4. Discussion
Small HSPs are known to function as molecular chaperone, protecting
plants against abiotic stress in plant growth and evelopment. Earlier,
sHSPs were thought to express almost exclusively in vegetative tissues
only under heat stress, but recent studies have demonstrated the role of
these proteins in diverse stresses like cold, drought, salinity and
oxidative stress 29, 30, 33,48-52.The tea plant is an
economic woody crop that is cultivated worldwide. However, the shade
tolerance and substantial water requirements of tea plants cause them to
be more vulnerable to environmental stresses. A decrease in tea yield
and quality necessarily affects the tea industry, especially production
firms. Therefore, studies have focused on ameliorating tea plant
adversity53-55. In this study, Small heat shock
proteins with different expression were screened out in tea plant
genome. What function do they play ? However, the function of sHSPs in
tea plants has been inadequately studied. Therefore, a genome-wide
analysis of the CssHSP superfamily in the tea plant was performed, and
the results provide a strong theoretical basis for future functional
studies.
Although 22 CssHSP genes had been discovered in C. sinensis based
on its genome, no systematic analysis of the sHSP gene family has
been completed. Because these 22 genes were identified only through
BLASTP searches, which used the sHSP protein sequences of A.
thaliana as queries against two C. sinensis genomes, some
CssHSPs may have been overlooked. Therefore, additional searches for
genes that encode CssHSPs in C. sinensis genomes were conducted
using HMM profiles. A total of 54 CssHSP genes were identified inC. sinensis , which is a higher number of CssHSP members
than identified previously56.
On the basis of conserved domain and
phylogenetic tree analyses, the 54 CssHSP genes were divided into
12 distinct subcellular localization groups. No C. sinensis sHSPs
were found in Px (peroxisome) groups, which was consistent with the
results of Chen et al .56. Because gene
expression analysis provides valuable information regarding gene
function57-58, the expression levels of theCssHSP genes in different tissues and the levels under abiotic
and biological stress (low temperature, NaCl, drought, and MeJA) were
investigated. The results revealed that most CssHSP genes were
expressed in at least one tissue. Some genes were highly expressed in
all tissues. Consistent with existing research, sHSPs were found to be
induced by stress33, 59-60. The results of the genome
analysis in this article also reveal that sHSPs can be involved in one
or more abiotic and biotic stresses.
According to genome analysis, we clonedCsHSP24.6 from the tea
plant. CsHSP24.6 belongs to a class of genes responsible for
chloroplast localization. The expression of this gene is typical for
various stress-inducing characteristics and can quickly respond to
stresses. Induced stress substantially increases the expression of all
sHSP genes (Figures 3 and 4 ). AtHSP21 was revealed to
be the only chloroplast-localized sHSP in A. thaliana.27-28, 61-62. Tea plant sHSP CsHSP24.6 and A.
thaliana AtHSP21 are both classified in the same branch of the
evolutionary tree, have high homology, and have three conserved domains
(Figures 1 and S2 ).In this study, CsHSP24.6transgenic A. thaliana seedlings were treated with NaCl. After
treatment, the transgenic seedlings were observed to have better growth
than the controls. However, there was no difference compared with
controls for the same treatment with CspTAC5 transgenic seedlings
(Figure 7 ). CsHSP24.6 showed distinctly salt tolerance.
As the photosynthetic organ of plants, chloroplasts are vulnerable to
damage when plants are under stress. Many studies have shown that under
stress, chloroplast sHSP can maintain the physiological state of the
chloroplast and preserve normal function 61, 63-64. In
the gun5 mutant with a yellowing phenotype, the expression ofHSP21 was inhibited. Under heat stress, the mutant had
disintegrated thylakoids and lacked stacked basal particles; when the
gun5 mutant expressed AtHSP21 protein, the thylakoid membrane
structure was normal and heat tolerance was
restored27. Therefore, the A. thaliana AtHSP21
protein can respond to high temperature stresses by protecting the
thylakoid membrane structure in the chloroplast and thereby resisting
damage.
It was reported that AtHSP21 and pTAC5 are required for
chloroplast development under heat stress by maintaining PEP
function28. Here, the Arabidopsis of overexpressed theCsHSP24.6 and CspTAC 5 genes could maintain a higher Fv/Fm
value than controls at high temperatures and under strong light.
Electron transfer via PSII was maintained. CsHSP24.6 interacts
with CspTAC5 to help the plant resist external stress, so the
same function was authenticated. CsHSP24.6 improves the stability
of growing development under heat stress (Figure 6 ).
Our results indicate that overexpression of CsHSP24.6 improvesA. thaliana seeding resistance to high temperature. Meanwhile, it
can protect plant from the strong light. In salt stess experiments, the
Arabidopsis of overexpressed the CsHSP24.6 grew better than the
control.(Figure 7 ) In this context, the CsHSP24.6 plays
an essential role in maintaining plant growth and development under
abiotic stress.