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.