Biotechnology approaches to overcome biotic and abiotic stress constraints in legumes - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 252

Glutathione reductase, superoxide dismutase, catalase,
carotenoids, ascorbate, cyanins, tocopherol, glycine and
sugars are among the many ROS responsive elements
(Becana et al., 2010; Gill et al., 2013; Liu et al., 2013). All
of these chemicals are considered to play important
roles in counteracting the stress. Additionally, in
response to salt stress and a chilling environment,
certain legumes produce amines like glycine betaine
that help in protection of plant. Sugars are found in
increased quantities in stressed plants. Mannitol, for
example, has been associated with the ability to scav-
enge reactive oxygen species (ROS) that are deemed
harmful for legume health. Similarly, proline, an amino
acid, has been found to protect the plant from salt stress
in saline conditions (Kohl et al., 1990). Although a
similar mechanism may be deployed by legumes in case
of biotic attack, the secondary metabolites are usually
different compared to those produced in response to
abiotic stress factors. Chief among these are the alka-
loids, cyanins, terpenes, lipids and related compounds.
These chemicals are potent antibacterial, antiviral, anti-
fungal, antihelminthic and antiprotozoal entities.
Apart from the physiological response of leguminous
plants to the stressed conditions, legume-rhizobia
interactions are also adversely affected by these biotic
and abiotic constraints. Some 70 bacterial species,
classified into the genera Sinorhizobium , Rhizobium ,
Mesorhizobium , Bradyrhizobium , Azorhizobium and
Allorhizobium , are involved in this legume-bacteria
symbiotic relationship (Oldroyd et al., 2011). Any
physical phenomenon that adversely affects the bacte-
rial growth causes disruption of this relationship, thus
diminishing the nitrogen fixation process and the
benefits associated with it. The most threatening of the
various physical constraints for this symbiotic relation-
ship are drought and high temperatures. In case of
drought, for instance, shoot photosynthesis is compro-
mised leading to a lack of carbohydrate supply to the
rhizobia. This is followed by decreased biological
nitrogen fixation capacity and, hence, a decrease in the
overall ammonia output. Rhizobia have developed very
efficient mechanisms to protect themselves and the
plant nodules from drought. Firstly, they are them-
selves more tolerant to physical parameters compared
to the legumes. Moreover, through a number of mecha-
nisms they provide protection to the nodules of the
plants. Trehalose-phosphate synthase is one such system
involved in providing osmoprotection to legumes in

Biotic stress factors

Abiotic stress factors

Cold
Heat
Frost
Drought
Salinity

Viral diseases

Bacterial diseases
Fungal diseases
Insect and herbivore
attack
Weed attack

Stress matrix

Oxidative stress

Signal sensing

Disease
resistance

Protein
expression

Detoxi cation Osmoprotection

Stress tolerance or resistance

Figure 15.2 Legume response to biotic and abiotic stress factors.

high temperatures, legumes respond physiologically by
decreasing the rate of photosynthesis and increasing
the rate of respiration, stomatal conductance and leaf
temperature. Hence, multiple responses are observed in
response to a single physical constraint. Another type
of response is the regulation of cellular transporters
and pumps. These are mainly involved in ion exchange
and regulation across membranes. An example of this
osmoregulatory mechanism, in case of drought for in-
stance, is the Na + /H + antiporter system, which involves
regulating the cytoplasmic pH, sodium ion levels in the
cell, cell turgor and, consequently, the homeostasis of
the cellular environment. Secondary to the process is
the change in osmotic gradient, signalling cascade and
production of certain metabolites.
The primary response, generally observed in reaction
to any stress condition, is the regulation of plant metab-
olites, both primary and secondary. Additionally, certain
metabolites are produced by the plant to detoxify or
neutralize the effects of the damage caused by the stress
factors. These metabolites are naturally produced but
stress conditions lead to their overproduction. Principal
among these is oxidative stress. A variety of neutralizing
species including enzymes, non-enzyme systems and
secondary metabolites, are produced as a result.