1 INTRODUCTION
Due to climate change, the global temperature will continue rising in
the future, which will cause great challenges to crop production (Lesk,
Rowhani, & Ramankutty, 2016). Ambient temperature of plants increases
rapidly by 10℃ will pose a threat to plants and cause heat stress (HS).
In order to survive, plants form a complex and efficient regulatory
network to resist and adapt to HS, understanding these protective
responses has important agronomic value for maximizing agricultural
production and maintaining yield (Huang, Zhao, Burger, Wang, & Chory,
2021). Under HS, heat shock transcription factors (HSFs) are induced
rapidly to active the expression of heat shock proteins (HSPs). HSPs
mainly function as molecular chaperones to protect against thermal
denaturation of substrates and facilitate refolding of denatured
substrates, by regulating heat shock proteins, plants can tolerate
certain heat levels (McLoughlin, Kim, Marshall, Vierstra, & Vierling,
2019).
In the natural environment, plants often suffer from repeated and
changeable heat shock, these sub-lethal HS induce plants to produce
thermal memory (Balmer, Pastor, Gamir, Flors, & Mauch-Mani, 2015). HS
priming endows plants with a stronger thermal response to a second
stress exposure. Hs memory involves an initial priming phase and a
memory period that lasts for several days (Ohama, Sato, Shinozaki, &
Yamaguchi-Shinozaki, 2017). The initial phase of the HS memory involves
interacting transcriptional regulatory networks, HSFs play fundamental
role among these transcription factors. In addition, specific
modifications in the chromatin enable these memory genes to be induced
rapidly to cope with next stress in the memory phase (Lamke, Brzezinka,
Altmann, & Baurle, 2015). During the HS memory state, several HSPs with
chaperone activity maintain high protein levels for several days after
priming (Wu et al., 2013).
As efficient and accurate signal molecules, phytohormones actively
participate in HS response. Exogenous application of abscisic acid (ABA)
increases hydrogen peroxide (H2O2)
accumulation by inducing the expression of respiratory burst oxidase
Homologs (RBOHs), and thus enhances antioxidant capacity (Suzuki et al.,
2011). In addition, ABA also induces the expression levels of HSPs to
maintain protein in functional conformations (X. Wang, Zhuang, Shi, &
Huang, 2017). In flower tissue of Arabidopsis , cytokinin
treatment increases resistance in response to harmful high temperature,
protecting developing flowers exposed to HS (K. Wang, Zhang, & Ervin,
2012). Salicylic acid (SA) reduces heat stress-induced membrane damage,
enhancing the activities of antioxidant enzymes (Shah Jahan et al.,
2019). In addition, SA treatment maintains the level of HSP21 protein in
chloroplast, facilitates the recovery of photosynthesis after stress (L.
J. Wang et al., 2010). High temperatures lead to accumulation of
jasmonate (JA), loss of function in JA signaling or biosynthesis impairs
the thermotolerance of Arabidopsis seedlings, indicating that JA
is required for effective heat tolerance (Clarke et al., 2009). JA
treatment did not induce the expression of HSPs, and a defect in JA
signaling has no impact on HSPs transcription level (Manvi & Ashverya,
2016). In contrast to JA, ethylene signal transduction pathway promotes
the expression of HSFs and HSPs via ETHYLENE RESPONSE FACTOR 95/97
(ERF95/97), the direct target of ETHYLENE INSENSITIVE 3 (EIN3) (Huang et
al., 2021). Despite overwhelming evidences supporting the vital role
phytohormones act in HS response, limited knowledge of the molecular
mechanism restricts the application in crops, it is urgent to identify
pivotal factors in response to heat shock in phytohormones signaling.
Brassinosteroids (BRs) are a class of plant steroid hormones that play a
central role in the control of plant growth and development (Nolan,
Nemanja, Liu, Eugenia, & Yin, 2019). BRs are perceived by the
transmembrane receptor kinase BRASSINOSTEROID INSENSITIVE 1 (BRI1), and
the binding of BRs activates BRI1, thus triggering a phosphorylation
cascade and finally inactivating BR-INSENSITIVE 2 (BIN2), a GSK3-like
kinase that negatively modulates BRs signal by phosphorylating
BRI1-EMS-SUPPRESSOR 1 (BES1) and BRASSINAZOLE RESISTANT 1 (BZR1), the
key regulators of BR-regulated gene expression (Guo, Li, Aluru, Aluru,
& Yin, 2013). Transcriptome analysis showed that BES1/BZR1 up- or
down-regulate thousands of BR-responsive genes (Sun et al., 2010; Yu. et
al., 2011). In the spring barley (Hordeum vulgare L .), improved
functioning of PSII was observed under HS upon 24-epibrassinolide (EBR)
application (Janeczko, Okleková, Pociecha, Kocielniak, & Mirek, 2011).
In rice (Oryza sativa ), EBR or 7,8-dihydro-8-20-hydroxyecdysone
(DHECD) application improves photosynthetic efficiency and raises the
rice yield under HS (Diaz et al., 2021). In tomato, epi-brassinolide
(EBL) induces thermotolerance and peroxidase (POD) activity (Mazorra,
Holton, Bishop, & Nunez, 2011). HS promotes the dephosphorylation of
BES1 through repressing the activity of type 2C phosphatases (PP2Cs),
activated BES1 directly binds to heat shock elements (HSEs) thus
contribute to HS signaling (Albertos et al., 2022). These studies
demonstrate that BRs signal play a positive role in HS response.
Nevertheless, little is known about whether BRs signal transduction
modulates thermal memory process, so far there are no reports involved
in BR-mediated thermomemory.
In this study, we report that BRs act positively in HS memory and that
BES1 is required for BRs-enhanced thermomemory. Heat priming induces the
nucleus accumulation of BES1 and BES1 protein sustains for high levels
several days after priming. BRs treatment further activates BES1 during
the memory phase. The ability to maintain high levels of BES1 protein in
the nucleus after priming is required for sustained activation of HS
memory‐associated genes and BES1 activates memory genes in a direct or
indirect manner. Our results thus demonstrate that BES1 lays the
foundation of thermomemory, while BRs further pull up the upper limit of
it.