Essentiality of Nickel for Plants

Nickel (Ni) is an essential element for all plants due to its role in urease, the enzyme that catalyzes the conversion of urea into NH₄⁺ and CO₂. R. V. Klucas addresses few criteria to establish essentiality, that is, that other metals cannot substitute for Ni, thus establishing Ni firmly as an essential element. Studies on the Ni requirement of plants can be understood best by separating those studies that used mineral N as the sole N source as opposed to studies where urea is the sole N source. J. C. Polacco showed that growth of soybean in tissue culture with urea as the sole source of nitrogen could be strongly stimulated by low concentrations of Ni. The evidence that Ni is required for urease is firmly established, but this raises the question of whether urease itself is essential for plants. Numerous mutants deficient in urease can give further evidence regarding the role of urease.