Essential Roles and Hazardous Effects of Nickel in Plants - Reviews of Environmental Contamination and Toxicology - page 138

nucleic acids, when present in soils at excessive levels (Maheshwari and Dubey
2007 ). When high levels of Ni is involved, these effects derive from the direct inlu-
ence on key metabolizing enzymes, which induce alterations in levels of essential
biomolecules such as sugars, amino acids, proteins, and nucleotides in germinating
seeds and growing plants (Veer 1989 ). Hence, the concentration of sugars (total,
reducing and nonreducing ones) decreases under Ni stress. The effects are direct ones
by Ni on the activity of the major hydrolytic enzymes involved in sugar
metabolism.

8.6 Proteolytic and Ribonucleolytic Enzymes

Although several enzymes are involved in the reserves mobilization that takes place
during seed germination, proteolytic and ribonucleolytic enzymes are also activated
during the process, and their activity is important for seedling establishment (Bewley
and Black 1994 ). These enzymes regulate both RNA turnover and mobilization
from seed storage tissues (Gomes-Filho et al. 1999 ). They also exert a major inlu-
ence on gene expression under certain abiotic and biotic stressful conditions (Booker
2004 ). In addition to the activity of RNases, the regulation of protein breakdown
and recycling is essential for control of seed germination (Palma et al. 2002 ). Such
mobilization of protein reserves is controlled by the enzyme protease , which
degrades storage proteins and mobilizes amino acids to support the growing embry-
onic apices of germinating seeds (Yamauchi 2003 ).
Plants mobilize and utilize biomolecules (e.g., proteins, starch, and nucleic
acids), and activate metabolic enzymes in response to stresses (Syros et al. 2005 ).
The activated RNase and protease in germinating seeds affect the protein pool and,
therefore, seed germination. The activities of RNase and protease are altered when
exposed to high Ni levels (Kirchgessner and Schnegg 1979 ; Booker 2004 ). Pena
et al. ( 2006 ) exposed sunlower plants to various levels of Pb, Al, Ni, Cd, Hg, Co,
Cr, and Cu. All of these metals increased protein oxidation levels, although Ni was
the most effective in doing so (112%), followed by Cd and Hg (74%) and Co
(68%), Cr (64%), Pb (62%), and Al (57%). Copper treatment had the lowest effect
on protein oxidation (40% more than the controls). Different metals affect protease
activity differently, with Ni being the most toxic in reducing protease activity on
sunlower plants. Maheshwari and Dubey ( 2007, 2008 ) have recently shown that
rice seed germination was signiicantly reduced when exposed to 200 and 400 mM
NiSO 4. In addition, Ni stress in germinating seeds produced signiicantly increased
levels of RNA, soluble proteins, and free amino acids in endosperm and embryonic
axes. Maheshwari and Dubey ( 2007, 2008 ) suggested that the presence of high Ni
levels in the medium of the germinating rice seeds induced stress, which decreased
hydrolysis and delayed mobilization of endospermic RNA and protein reserves,
which, in turn, caused an imbalance in amounts of certain biomolecules (e.g.,
RNA, proteins, and amino acids) in the growing embryo axes, thereby reducing
seed germination.