Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 130

corticular region. This distribution may explain why Ni has good mobility in xylem
and phloem tissues (Marschner 1995 ; Page and Feller 2005 ; Riesen and Feller
2005 ). However, the distribution of Ni varies in stems and leaves, where it is distrib-
uted preferentially in epidermal cells, and probably in vacuoles, rather than in the
cell wall (Küpper et al. 2001 ). The distribution of nickel in leaf organelles and in the
cytoplasm is different. Herein, Ni's distribution is higher in the cytoplastic luid and
in vacuoles (where it may exceed 87%), and lower in chloroplasts (a content of
8-9.9%), mitochondria, and ribosomes (which contain 0.32-2.85% of total nickel)
(Brooks et al. 1981 ).
Nickel is translocated to fruits and seeds through the phloem (McIlveen and
Negusanti 1994 ; Welch 1995 ; Fismes et al. 2005 ; Page et al. 2006 ). However, its
distribution within the seed varies greatly, depending upon the plant species involved,
and on other factors such as presence of pathogens or insects (Boyd et al. 2006 ). For
example, in the seeds of Stackhousia tryonii , a metal hyperaccumulator species, Ni
partitioned to the pericarp (fruit wall), while little entered endospermic and cotyle-
donary tissues. However, the high amounts of Ni that partitioned into the fruit wall
had no effect on seed germination of S. tryonii (Bhatia et al. 2003 ). Thus, exclusion
of Ni from embryonic tissues may ensure high reproductive success of such hyper-
accumulating species when they are grown on metal-enriched soils.

5 Role of Nickel as a Micronutrient

The discovery of nickel's metabolic role in plants occurred early in the twentieth
century. At that time nickel was discovered to be a constituent of plant tissue ash
residue. The evidence for Ni's metabolic role was not discovered until Roach and
Barclay ( 1946 ) disclosed the results of their research into this topic. Later,
Dobrolyubskii and Slavvo ( 1957 ) further strengthened the understanding of nickel's
metabolic role in plants, as has many more recently published papers (Shimada
et al. 1980 ; Welch 1981 ; Eskew et al. 1983, 1984 ; Brown et al. 1987a, b, 1990 ;
Gerendás et al. 1999 ; Gerendás and Sattelmacher 1997a ; Mulrooney and Hausinger
2003 ; Wood et al. 2006 ).
Nickel's role as an essential plant nutrient came from the work of Dixon et al.
( 1975 ), who proved the essentiality of it to the urease activity of Jack-bean (urea ami-
dohydrolase, EC 3.5.1.5). This entity is a metalloenzyme that requires nickel as an
integral part for its proper functioning. Since this discovery, nickel has proven to be an
essential micronutrient for proper functioning of many other metalloenzymes (Klucas
et al. 1983 ; Brown et al. 1987a ; Sakamoto and Bryant 2001 ). It is estimated that there
are ~500 proteins and peptides in living system that have the ability to bind Ni (Maier
et al. 2007 ). Therefore, Ni is now understood to be an integral component of many
biomolecules (including metalloenzymes), where its presence is required to maintain
normal structure and functioning (Won and Lee 2004 ; Seregin and Kozhevnikova
2006 ; Benoit et al. 2007 ). The following are major enzymes that require Ni for cataly-
sis, either in lower or higher plants: urease, superoxide dismutase, NiFe hydrogenases,