Genetic and molecular responses of legumes in a changing environment - Legumes Under Environmental Stress: Yield, Improvement and Adaptations - page 209

stress are glutamine, betaine, sterols, glucosides and
polyamines (Signorelli et al., 2013).
Alteration of photosynthesis rate is one of the most
common adaptations a legume makes to survive a cold-
stressed environment. However, the most significant
aspect is the change in carbohydrate metabolism. There
is often an increase in the soluble sugar content in the
plant body, which demonstrates their role in the signalling
process, temperature maintenance and osmoregulation.
Moreover, sugars help in osmotic regulation, by increasing
the solvent retention, resulting in decreased availability of
free water for freezing. Sugar molecules also interact
with the lipoproteins of the plasma membrane and sta-
bilize it. Carbohydrates are also implicated in antioxidant
phenomena and signalling processes. Among the var-
ious sugars, trehalose, stachyose and raffinose are
considered important in inducing cold tolerance. Hence,
a number of genetic and molecular mechanisms are
available for inducing tolerance in susceptible plants
against low temperatures.

instance, certain bacterial and fungal species tend to
infect plants with much greater ease. Pseudomonas syrin-
gae contributes to the ice nucleation process (Attard
et al., 2012), which results in failure of legumes to sense
the freezing temperatures and initiate mechanisms to
survive the drop in temperatures. In contrast, certain
species of nitrogen-fixing bacteria, plant growth pro-
moting rhizobacteria (PGPR), are involved in the
reduction of chilling-induced damage to the plant body.
These bacterial species manipulate a variety of stress-
induced genes (chitinase, glucanase) and cause the
production of a number of metabolites (lipids, aldehydes,
sugars, ROS) to exert their effects.
It is therefore evident that there is no single, uniform
survival mechanism that can be adopted by legumes to
protect themselves from various environmental and
physical constraints. All the pathways adopted by these
plants tend to connect to one another at a certain stage.
The efficiency of any process depends upon a number of
factors. The intensity and time of exposure of a particular
stress stimulus is the first controlling factor in regulating
the response to that threat. The sensitivity with which a
plant can detect a stress factor also affects the degree of
the genetic and molecular changes occurring in the
plant body. Additionally, the correct sequence of signal
cascade components must be activated in response to a
threat. All the necessary proteins, enzymes and other
chemical moieties have to play their role in the cascade.
Moreover, the regulatory and effector molecules pro-
duced as a result of the process need to act on specific
tissue types in the plant body so as to bring about
required morphological and physiological changes.
These phenotypic changes are the final manifestations
by which a plant responds to any environmental threat.
Legumes tend to respond to various physical and envi-
ronmental constraints by the concerted effort of many
genes, proteins and other biomolecules, ionic species,
solvent systems, and cell and tissue types. Development
of resistance to various stress factors thus relies on the
efficient working of all these processes and molecules
within the plant body.

12.7 plant defence mechanisms and
their efficiency

Legumes respond to various stress conditions by
invoking a variety of effector mechanisms. The specific
ways by which plants render themselves resistant to the
major physical constraints have been discussed. Apart
from salinity, drought and temperature extremes, a
variety of other stress factors are also responsible for
causing major damage to the leguminous systems.
Natural disasters, for instance, result in significant
abrupt changes in the ecological parameters of the plant.
The plant usually does not get enough time to develop
resistance or adapt to tornadoes, floods, wild fires or
high-speed winds. In contrast, exposure to high-inten-
sity light is an ecological factor that changes very slowly
and gradually. Legumes tend to adapt to this factor over
a prolonged period of time, i.e. over several generations.
Hence, plants can only develop tolerance to the stress
factors that tend to show a constant presence of a period
of several weeks to months.
Environmental stress factors do not, necessarily, have
a direct effect. They may be involved in changing the
overall morphology and physiology of the legume, pro-
moting the ability of biotic stressors, such as pathogens,
to infect easily and effectively. In case of a cold stress, for

12.8 Conclusion and future prospects

Many of the changes exhibited by legumes in response
to environmental constraints involve the interplay of a
number of genetic and molecular entities. Many levels