et al. highlights the importance of trehalose and trehalose metabolism in ectomycorrhizal symbiosis. With their painstaking work, they have clearly shown that trehalose accumulates strongly in the Hartig net, most probably because of the combined actions of TPS and TPP, and possibly that of TP. It will be a future challenge to define the biological role of trehalose accumulation in the Hartig net.