There there is an increase in the rate of

There exist some abiotic stresses which include
salinity, drought and temperature that affect the growth, survival and
reproduction of plants. As a results, plants respond to such unfavorable
conditions by physiological, developmental and biochemical ways. In order to
respond appropriately to these changes, it demands expression of stress-
response genes. These stress response genes are regulated by a network of
transcription factors (TFs). These include the heat stress transcription
factor, abbreviated as HSFs). This factor plays an important role in response
of plants to various abiotic stresses. They accomplish this by regulating the
expression of stress responsive genes like the heat shock proteins (Hsps).

The stresses that plants are subjected have either
direct or indirect effect on their productivity. Some of the abiotic stresses
like high temperature, salinity and drought results in a deadly economic loss
in agricultural sector. According to Rodziewicz et al., (2014), they
established that there is an estimate of 50% losses in crop yield worldwide as
a result of these stresses. In addition, Edmeades (2009) established that
abiotic stresses that result from either cold, salinity; high temperature or drought
hinders crops from realizing their full yield potential.

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It is a complex phenomenon for plants to respond to
these abiotic stresses because of plants at different development stages can be
affected by a particular stress and therefore, simultaneous occurrence of
stresses can affect the plants, as established by Chinnusammy et al, (2004).
Heat stress affect the photosynthetic rate of plants directly and indirectly
hence changing the structural organization and psycho-chemical properties of
thylakoid membrane of the plant, as suggested by Lichtenthaler et al., (2005). According
to Sage et la. (2007), there is an increase in the rate of photorespiration
with the increase in heat (temperature) which then results in reduction of
photosynthesis.

Flowing stage in a plant is the most sensitive stage
that is affected by high temperature as high temperatures damage it. This is
because of the high level of vulnerability in the event of development of pollen;
fertilization and anthesis in a plant which will alter reduce the final yield
of the plant (Kumar, 2015). He found out that for every unit increase in
temperature result in a reduction in the plant yield by about 3 to 7% (Kumar,
2015).

With the increasing incidences of abiotic stresses, it
has necessitated the creation of a new genotype or screen that can be used for
the existing germplasm that is favorable for the changing conditions.

Stress

Stress can be defined as a change in the environment
that is sudden and the change can exceed the optimal conditions of the organism
and the change result in a change in homeostatic imbalance (Taiz et al., 1991).
Plants that grow under the field conditions experience various environmental
conditions that are exposed to them and these conditions affect their macro and
microenvironment as suggested by Larcher, (2003). Stress can be caused by
different factors including abiotic factors which include variation in temperature,
strong light, and salinity among others. It can also be caused by biotic
factors like insects, bacteria, virus among others. Plants can at times face a
condition of combination of both biotic and abiotic factors. Yadav et al (2014)
found out that abiotic stress is the main agents that cause failure in most of
plants. They found out that they lower the average production of plants by
about 60% which then threaten sustainability and food security.

Abiotic Stress

These are non-living factors that affect negatively
plants on a specific environment. These factors influence the environmental
conditions beyond their normal range required by plants hence affective
adversely the performance of the population of plants. They include extreme
temperatures which can be too much heat or freezing, drought, reduction in the
nutrients in the soil, too much light and excessive toxicity in the soil.

The response of plants to stress depends on the area
it was affected by the stress. Several signaling pathways come up due to the
molecular response of the plant to any stress. These include RNS or ROS and the
hormonal changes which include ethylene and ABA as established by Cramer et
al., (2010). He also commented on the use of time series analysis in order to
study multiple phases that relate to stress responses that were important in
analysis that aims at distinguishing between the primary and the secondary
stress responses by plants. Some of these mechanisms that can be used to
respond to stress include signal transduction, stress perception and
transcriptional activation of stress response genes. Others include synthesis
of proteins that relate to the stress and other molecules that can help plants
to cope with adverse environmental conditions.

Heat Stress

According to Rao et al (1992), they found out that
high temperature enhances development in plants and result in abortion of
flower leading to a significant loss in the yield of seeds. They suggested that
the duration of flowering in plants have a strong impact on the yield of seeds
and a rise in temperature result in causes a decline in the yield. These
findings were obtained from a study of Indian Mustard seed.  They also agreed that flowering is the most
sensitive stage of a plant and any change in environmental conditions result in
stress damage.

In addition, Cramer et al. (2010) established that the
main cause of the rise in sterility in plants when subjected to heat stress are
as a result of the impaired meiosis in both the female and male organs of
plants. It is also due to the impairment of the pollen germination and the
growth of pollen tube, anomaly in position of style and stigma and disturbance
in the process of fertilization among others.

According to the research done by Mishra et al. (2011),
they reported that in case of a progressive shortfall in precipitation in conjunction
with high level of evapotranspiration rate that is caused by high heat result
in agricultural drought. Drought adversely affect the growth of plants and
their development which in turn lower the rate of growth in plants and hence
biomass accumulation. Mishra et al. (2011) further argued that heat stress is
one of the main factors that limit the production of crops in the world.

Araus et al., (2002) concluded that heat stress is one
of the main abiotic factors that seriously affect crop productivity worldwide. He
found out that long term exposure of plants to high temperatures affect their
biochemical, metabolic and molecular functioning of plants. This have a serious
effect on various parts of the plant like the leaves, flowers, roots and the
buds hence affect productivity.

