Nutrient deficiencies under stress in legumes: An overview - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 58

intermediate complexes that help in transporting mate-
rials across membranes in legumes (Hansch & Mendel,
2009).

3.5.7 Manganese
Manganese has an important role in the detoxification
of active oxygen species. It also is important in the
formation of polyamines necessary for plant growth and
development (Evans & Malmberg, 1989). This element
is best known for being a cofactor of superoxide dis-
mutase, whose function is to scavenge superoxide
radicals. This scavenging action is necessary to guar-
antee the stability of the symbiotic relation between the
plant and bacteria during stress. Maganese is also
responsible for activation of enzymes of the shikimic
acid cycle and of the Krebs cycle (Hansch & Mendel,
2009). Signalling events between the bacteria and host
plant also involve manganese through its role in main-
taining levels of carboxylic acids and flavonoids by
controlling Krebs cycle enzymes. Manganese is also a
cofactor in the water-splitting reactions of photosystem
II of plant cells.

3.5.5 Zinc
Plants require zinc for various physiological processes
and its deficiency can lead to severe problems (Weisany
et al., 2011). It plays an important role in transcriptional
regulation of the Ros-type regulator MucR, which is
necessary for establishing and maintaining cell envelope
integrity, polysaccharide biosynthesis, iron homeostasis,
genome plasticity and transcriptional regulation in
many legumes (Caswell et al., 2013). Zinc also acts as a
cofactor for different enzymes, including metalloprote-
ases, aminopeptidases, nucleases and CuZn-SOD
(Chauhan & O'Brian, 1995; Hansch & Mendel, 2009).
Zinc is believed to bring changes in both host and rhizo-
bia at genetic levels and has no direct role in nodule
development. In protein-protein interactions it is a
constituent of zinc finger motifs.

3.5.8 Nickel
Nickel is required for various metabolic processes like
enzyme activities, maintaining a proper redox state of
cells and various other biochemical, physiological and
growth responses, intensive industrial and agricultural
activities have often raised its levels in soils, resulting in
toxicity (Patrick et al., 1987; Yusuf et al., 2011; Hussain
et al., 2013) and hence provoking oxidative stress
(Rao & Sresty, 2000; Gonnelli et al., 2001; Boominathan
& Doran, 2002; Gajewska et al., 2006). Increased nickel
concentrations cause chlorosis, inhibition of root and
shoot growth, photosynthesis, protein synthesis and
enzyme activity, and alteration of membrane integrity
in plants (Hussain et al., 2013). Plants grown in soil con-
taining an optimum amount of nickel show increased
weight of the root nodules and enhanced yield. At
optimum concentrations, nickel regulates hydrogenase
activity, and maximal efficiency of this enzyme results
in enhanced nitrogen-fixing activity of rhizobia. Nickel
has been found to have a link with plant urease and
Ni-metallochaperones. Plants use ureides as nitrogen-
transporting molecules, which play a significant role in
nodule formation (Dalton et al., 1985). In some rhizobia,
the enzyme NiFe-dehydrogenase oxidizes hydrogen gas
to obtain energy. Nickel constitutes an important com-
ponent of NiFe-nitrogenase, thereby having an active
role in nitrogen fixation. Hence for the proper func-
tioning of plant systems an optimum concentration of
nickel is important.

3.5.6 Iron
Iron is required for nitrogenase and hydrogenase
enzyme complexes, and is one of the key components of
leghaemoglobin, the most abundant protein in the
nodule. Leghaemoglobin maintains the anaerobic envi-
ronment inside the root nodule so that the activity
of nitrogenase is maintained. Iron deficiency hinders
nitrogenase activity and hence nitrogen fixation. Besides
this, iron is also an important component of the electron
carrier ferredoxin, and hence important in energy
transfer pathways. It has been shown that nitrogenase
and leghaemoglobin constitute about 30% of the total
protein content of root nodules, and both these enzymes
contain iron as an essential component (O'Hara et al.,
1988; Bonilla et al., 2010). As mentioned above, iron is
a cofactor of many enzymes of both bacteria and plants
that play a vital role in nitrogen fixation (Tang et al.,
1990). Nitrogenase, nitrate reductases and the enzymes
involved in biosynthesis of nucleic acids and detoxifica-
tion of ROS also require iron (O'Hara et al., 1988).
Some iron (Fe 2+ ) must be present in the vicinity of
root nodules so that the enzyme iron-chelate reductase
can perform chelation of iron from soil to either host
plant cell or to bacteria. This process is a reduction reac-
tion in which Fe 2+ is converted to Fe 3+ with the help of
NAD + or NADP + in order to maintain the optimum pH
necessary for chelation (Slatni et al., 2012).