Significance of nickel for plant growth and metabolism

Ni is the most recent candidate to be added to the list of 13 essential min eral elements for higher plants although failure to complete the life cycle in the absence of Ni has only been demonstrated in a few plant species. Ni is considered an essential element primarily because of its function as an irreplaceable component of urease which is responsible for the hydrolysis of urea N, and which seems to be the only proven nutritional function of Ni in higher plants. For production of full urease activity and growth on ure a N a critical deficiency level of around 100 mu g kg(-1) DW seems appropri ate, while plants depending on mineral N may have a lower Ni requirement. N i has also other effects on plant growth, of which the phytosanitary action is possibly most significant in the field. The incorporation of Ni into ur ease apoprotein requires the active participation of several accessory prot eins, and mutations in genes coding the accessory proteins as well as the u rease apoprotein have been exploited to characterise aspects of urease acti vation. The mobility of Ni within the plant, as compared to other heavy met als, is usually high, although little is known of the uptake mechanisms and the form of transported Ni under Ni-deprived conditions. This as well as o ther effects of Ni that cannot be related to its structural component of ur ease, remain to be elucidated.