Nickel is the latest element to be classified as essential for plant growth, however, its biological role is poorly understood. This is mainly because plants require low levels of Ni while it is relatively abundant in soil (Marschner 1995). The knowledge of Ni uptake by plants is indeed very limited, and apart from the observation that Ni is quite mobile as compared to other heavy metals, little is known about the uptake mechanism and translocation under Ni-limiting conditions (Brown 2007) . There are several enzyme systems (NiFe-hydrogenase, carbon monoxide dehydrogenase, acetyl-CoA decarbonylase synthase, methyl-coenzyme M reductase, superoxide dismutase, Ni-dependent glyoxylase, acireductone dioxyge-nase, and methyleneurease) in bacteria and lower plants (Mulrooney and Hausinger 2003) that are activated by Ni. Activation of urease with two Ni ions at the active site (Ciurli 2001), however, is the only known biological function of Ni in higher plants (Gerendas et al. 1999).
The metabolic effects of Ni deficiency have been reported in cereals (Brown et al. 1990), legumes (Gerendas and Sattelmacher 1997) and perennial species (Bai et al. 2006). These include reduced urease activity, induced metabolic N deficiency, disruption of N metabolism because of alterations in the ureide catabolism and metabolism of amino acids and ornithine cycle intermediates.
Function of citric acid cycle disrupts also in Ni-deficient plants. Under Ni-deficiency conditions, leaves contained low levels of citrate compared to Ni-sufficient leaves and accumulated lactic and oxalic acids (Bai et al. 2006).
According to these results, Ni deficiency results in distinct biochemical symptoms even before development of morphological symptoms and disruption of vegetative growth. The wide spectrum of metabolic disruption in Ni-deficient plants is an evidence for the existence of unidentified physiological roles for Ni in plants. This finding in combination with the diverse known functions of Ni in bacteria suggests that Ni may indeed play a role in many, yet undiscovered processes in higher plants (Brown 2007) .
Improvement of our knowledge of the biochemical role of Ni in plants may bring new insights into how Ni nutrition affects plants stress responses. Genetics and molecular biology approaches may be useful in identification of the roles of Ni in the biochemical processes particularly under stressful conditions similar with the studies on Mo and its effect on the plants stress response via ABA metabolism.
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