A native promoter was employed for the
expression of Bacillus phytase from B. subtilis resulted in higher yield [123].
Transgenic expression of phytase in transgenic seeds is becoming a cheap source to
improve the bioavailability of phosphorus in food/feed compared with direct supplementation
of microbial phytase. The efficient use of transgenic technology for the
improvement of phosphorus nutrition in crop plants may require the expression of
introduced genes only under conditions of phosphate deficiency and only in specific
tissues. A phosphate transporter promoter from Arabidopsis was used to drive the
expression of a secretable A. niger phytase gene only in the root epidermis of phosphatedeprived
plants [126]. Similarly, an A. niger phytase gene has been successfully expressed
in rice using a ubiquitin promoter from maize. Here, the recombinant target gene
expression appeared not only in the seeds but also in other tissues such as leaves and
culms [127]. The sweet potato sporamin promoter is also used to control the expression
in transgenic potato of the E. coli appA gene. This promoter is highly active in the leaves
and stem with levels of phytase expression ranging from 3.8% to 7.4% of total soluble
proteins [128]. Escherichia coli appA can be expressed under the control of the proximal
PSP promoter in mouse saliva. The salivary appA phytase in these mice leads to a significant
reduction in fecal phytate levels. In 2013, a transgenic Enviropig was reported in
which E. coli appA was expressed in the saliva regulated by the mouse parotid secretory
protein promoter