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.