Nutrient deficiencies under stress in legumes: An overview - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 57

3.5 Micronutrient deficiency in plants

the bacterial enzyme complexes also requires cobalt.
Deficiency badly affects nodule formation and hence
nitrogen fixation (Dilworth et al., 1979). Cobalt is also
required for the synthesis of vitamin B 12 , which is needed
for fatty acid metabolism, for the production of haem
group cytochromes and for the leghaemoglobin that acts
with the help of methylmalonyl coenzyme A mutase.
Rhizobia use cobalt for functioning of two enzymes,
homocysteine methyltransferase, required for the
formation of methionine, and ribonucleotide reductase,
which is involved in the catalysis of deoxyribonucleotide
formation (Licht et al., 1996). Experiments show that
elevated concentrations of cobalt (usually >30 μg/mL)
play a very important role in protecting legumes under
drought stress by inhibiting the production of ethylene,
which is a stress hormone. Cobalt has the ability to
reduce polyamine content as well as antioxidamt
enzyme activity. This, in turn, increases the ROS level
and also increases cell membrane damage by inhibiting
ethylene. Higher concentrations of cobalt (>50 mg/kg of
soil) caused decreases in antioxidant enzyme activity,
reducing sugars, starch, free amino acids and protein
content in Vigna radiata (Jayakumar et al., 2008).
However, Gad (2012) reported that a lower concentration
(8 ppm) caused a significant increase in height, number
and area of leaves, nitrogenase activity, number of nod-
ules, and nitrogen, phosphorus and potassium content
in Arachis hypogaea .

3.5.1 Boron
Boron is one of the essential micronutrients required for
proper working of the symbiotic association in legumes
(Yamagishi & Yamamoto 1994). Boron plays an impor-
tant role in maintaining the structural integrity of cell
walls and cell membranes (Marschner, 1995; Bolanos
et al., 1996). Its deficiency also affects the activity of
indole acetic acid oxidase, sugar translocation and
nucleic acids resulting in stunted growth.
Boron plays a role in the synthesis of the polysaccha-
ride capsule required for the recognition of bacteria by
the legume. When boron is absent, a mutant polysac-
charide capsule is formed, which is not properly
recognized by the root and hence the plant defense
mechanism becomes activated (Abreu et al., 2012).
Inside the infection thread, boron controls the inter-
action between a matrix glycoprotein called
arabinogalactan protein-like extension (AGPE) and
RG-2 (rhamnogalacturonan 2). This binding of AGPE
and RG-2 keeps the bacteria entrapped within the
thread and also promotes the apical growth of the infec-
tion thread (Reguera et al., 2010). During nodule
organogenesis, boron plays an important role in signal-
ling (Reguera et al., 2009). No enzyme discovered so far
has been found to use boron as a cofactor, but still the
deficiency of boron has a negative effect on the formation
and growth of the nodule.

3.5.4 Copper
Wilting and the appearance of grey patches in leaves are
the major signs of copper deficiency in plants. Copper is
the major component of the respiratory proteins
required for nitrogen fixation by rhizobia (Delgado et al.,
1998). Copper is also required for a particular protein
that works in co-operation with nitrogen-fixing genes
of rhizobia (Snowball et al., 1980). Because of its role as
a cofactor for cytochrome c oxidase, it is important in
the energetic processes occurring in the cells (Delgado
et al., 1998). Copper is also important for the metabo-
lism of carbon and nitrogen, formation of cell walls and
the detoxification of ROS. At times of stress, the level of
ROS is quite high. The copper chaperone, a metallopro-
tein, delivers copper ions to the Cu-superoxide
dismutase (CuSOD) and hence mediates detoxification
of ROS. Copper also activates the enzyme polyphenol
oxidase (PPO), hence inhibiting membrane lipid peroxi-
dation. Copper along with molybdenum and a petrin
molecule plays an important role in the synthesis of

3.5.2 Molybdenum
In legumes, molybdenum is required for the synthesis
of proteins associated with nitrogen metabolism
(Wiedenhoeft, 2006). It acts as a cofactor for the nitro-
genase enzymes (Hansch & Mendel, 2009). Usually soils
having very low pH are deficient in molybdenum and
therefore have reduced nitrogen-fixing activity. Mo
occupies the central position in the cofactor FeMoCo,
which is involved in a number of oxidation-reduction
processes during nitrogen fixation. Any fluctuation in
Mo concentration can affect the activity of the enzyme
complex and hence nitrogen fixation (Shah et al., 1999),
and upon addition of Mo, nitrogen fixation is signifi-
cantly enhanced (Delgado et al., 2006).

3.5.3 Cobalt
Cobalt is the cofactor of vitamin B 6 (cobalamine), which
acts as a coenzyme in nitrogen fixation and nodule
growth (Jordan & Richard, 1998). Proper functioning of