Behaviour of plant‐derived extracellular phytase upon addition to soil

The behaviour of phytase after addition to three soil types with different sorption capacities was investigated. Phytase was collected from the roots of transgenic Arabidopsis thaliana that express a phytase gene from Aspergillus niger. Phytase activity in solution and on the solid phase of the soil was monitored over time. Phytase added to the solution phase of a soil suspension (1:20, w/v) was almost completely lost within 10 min in all soil types, while phytase in non-soil controls remained active in solution. Phytase activity lost from solution was recovered on the soil solid phase, suggesting rapid adsorption of the enzyme. Adsorption of phytase was less in soil taken from the rhizosphere of transgenic plants expressing phyA, indicating that the rhizosphere environment may help maintain phytase activity in solution. The activity of adsorbed phytase declined with time at a rate 2-4 times slower than that in the absence of soil. Adsorption of phytase in soils was highest at pH 4.5, which is below the reported isoelectric point (pI) of the Aspergillus phytase. As soil pH increased, adsorption decreased until, at pH 7.5, all phytase was in solution. Where phytase remained in solution, activity was maintained for at least 8 d. In contrast, the activity of adsorbed phytase was increasingly inhibited with time, particularly at low pH. By increasing the pH in soil suspensions, phytase that had remained active on the soil solid phase for 28 d was almost totally desorbed. Rapid immobilisation of phytase in soil may limit its capacity to interact with phytate, and this may compromise the ability of transgenic plants which exude phytase from their roots to acquire P from endogenous soil phytate.