Lokalisation und Regulation der sauren Invertase in mykorrhizierten Kurzwurzeln

Abstract

Most vascular plants live in a symbiotic association with soil fungi, the mycorrhiza. In this symbiosis, the fungus takes up nutrient salts and water from the soil and makes them accessible for the plant partner, while the plant supplies the fungus with essential carbohydrates produced in photosynthesis. As the fine fungal hyphae can penetrate and exploit the soil to a much greater extend than the plant's own root hairs, the mycorrhizal symbiosis increases both the ecological and physiological fitness of the plant. The mycorrhiza has a tremendous impact on agriculture and forestry by increasing plant growth, plant health and crop yield.The transport form of carbohydrates in most plants is sucrose, which mycorrhizal fungi are unable to take up. Invertases can convert sucrose into its component hexose sugars, glucose and fructose. They are key enzymes for sugar distribution in the plant and for nutrient exchange between organisms in the ectomycorrhizal symbiosis. Several different forms of invertases are known in plants and in saprophytic fungi, whereas most ectomycorrhizal fungi are deficient in these enzymes. The enzymatic action of plant invertases supports the fungal partner with a stable supply of hexose sugars, as these fungi are unable to use the plant sugar sucrose directly. From previous studies, the presence of invertases in the contact zone of an ectomycorrhiza, the apoplastic space, has been suspected, but was not directly demonstrated. This work focusses of a direct immunohistochemical detection of invertases in the interfacial apoplast of an ectomycorrhiza, combined with invertase activity measurements in different kinds of plant roots.Acid invertases from poplar roots (Populus tremula x Populus tremuloides, Clone T89) exhibit reaction optima in the temperature range of 50Ã ° - 65Ã °C. When assayed at their resepctive optimal temperature, invertase activity is highest in long roots and lowest in both mycorrhizal and non-mycorrhizal short roots. The difference between mycorrhizal and non-mycorrhizal short roots is only slight, but shortly after fertilization, cell-wall invertase activity is slightly higher in myorrhizal roots. This is interpreted as a sign of active transport processes across the apoplastic interface in the ectomycorhizal symbiosis.For immunization and specific antibody production, a synthetic polypeptide from a highly conservered domain of plant acid invertases was used. Both raw antiserum and affinity-purified antibody from this immunization detected poplar invertase proteins in Western Blots. The same invertase proteins were also detected by other invertase antisera obtained from other researchers for comparison. The antibodies produced here did not cross-react with fungal (Amanita) cell-wall proteins, thus enabling a specific detection of invertases at the plant-fungus interface.In semi-thin sections of a poplar-Amanita muscaria ectomycorrhiza, the invertase-specific antibodies produced a distinct positve signal at the inner boundary of the Hartig Net, precisely at the cell walls. This signal did not occur at other cortical or fungal cell walls and was most pronounced in the median part of the ectomycorrhizal root, the main zone of nutrient exchange in the symbiosis. Such a distinct reaction was not observed in non-mycorrhizal short roots.This work supports the current models of nutrient exchange in the ectomycorhizal symbiosis, which postulate the presence of an apoplastic invertase in the plant-fungus interface.