Another research conducted by Al-khatib et al. (2004)
found out that in case of an increase in temperature, it results in premature
senescence of plants which can lead to an increased rate of photorespiration. They
also observed that there is an increased in the rate of photorespiration with
an increase in the temperature. This in turn lowers the rate of photosynthesis which
in turn affects the yield.

Heat Stress Transcription Factor
(HSFs)

The research conducted by Nover et al. (2001) came up
with a conclusion that HSFs act as a terminal component of a signal
transduction chain that can be used to mediate the expression of genes that are
responsive to different abiotic stresses. This statement was supported by
Scharf et al. (2012) who reported that HSFs play a critical role in
manipulation of various abiotic stresses, heat stress inclusive. In addition,
heat stress transcription factors is an important tool in conversion of stress
signal perception to stress responsive gene that can be expressed by interacting
with cis-acting elements present in the promoter region of stress responsive
genes in the process of signal transduction. This results in activation of
signaling cascade (Akhtar et al., 2012).

Further findings were found by Nakashima et al. (2012)
who concluded that there is a possibility of HSFs to enhance tolerance of
plants to overcome environmental conditions that are harsh. Guo et al. (2016)
came up with a modification of a schematic representation of the HFSs which is
a key component in transcriptional regularity networks during heat stress as
shown in the diagram below.

 

They were able to observe that plant respond to
changes in the environment that are not favorable to their development,
biochemical and physiological ways.  These
responses by the plant need an expression of stress-responsive genes. These
genes are regulated by a network of TFs (transcription factors) with inclusion
of HSFs (Guo et al., 2016). In addition, they further found out that in the
genomes of a plant, about 7% of the overall coding sequences are assigned to
transcription factors against the stress condition (Guo et al., 2016).

Stress tolerance genes in plants

There if no expression of Arabidopsis HSFA2 in control
of cell cultures but this was strongly detected when it was treated with HS
(Nover et al., 2001). It should also be noted that with the introduction of
molecular strategies like whole genome transcriptome analysis and microarray
analysis, it makes it possible to identify a great number of genes that are
responsive to abiotic stress (Nakashima et al., 2009). According to the
research conducted by Mishra et al. (2011), they were able to observe that SlHSFA2
that is found in a tomato was affected by high temperature by up-regulating
them.

For the case of rice, 23 HSF encoding genes found are
found in rice, as reported by Park et al. (2013). In wheat, a study was
conducted by Xue et al. (2015) and they found out that there exist 56 HSF
encoding genes in them. GhHSF3, 24, 37 and 40 genes are found in cotton and
they are used in regulation under heat stress (Wang et al., 2014).

According to the findings by Guo et al. (2015), they
were able to identify 23 rice OsHSF genes, where among them, 16 OsHSFs were
up-regulated by two-folds in response in heat stress. Among them are eight
genes that are up-regulated, by only two folds at the period of occurrence of early
heat shock (HS for 10 min). In addition, 8 genes were up-regulated at both the
short HS at around 10 min and at prolonged that take HS for 30 minutes in the
occurrence of heat stress (HS) treatment.

The main reason for the low tolerance of rice crop to
salinity is because of the high permeability of its roots to sodium ions. Ions
of sodium can easily penetrate the apoplast and its subsequent cells to rapidly
resulting in concentration of intracellular toxic. Due to the vast land area,
it makes it prone to high salinity effect (Hadiarto and Tran, 2011). It
therefore makes it of economic significance to understand the salt tolerance of
crops. Response to salt by rice crops involves expressional changes of genes
that relate to stress. These include protein kinases, transcriptional factors
like OsHsfC1a and iron transporters. In the case of rice, it has several
transcriptional changes as a result of stresses. These include NAC, bZIP, MYB
and AP2. These transcriptional factors contribute to crop adaptation to stress
by regulating the expression of stress-responsive genes (Hu et al., 2006).

HSFs are classified into three classes namely A, B and
C. these factors are made of N-terminal DNA-binding domain. In rice, 13 HSFs
are assigned to class A which include subclasses A1, A2 and A4. 8 of the HSFs
are assigned to class B and the remaining 4 HSFs are then assigned to class C
(Guo 2008). Heat shock factors control expression of genes. They accomplish
this by binding to the heat shock element which is an inverted 5-bp repeat of a
sequence. These factors also operate as regulators of any other HFS gene that
is demonstrated by the HsfA1d (Nishizawa-Yokkoi et al., 2011).

Rice crops overexpress OsHsfA2e making them more
tolerant to stresses caused but heat and salt (Yakotani et al., 2008). Overexpression
of OSHsf7 in rice also leads to an increase in heat tolerance of plants as
commented by Liu et al. (2009). However, the role of class C HSFs in response
to stress has not been discovered yet. On the other hand, expression patterns
of class C HSF gene from rice is given as an addition to the role it play in
heat shock response. They also take part in response of other none thermal
stresses like salt, oxidative and drought stresses. Hu et al. (2009) found out
that OsHsfC1b and OsHsf2b as the most effective responsive factors to drought
and salt stress.