THE BINDING OF ZINC IN ROOT-CELLS OF CROP PLANTS BY PHYTIC ACID

Appreciable quantities of Zn are bound as Zn phytate (myo-inositol kis-hexaphosphate) within small vacuoles of cortical cells in the elongation zone of root tips of zinc tolerant Deschampsia caespitosa. These Zn/P-containing globular deposits have now been shown to occur in the roots of soybean, lucerne, lupins, tomato, rapeseed, cabbage, radish, wheat and maize. The globules are most frequent in the endodermis and pericycle but may also occur in the stele and inner cortex. The X-ray data again confirmed the presence of phytate with a relatively stable proportion of Zn and a species-dependent, variable, proportion of K, Mg and Ca to P. Analysis of soybean plants by atomic absorption spectroscopy showed that the Zn concentration in the shoots doubled in response to an increase in Zn supply from 1 to 100 mu M while the concentration of Zn in the root symplast was approximately 22 times greater than in the shoot, suggesting restricted transport to the shoot. It is suggested that the genetic expression of the capacity to bind heavy metals by means of phytate in endodermal cells may provide a strategy for keeping the above-ground content of heavy metals low. It may be possible to incorporate the trait into transformed roots that can be utilized for the treatment of industrial wastes.