Partner und Internationale Organisationen
(Englisch)
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AT, BE, BG, CY, CZ, DK, FI, FR, DE, EL, HU, IE, IT, NL, NO, PL, SK, SI, ES, SE, CH, UK
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Abstract
(Englisch)
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Inositol hexa and penta phosphates represent the most important fraction of soil organic phospho-rus (Po). The release of orthophosphate (Pi) from myoinositolhexakisphosphate (IHP or phytate) is catalyzed by the activity of extracellular enzymes, the phytases that can be produced by plant roots and soil microorganisms. The objective of this project was to improve our understanding of the sig-nificance of extracellular phytases released by plant roots and microbes in relation to plant P up-take. To answer this question the group of plant nutrition of the ETH and the laboratory of microbiol-ogy of the Unviersity of Neuchâtel conducted three series of experiments: (i) the ETH group com-pared the uptake of P from phytate by a potato that had been genetically engineered to exude large quantities of phytase in its rhizosphere (phy+) to the uptake of P by the non-transformed parent (phy-) which secreted a very low phytase activity in its rhizosphere; (ii) the group in Neuchâtel es-tablished a method to screen bacteria able to produce extracellular phytase and finally (iii) the ETH group started to assess phosphatase exudation from the roots of two Brachiaria species which are tropical grasses adapted to low P soils. The results of these 3 parts can be summarized as follows. P Uptake by phy+ and phy- potato plants: The phy+ potato plants did not took up more P from phy-tate as the phy- potato plants when grown in a low P soil. However the phy+ plants took up more P from phytate when the soil was mixed with sand in a 1 to 1 ratio. The lack of reponse of the phy+ in the undiluted low P soil is probably related to its sorption properties which led to the immobilization of the exuded phytase and of the added phytate hindering any phytate hydrolysis. Production of phytase by soil bacteria (2nd year): The objectives of this second part were to develop a culture medium specific to phytase-producing rhizobacteria; to evaluate a technique to detect phy-tase activity which would be sensitive enough and suitable for the screening of a great number of isolates; to determine the proportion of phytase-producing strains in roots, in the rhizosphere, and in the soil and to make hypotheses concerning the possible interaction between siderophores- and phytase-producing rhizobacteria. The phytate medium was developed, two colorimetric methods and a plate assay for the detection of phytase activity were evaluated and the screening of the col-lection of strains has begun. Phosphatase exsudation by Brachiaria spp (3rd year) : Two Brachiaria species (B. decumbens and B. ruzizienzis) were grown in nutrient solution for 14 and 26 days in the presence of very low P con-centrations. Both species exsuded large quantities of phosphatase after 26 days in the solution, but the phosphatase production increased strongly between 0 and 14 days in B ruzizienzis and re-mained constant afterwards, while the phosphatase production remained low in B decumbens be-tween 0 and 14 days, and then increased. These results show that root phosphatase of Brachiaria spp can be used as a biochemical marker of P limitation. The phytase activities in these exsudates as well as their properties (optimal pH etc) are now being studied.
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