Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 147

Most heavy metals inhibit protein activity by altering their structure (Lee et al.
2002a ). Therefore, decreased protein content is a sign of metal toxicity to crop
plants (Seregin and Kozhevnikova 2006 ). Reduced titers of plant proteins may occur
through several mechanisms. First, Ni may produce oxidative stress, by enhancing
ROS production, which, in turn, causes direct protein damage (Baccouch et al.
1998b, 2001 ; Gajewska et al. 2006 ). Second, Ni may deplete the abundance of low-
molecular-weight proteins, and, thereby, induce oxidative stress (Rao and Sresty
2000 ; Kukkola et al. 2000 ). Third, heavy metals may bind to functional groups,
mainly protein SH-groups, and thereby modify protein structure. This last mecha-
nism may reduce the activities of enzymes that contain SH-groups (Seregin and
Kozhevnikova 2006 ).
Proteins, polypeptides and ligands play a vital role in the tolerance of plants to
Ni (Kim et al. 2005 ; Seregin and Kozhevnikova 2006 ). First, such entities bind Ni
to N-, O-, or S-ligands (Van Assche and Clijsters 1990 ; Clemens 2001 ; Vacchina
et al. 2003 ; Montarges-Pelletier et al. 2008 ). Second, Ni binds to polypeptides such
as phytochelatins (Kim et al. 2005 ), which play a crucial role in heavy metal toler-
ance of plants (Steffens 1990 ; Cobbett 2000 ; Vacchina et al. 2003 ). Third, some
proteins are able to complex with Ni, which mitigates its toxic effects to plants.
Examples of such complexing agents are permeases (Wolfram et al. 1995 ; Eitinger
and Mandrand-Berthelot 2000 ), metallothioneins (MT) (Schor-Fumbarov et al.
2005 ), and metallochaperones (Hausinger 1997 ; Olson et al. 1997 ; Watt and
Ludden 1998 ).

9.10 Nutrient Accumulation

High concentrations of Ni in a plant growth medium interfere with the uptake of
many essential macro- and micro-nutrients (Kochian 1991 ; Hasinur et al. 2005 ).
As mentioned above, when plants are stressed by the presence of excessive
amount of Ni, they may take up reduced amounts of N, P, K, and S (Singh 1984 ;
Dahiya et al. 1993 ; Aziz et al. 2007 ; Ali et al. 2009 ). Similarly, Palacios et al.
( 1998 ) reported that Ni stress can signiicantly reduce the absorption and translo-
cation of Na in plant tissues. Furthermore, plant-leaf photosynthesis studies have
shown that Ni competitively removes Ca ions from its binding site in the oxygen
evolving complex (Boisvert et al. 2007 ) and replaces the Mg ion in chlorophyll
pigment (Küpper et al. 1996 ). When this occurs, PSII electron transport is eventu-
ally inhibited, which reduces the available energy supply for nutrient uptake.
Reduced nutrient uptake may result, thereby leading to nutrient deiciency
in plants tissues (Khalid and Tinsley 1980 ; Moya 1995 ; Ahmad et al. 2007 ;
Liu 2008 ).
When Ni competitively mitigates the absorption of certain other micronutrients
(e.g., Mn, Fe, etc.), selective micronutrient deicits may occur in plant tissues.
Therefore, when excessive Ni levels exist, plants are likely to show symptoms of
nutrient deiciency (Gabbrielli et al. 1990 ; Rubio et al. 1994 ; Rahman et al. 2005 ;