Essentiality and Function of Nickel in Plants

The first evidence of a response of a field crop to application of a nickel fertilizer was demonstrated in 1945 for potato (Solanum tuberosum L.), wheat (Triticum aestivum L.), and bean (Phaseolus vulgaris L.) crops (25). In these crops, the application of a dilute nickel spray resulted in a significant increase in yield. These experiments were conducted on the ‘Romney Marshes’ of England, a region that is well known for its trace mineral deficiencies, particularly of manganese and zinc. These experiments were conducted very carefully and excluded the possibility that the nickel applied was merely substituting for manganese, zinc, iron, copper, or boron, suggesting that the growth response was indeed due to the application of nickel. Interestingly, the soils of this region may be low in nickel since the conditions that limit manganese and zinc availability in these soils (acid sands of low mineral content) would also limit nickel availability to crops, and the concentrations of nickel provided were appropriate based on the current knowledge of nickel demand. These same soil types also dominate the region of southeast United States where Ni deficiency is now known to occur.

Mishra and Kar (26) and Welch (27) reviewed the evidence of the role of nickel in biological systems and cited many examples of yield increases in field-grown crops in response to the application of nickel to the crop or to the soil. The significance of these purported benefits of field applications of nickel is difficult to interpret since the majority of the reported experiments used very high nickel application rates. None of these reports considered the possibility that nickel influenced plant yield through its effect on disease suppression, nor was the nickel concentration in the crops determined. Indeed, prior to the availability of graphite-furnace atomic absorption spectrophotometers and inductively coupled plasma mass spectrometers (in the mid-1970s), it was exceedingly difficult to measure nickel at the concentrations (^-0.1 mg Ni kg^-1 dry weight) later shown to be critical for normal plant growth. In the absence of information on tissue-nickel concentrations, it is impossible to conclude that the observed yield increases were the result of a correction of a nickel deficiency in the plant.

Clear evidence that nickel application benefited the growth of nitrogen-fixing species of plant was demonstrated by Bertrand and DeWolf (28), who reported that soil-nickel application to field-grown soybean (Glycine max Merr.) resulted in a significant increase in nodule weight and seed yield. The authors suggested that the yield increase was the result of a nickel requirement of the nitrogen-fixing rhizobia. A specific role for nickel in nitrogen-fixing bacteria is now well established with the determination that a nickel-dependent hydrogenase is active in many rhizobial bacteria (20) and is thus essential for maximal nitrogen fixation (29). Nickel is also known to be essential for nitrogen fixation of the free-living cyanobacterium, Nostoc muscorum C.A. Adargh, though the specific mechanism has not been determined (30).

A role for nickel in plant disease resistance has long been observed and has been variously attributed to a direct phyto-sanitary effect of nickel on pathogens, or to a role of nickel on plant disease- resistance mechanisms. Mishra and Kar (26) concluded that nickel likely acted to reduce plant disease by direct toxicity to the pathogen. Nickel, however, is not particularly toxic when applied directly to microorganisms, and Graham et al. (31) demonstrated that nickel supplied to the roots of cowpea (Vigna unguiculata Walp.) that contained only 0.03 mg Ni kg^-1 dry weight effectively reduced leaf-fungal infection by 50%. Whether this effect was directly due to a role of nickel in plant defense reactions (possibly involving superoxide dismutase-mediated processes) or a consequence of the alleviation of deficiency-induced changes in nitrogen metabolites (urea, amino acids, etc.) is uncertain. Regardless of the mechanism, a positive effect of nickel supplementation on disease tolerance was clearly documented.

The discovery that nickel is a component of the plant urease in 1975 (3) prompted a renewed interest in the role of nickel in plant life. In 1977, Polacco (32) determined that tissue-cultured soybean cells could not grow in the absence of nickel when provided with urea as the sole nitrogen source. Subsequently, an absolute nickel requirement was demonstrated for tissue-cultured rice (Oryza sativa L.) and tobacco (Nicotiana tabacum L.) (26,27). This finding was followed in 1981 by a review of nickel in biology that suggested that leguminous plants might have a unique requirement for nickel (28).

Using a novel chelation chromatography technique to remove nickel as a contaminant from the nutrient media, Eskew et al. (9,33,34) and Walker et al. (11) demonstrated that, under nickeldeficient conditions, urea accumulated to toxic levels in the leaves of soybean and cowpea. Leaflet tips of nickel-deficient plants contained concentrations of urea as high as 2.4% dry weight. The accumulation of urea occurred irrespective of the nitrogen source used and was assumed to have occurred as a result of urease-dependent disruption of the arginine-recycling pathway. Eskew et al. (9) concluded that nickel was an essential element for leguminous plants though they did not demonstrate a failure of nickel-deficient plants to complete their life cycles. Recently, Gerendas et al. (12-14), in a series of elegant studies demonstrated a profound effect of nickel deficiency on the growth of urea-fed tobacco, zucchini (Cucurbita pepo L.), rice, and canola (Brassica napus L.), but observed no growth inhibition when nitrogen sources other than urea were used.

Confirmation that nickel was essential for higher plants was provided by Brown et al. (1), who demonstrated that barley seeds from nickel-deprived plants were incapable of germination even when grown on a nitrogen source other than urea. Significant restrictions in shoot growth of barley, oats, and wheat (Triticum aestivum L.) were subsequently demonstrated under nickel-deficient conditions when the plants were supplied with mineral nitrogen sources (15). Brown et al. (15) also observed a marked suppression in tissue-iron concentrations in nickel-deficient plants, a response that is also observed in nickel-deficient animals (7). Reductions in tissue-malate concentrations have also been observed in nickel-deficient animals and plants (15,24,35). Confirmation of the essentiality of Ni under field conditions was provided in 2004 by Wood et al. (2), who observed a marked and specific positive response to application of Ni fertilizer to pecan (Carya illinoinensis K. Koch) and other species (2) that could not be corrected with any other known essential element.

The demonstration of a role for nickel in diverse plant species, the presence of nickel in a discrete metabolic process, and the failure of plants to complete their life cycles in the absence of nickel, satisfies the requirement for the establishment of essentiality (4).

Although nickel has been accepted generally as an essential element, there is reason to be cautious about this conclusion, and some authors suggest that nickel may not fully satisfy the most stringent interpretation of the laws of essentiality primarily since its role in a specific essential metabolic function has not been identified. Furthermore, even though nickel has a clear role in metabolism, it is now clear that urease is not, by itself, essential for plant life as evidenced by the observation that urease-null soybean mutants can complete their life cycles (37). There has also been no independent replication of the effect of nickel on barley grain viability though Horak (36) did observe a marked increase in seed viability with the addition of nickel to pea (Pisum sativum L.) seeds grown in nickeldeficient soils.

Regardless of these apparent contradictions, nickel is still clearly required for normal plant metabolism. As a component of urease, nickel is required for urea and arginine metabolism, and both of these metabolites are normal constituents of plants (5). Nickel is also an essential component of hydrogenases involved in nitrogen fixation and other associative bacterial processes, and nickel clearly influences plant response to disease. Nickel is clearly a normal constituent of plant life.

Many of the reported effects of nickel on plant growth cannot be attributed solely to the role of nickel in urease, and many symptoms of nickel deficiency (disrupted iron and malate metabolism) are also observed in animals (7). It is likely, therefore, that additional nickel-dependent enzymes and proteins await discovery and will help resolve the remaining questions on the function of nickel in plants.