Fungal carbohydrate support in the ectomycorrhizal symbiosis: a review

Abstract

Ectomycorrhizal (ECM) symbiosis is a mutualistic interaction between certain soil fungi and fine roots of perennial plants, mainly forest trees, by which both partners become capable of efficiently colonising nutrient-limited environments. The success of this interaction is reflected in the dominance of ECM forest ecosystems in the Northern hemisphere. Apart from their economic importance (wood production), forest ecosystems are essential for large-scale carbon sequestration, leading to substantial reductions in anthropogenic CO(2) release. The biological function of ECM symbiosis is the exchange of fungus-derived mineral nutrients for plant-derived carbohydrates. Improved plant nutrition as a result of this interaction, however, has a price. Together with their fungal partner, root systems of ECM plants can receive about half of the photosynthetically fixed carbon. To enable such a strong carbohydrate sink, the monosaccharide uptake capacity and carbohydrate flux through glycolysis and intermediate carbohydrate storage pools (trehalose and/or mannitol) of mycorrhizal fungi is strongly increased at the plant-fungus interface. Apart from their function as a carbohydrate store, trehalose/mannitol are additionally considered to be involved in carbon allocation within the fungal colony. Dependent on the fungal species involved in the symbiosis, regulation and fine-tuning of fungal carbohydrate uptake and metabolism seems to be controlled either by developmental mechanisms or by the apoplastic sugar content. As a consequence of the increased carbohydrate demand in symbiosis, trees increase their photosynthetic capacity. In addition, host plants control and restrict carbohydrate flux towards their partner to avoid fungal parasitism. The mechanisms behind this phenomenon are still largely unknown but rates of local sucrose hydrolysis and hexose uptake by rhizodermal cells are thought to restrict fungal carbohydrate nutrition under certain conditions (e.g., reduced fungal nutrient export).

Amelioration of planting stress by soil amendment with a hydrogel–mycorrhiza mixture for early establishment of beech (Fagus sylvatica L.) seedlings

“…EMFs increase plant nutrient availability and protect root tips from various stresses but consume plant-derived carbohydrates (Nehls et al 2010). The beneficial effect of soil amendment may also have been caused by an increased EMF colonization since the cortex cells of nonmycorrhizal beech roots were distorted (not shown), which has been reported to affect their physiological activity (Winkler et al 2010), while the intrinsic structures of EMF root tips had a healthy appearance.…”

Expression Analysis of Aquaporins from Desert Truffle Mycorrhizal Symbiosis Reveals a Fine-Tuned Regulation Under Drought

“…It has been hypothesized that the presence of a fungal sink may stimulate the rate of C assimilation (Tinker et al 1994). However, evidence in support of this hypothesis remains limited due to conflicting results (Nehls et al 2010;Wright et al 2000). The A values decreased less in H. almeriense M-DS than NM-DS plants ( Table 2).…”

Phloem Transport of Resources

“…Moreover, invasion by symbionts (nitrogenfixing bacteria [Complainville et al 2003]; mycorrhiza [Nehis et al 2010]) and parasites (nematodes [Hoth et al 2008]; vascular parasites [Birschwilks et al 2007]) shift transport phloem unloading from apoplastic to symplastic routes. These observations imply shifts to symplastic phloem unloading are regulated to satisfy various physiological states and raises the query as to whether transport mechanisms contribute to phloem unloading.…”