Changes in carbon partitioning or allocation due to ectomycorrhiza formation: biochemical evidence

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Abstract
Based on the in vitro synthesis of ectomycorrhizas between spruce seedlings and different fungal partners we assessed biochemical parameters connected to the regulation of both formation and utilization of sucrose. Mycorrhization of 4-month-old seedlings resulted in an increase of the activity of sucrose phosphate synthase, a key enzyme in the cytosolic formation of sucrose in source leaves. In parallel, the concentration of fructose 2, 6-bisphosphate (F26BP), a potent inhibitor of fructose 1, 6-bisphosphatase (FBPase), was decreased in needles. Both changes indicate an increased capacity for sucrose formation in mycorrhizal seedlings. To examine the fate of sucrose in the root, we compared transport properties of protoplasts from mycorrhiza-forming fungi (Amanita muscaria, Cenococcum geophilum) with apoplastic sucrose metabolism. Protoplasts from A. muscaria preferentially took up glucose (KM, 1.25 mM; Vmax, 18 pmol/(106 protoplasts∙min)), while sucrose was not transported. This is in accordance with the restriction of invertase activity (sucrose hydrolysis) to the host tissue. A distinction of different zones of symbiotic interaction (analysis of 0.5-mm-wide sections from the tip towards the base of single Picea abies – A. muscaria mycorrhizas) showed a decrease of sucrose in the zones exhibiting the highest fungal proportion (ergosterol), which was paralleled by a concomitant increase in trehalose. In addition, these zones of symbiotic interaction were characterized by high levels of F26BP (up to 1.8 pmol/mg dry weight), which exceeded those resulting from a mere addition of the levels contained in the single partners. An assay of potentially F26BP stimulated enzymes (phosphofructokinase (PFK), PPi-dependent fructose-6-phosphate phosphotransferase (PFP)) revealed that in fungal extracts PFK was stimulated by F26BP while in host tissue it was PFP. This partner-specific difference in stimulation by F26BP could play a pivotal role in creating the strong sink activity in mycorrhizal roots. Possible implications on the partner-specific regulation of glycolysis are discussed. Key words: carbon allocation, ectomycorrhizal symbiosis, fructose 2, 6-bisphosphate, hexose transport, regulation of glycolysis, sucrose metabolism.