The fate of [C-14]glutamate and [C-14]malate in birch roots is strongly modified under inoculation with Paxillus involutus

The impact of inoculation with Paxillus involutus on the utilization of org anic carbon compounds by birch roots was studied by feeding [C-14]Glu or [C -14]malate to the partners of the symbiosis, separately or in association, and by monitoring the subsequent distribution of C-14, Inoculation increase d [C-14]Glu and [C-14]malate absorption capacities by up to eight and 17 ti mes, respectively. Six- and 15-d-old mycorrhizal roots showed about four-fo ld higher [C-14]Glu and [C-14]malate absorption capacities compared with 60 -d-old mycorrhizal roots, suggesting that the early stages of mycorrhiza fo rmation induced higher requirements for C skeletons. Moreover, the results demonstrated that inoculation strongly modified the fate of [C-14]Glu and [ C-14]malate. It was demonstrated that exogenously supplied Glu and malate m ight serve as C skeletons for amino acid synthesis in mycorrhizal birch roo ts and in the free-living fungus. Gin was the major C-14-sink in mycorrhiza l roots and in the free-living P. involutus, In contrast, citrulline and in soluble compounds were the major C-14 sinks in non-mycorrhizal roots, whate ver the C-14 source, If was concluded that mycorrhiza formation leads to a profound alteration of the metabolic fate of exogenously supplied C compoun ds. The ecological significance of amino acid and organic acid utilization by mycorrhizal plants is further discussed.