There plant, as suggested by Lichtenthaler et

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.