Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 145

metal stress. Homer et al. ( 1997 ) determined the accumulation pattern of amino
acids in three species of Ni-hyperaccumulator plants ( Dechampetalum geloniodes,
Phyllanthus palwanensis, and Walsura monophylla), and reported that, with one
exception, proline was the most abundant amino acid among all those studied; the
exception was for W. monophylla , which accumulated glutamine in higher quantity
than it did proline. Bhatia et al. ( 2005 ) studied the changes in the amino-acid proile
that occurred in the xylem sap of Stackhousia tryonii (Ni-hyperaccumulator). They
reported a slight decrease in the concentration of total amino acids under Ni stress,
which contrasted with earlier observations. The proportion of glycine declined con-
siderably from 48 to 22%. However, that of alanine, asparagine, and glutamine
increased under Ni stress. The authors suggested that asparagine and alanine may
contribute to Ni complexation in the xylem of hyperaccumulator species (Bhatia
et al. 2005 ). Freeman et al. ( 2004 ) reported that the accumulation of o -acetyl-L-
serine, cysteine and glutathione are strongly correlated with Ni accumulation and
tolerance ability in various Thlaspi hyperaccumulator plants. They concluded that
elevated levels of these amino acids play a causal role in Ni tolerance by enhancing
GSH-dependent antioxidant activity. Moreover, Ali et al. ( 2009 ) suggested that Ni,
although absorbed by roots as the free cation under control conditions, changes
under Ni stress conditions. Under Ni stress some canola cultivars synthesized high
amounts of amino acids (i.e., histidine and cysteine), which may have enhanced Ni
translocation from root to shoot and accentuated detoxiication (Hall 2002 ).
The foregoing results indicate that complexation between amino acids and Ni is
more stable than complexation with carboxylic acids (Krämer et al. 1996 ; Homer
et al. 1997 ; Kerkeb and Krämer 2003 ). Moreover, the accumulation of amino acids
under metal stress conditions is one of the most important detoxiication mecha-
nisms, particularly in hyperaccumulator plants (Clemens 2001 ; Hall 2002 ).

9.7 Proline

The accumulation of free proline under Ni stress has been reported in several stud-
ies. The protective functions of proline have been attributed to its roles as an osmo-
protectant (Paleg et al. 1984 ), osmo- and redox-regulator (Sharma and Dietz 2006 ),
membrane stabilizer (Bandurska 2001 ; Matysik et al. 2002 ), metal chelator (Cobbett
2000 ), ROS scavenger (Alia et al. 2001 ), and enzyme protector (Sharma and Dubey
2007 ). For example, an eightfold increase in the free proline content of Ni-treated
Chlorella vulgaris was observed by Mehta and Gaur ( 1999 ). Gajewska and
Skłodowska ( 2005 ; 2008 ) reported that Ni stressed pea and wheat plants accumu-
lated high quantities of proline that signiicantly reduced oxidative damage to mem-
branes and proteins. Similarly, Ghorbanli et al. ( 2006 ) reported that Ni treated
Brassica plants accumulated more proline than did normal plants. From these
reports it is inferred that proline accumulation augments plant tolerance to Ni stress,
and further, could be used as an important metabolic indicator of Ni tolerance.
However, because there are so few data, it is dificult to afirm that the enhanced
accumulation of proline in plants is an adaptive response to Ni-induced stress.