1.Introduction
Plants have difficulty avoiding threats due to changes in the environment, such as drought, cold, heat, and salt or heavy metal concentrations1-3. When plants are subjected to these stresses, their proteins denature, agglomerate, and lose inherent function, resulting in a substantial decline in crop yield and quality4. In general, plants cannot change their positions to escape these stresses; however, they have developed various defense mechanisms to resist them including synthesis of heat shock proteins (HSPs)5-6. HSPs are commonly observed in plants responding to heat stress, but studies have shown that HSPs are also involved in the response to other biological and abiotic stresses7. As a molecular chaperone, HSPs are involved in protein folding, refolding, assembly, transport, and degradation, facilitating the stabilization of protein and cell membranes under stressful conditions8-9. Plant HSPs are divided into five conserved families according to their molecular weights: the HSP100, HSP90, HSP70, HSP60, and small HSP (sHSP) families10-13. In particular, HSP20 is a group of ATP-independent HSPs that is a first line of defense for plants at risk of protein aggregation.
HSP20s are key proteins protecting plants from aggregation and enhancing the effectiveness of other HSPs14. The monomeric molecular masses of most HSP20s are approximately 15-42 KDa15-16. The function of an HSP20 protein is related to its structure, which includes three main functional components: 1) a conserved C-terminal domain called the α-crystallin domain (ACD) or HSP20 domain, which forms a compact axial sandwich structure to help oligomers disintegrate into dimers and bind to nonnatural proteins; 2) a C-terminal extension region, which may be involved in the stabilization and solubilization of oligomeric assemblies; and 3) a variable N-terminal region, which plays a role in transiting, leading, or signaling17-19. According to protein homology and intracellular localization, the sHSP group is divided into 12 subfamilies: cytoplasm and nuclei (CⅠ~ CⅦ), mitochondria (MⅠ, MⅡ), endoplasmic reticulum (ER), chloroplasts (P), and peroxide (Px)20. Different HSP20 proteins have different functions, but most of them are induced by heat, saltand drought stress. In addition, previous studies have shown that HSP20s positively promote plant tolerance to adverse environments. For example, riceOsHSP26 significantly enhances the tolerance of tall fescue to oxidative and heat stresses by protecting photosystem II (PSII) and maintaining photosynthesis21. In Arabidopsis thaliana , AtHSP21 plays a positive role in the thermotolerance of plants and in extending the thermomemory phase22. In addition, overexpression of the Malus sieversiiMsHSP16.9 gene in A. thaliana increased plant tolerance to heat stress by alleviating damage from reactive oxygen species and regulating the expression of stress-related genes23. Under adverse conditions, the OsHSP18.2 could promote the germination of rice seeds and cotyledon growth24. HSP20s are also involved in plant growth and development. AtHSP17.4 andAtHSP17.6 accumulate in maturing seeds and play a protective role in seed development for A. thaliana 25-26.
These studies indicate that HSP20s are key for plant resistance to abiotic stress. The sHSP gene families have been investigated for several plant species including A. thaliana 27-29, rice30, tomato31-32, maize33, soybean34, populus35, and grapes36. The functional mechanisms of sHSPs in plant stress response have become a common research topic.
The tea plant, Camellia sinensis (L.) O. Kuntze, is the source of one of the most widely consumed beverages in the world. The tea plant often suffers biological and abiotic stressors (high temperatures, drought, pests, etc.), affecting its normal development as well as the yield and quality of tea24. Therefore, substantial benefits to tea production could be achieved by producing stress-resistant tea plant varieties by using molecular breeding technology. Obtaining these varieties requires studying the mechanisms of tea plant resistance to high temperature and drought and then identifying the related genes. Wang et al. confirmed thatCsHSP17.2 , CsHSP17.7 , CsHSP18.1 , andCsHSP21.8 improve tea tree tolerance to high temperature and drought stresses37. However, few other studies have investigated sHSPs in tea plants. The genome of the tea plant has been sequenced and published; thus, the identification of genes within the CssHSP superfamily is simple and reliable38.
In this study, 54 CssHSP genes were identified, and a comprehensive analysis that included analysis of phylogenetic relationships and conserved motifs was performed. In addition, the responses of all CssHSP s to biological stress (methyl jasmonate, [MeJA]) and abiotic stress (cold, drought, salt) were analyzed based on the tea plant genome data. We discovered that only two genes are significantly upregulated for all stresses. The results of this study revealed the molecular characteristics of the CssHSP gene superfamily and provide a theoretical basis for future studies of the biological functions of CssHSP s under abiotic stresses.