Is nickel a universal component of plant ureases?

Abstract

A simple assay for ureolytic activity in intact callus was devised to screen rice, tobacco and soybean for the role of nickel in urease synthesis. When callus is grown with citrate (10 mM) urease levels are reduced approx. 5-fold (tobacco), 10-fold (rice) and 28-fold (soybean). It appears that citrate is depressing urease synthesis by chelating trace nickel in the medium thus making it less available to the callus cells: a 10 −2 mM Ni 2+ addition to citrate-containing growth medium stimulates urease levels approx. 11-, 8- and 17-fold respectively in tobacco, rice and soybean cultures. The antagonistic effects of citrate and nickel on callus urease levels are not correlated with effects on urea uptake or on callus growth. However, when urea is nitrogen source Ni 2+ consistently stimulates growth. In light of the conclusions of Dixon et al. [1], viz. that nickel is an act site component of jack bean urease, the observed stimulation of urease synthesis in soybean, tobacco and rice callus by nickel strongly suggests that this metal is a component of their ureases as well.