Agriculture Reference
In-Depth Information
mediate their effects. They may, directly, produce certain
effector elements or induce the production of entities
that are involved in the regulation of cellular, metabolic
and biochemical responses. The genes involved in
providing tolerance against drought stress include the
responsive to dehydration ( rd ) and early responsive to
dehydration ( erd ) series (Nakashima et al., 2012; Rai et al.,
2012). These are stress-induced genes and can function
in response to a variety of stress factors including drought,
salinity and very low temperatures. The main mecha-
nism involved in responding to drought is the presence of
ABA. Whether endogenous or exogenous, ABA has been
proved to induce and regulate the expression of a number
of drought response elements and to help produce toler-
ance against this stress factor. However, certain pathways
(involving rd29A ) have been identified that do not, nec-
essarily, depend upon ABA for the production of stress
response elements (de Paiva Rolla et al., 2014). In many
plant species, including legumes, the endogenous levels
of ABA are significantly increased in response to drought,
and ABA is responsible for inducing many drought-
induced response elements. These genetic elements
possess specific ABA-responsive elements (ABRE) in
their promoter regions that are activated on synthesis of
ABA in drought conditions (Fujita et al., 2013). A number
of genes ( rd22 , rd29 ) and transcription factors (MYC,
MYB) are involved in inducing ABA responses in legumes
under drought (Ambawat et al., 2013; Bakht et al., 2013).
Dehydration Responsive Elements (DRE) are the ge-
netic sequences involved in inducing stress responses to
dehydration. DREs contains a highly conserved 9 bp
sequence that is considered essential for the binding of
cis -acting elements. A separate class of proteins known
as DRE-binding proteins (DREB) has also been identi-
fied. They contain a conserved DNA binding motif (AP2
domain) that has sequence homology in the binding
region but varies significantly in the region outside the
protein-binding groove (Jain & Chattopadhyay, 2013).
This genetic difference has formed the basis for
classification of DREB into two distinct classes, DREB 1
and 2; it is DREB 2 that are mainly involved in respond-
ing to dehydration conditions. Transgenic legumes
encoding DREB 2 have shown excellent capability to
survive in dehydrated conditions (Acharjee & Sarmah,
2013; Jovanovic et al., 2013). Hence, plant responses to
drought stress rely greatly upon the genetic and molec-
ular players involved in the signal transduction
pathways (Irving & Gehring, 2013; Osakabe et al., 2013)
12.6 Genetic and molecular responses
to extremes in temperatures
Sudden elevations and depressions in temperature sig-
nificantly affect legume output. Most, but not all,
legumes prefer moderate to warm temperature condi-
tions for their growth. However, the leguminous plants
that grow in colder environments are sensitive to rises
in temperature. Otherwise, a rise in temperature does
not significantly affect plant health in temperate regions.
In response to an increase in temperature, the plants
may respond by regulating their cell cycle, metabolic
rate and growth patterns. However, if the exposure is
prolonged, the size and thickness of aerial parts, sto-
matal density and chloroplast activity are adversely
affected (Theocharis et  al., 2012). Usually, though, if
cool temperate legumes are grown in a warmer climate,
they tend to survive and efficiently carry out their life
processes.
In contrast, a fall in temperature is considered a major
stress factor for many legumes because of the changes
accompanying the temperature drop. Decreased tempera-
ture may be accompanied by frost, with its attendant
dehydration and reduced nutrient supply to the plant. The
plant, hence, has to develop means to survive in these
harsh conditions. On perceiving a change in the growth
environment, the plants tend to generate signals that, ulti-
mately, help in the production of regulatory and effector
elements. In many cases, the mechanisms and elements of
resistance to drought and cold are the same (Table 12.3).
The physiological and morphological changes
observed in response to a cold stress vary widely among
different legume species. Figure  12.3  represents the
general mechanisms that are involved in responding to
a cold stress, and certain changes are observed in all
plants exposed to cold temperature. Generally, alter-
ation in cell membranes, increased ion concentration in
the cell, generation of reactive oxygen species and free
radicals, activation of cold-response genes and produc-
tion of secondary metabolites are noted. Modification of
cell membranes involves increasing their rigidity by
increasing the content of unsaturated fatty acids. This
helps in stabilizing the membrane and providing
increased resistance of the cell to extremely low temper-
atures. Decreases in temperature result in a reduced
ability of the plant body to carry out its metabolic
processes. This is coupled to a diminished rate of photo-
synthesis (Theocharis et  al., 2012). Moreover, plant
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