STABLE ACCUMULATION OF ASPERGILLUS-NIGER PHYTASE IN TRANSGENIC TOBACCO-LEAVES

Phytase from Aspergillus niger increases the availability of phosphoru s from feed for monogastric animals by releasing phosphate from the su bstrate phytic acid. A phytase cDNA was constitutively expressed in tr ansgenic tobacco (Nicotiana tabacum) plants. Secretion of the protein to the extracellular fluid was established by use of the signal sequen ce from the tobacco pathogen-related protein S. The specific phytase a ctivity in isolated extracellular fluid was found to be approximately 90-fold higher than in total leaf extract, showing that the enzyme was secreted. This was confirmed by use of immunolocalization. Despite di fferences in glycosylation, specific activities of tobacco and Aspergi llus phytase were identical. Phytase was found to be biologically acti ve and to accumulate in leaves up to 14.4% of total soluble protein du ring plant maturation. Comparison of phytase accumulation and relative mRNA levels showed that phytase stably accumulated in transgenic leav es during plant growth.