MdWRKY33
INTRODUCTION
The sixth assessment of the Intergovernmental Panel on Climate Change
(IPCC) stated that the global mean temperature is expected to rise by
2.8±0.7°C at the end of this century (IPCC, 2021). The frequency and
amplitude of heat episodes are projected to increase as global warming
intensifies (Ummenhofer and Meehl, 2017). These problems negatively
effects fruit production capacity and quality. Consequently, plants have
evolved complex and diverse responses to short or periodic exposure to
extreme temperatures. These responses include acquired and basal
thermotolerance. Acquired thermotolerance is the increase of
thermotolerance after pretreatment at high temperature (Sung et
al. , 2003), while basal thermotolerance is the ability to survive high
temperature without preacclimation (Yeh et al. , 2012).
Heat stress triggers a series of metabolic alterations in plants. When
subjected to high temperatures, the dynamic balance between the removal
and production of reactive oxygen species (ROS) is disrupted, resulting
in an excess of ROS (such as O2−,
H2O2, -OH˙,1O2) (Dong et al. , 2021). This
excess could induce damage to DNA, proteins, lipids, and cell membranes
in plants, ultimately leading to oxidative stress (Gill and Tuteja,
2010). In response, plants have developed efficient enzymatic
(peroxidase, POD; catalase, CAT; superoxide dismutase, SOD) and
non-enzymatic defense mechanisms to counteract the harmful effects of
ROS (Gill and Tuteja, 2010; Hoffman et al. , 2012; Sheikh-Mohamadiet al. , 2018). Furthermore, plants regulate cell osmotic pressure
by accumulating osmoprotectants, including soluble sugars, amino acids,
soluble proteins, and lipids (Hare et al. , 1998; Hasanuzzamanet al. , 2013; Jia et al. , 2021).
Notably, plant photosynthesis, one of the most fundamental biochemical
processes, is hypersensitive to heat stress at the subcellular level. In
the early stage, high temperature leads to the stomata closure and
reduces stomatal conductance (GS), directly limiting the
photosynthetic function (Wise et al. , 2004). Under strong heat
stress, the thylakoid membranes integrity and chloroplasts
ultrastructure are damaged, damaging the oxygen-producing complex of
photosystem II (PSII) and impairing electron transfer within the
reaction center of PSII (Kouřil et al. , 2004; Lichtenthaleret al. , 2005).
MT (N-acetyl-5-methoxytryptamine) is a versatile signaling molecule that
responds to various abiotic stresses during plant development (Arnao and
Hernández-Ruiz, 2015; Zhang et al. , 2015).
Since
its detection in plants, it has been shown to provide physiological
protection against environmental stresses, including temperature stress
(Ahammed et al. , 2018; Altaf et al. , 2021; Byeon and Back,
2014). Byeon and Back (2014) showed that endogenous MT levels increase
in rice (Oryza sativa ) seedlings when exposed to heat stress,
through increased activities of N-acetylserotonin methyltransferase
(ASMT) and serotonin N-acetyltransferase (SNAT), two enzymes involved in
the MT synthesis in plants. Exogenous MT application in tomato
(Solanum lycopersicum ) plants enhance cellular protein protection
by inducing autophagy and heat shock proteins, which help degrade or
refold heat-denatured (Xu et al. , 2016). Additionally,
overexpression of SlSNAT in tomato plants improves the maximum
photochemical quantum yield of PSII
(FV/FM) and increased the
transcription of heat shock factors under high-temperature stress.
Previous research found that MdASMT9 overexpression in apple
increased endogenous MT levels and improved water use efficiency (WUE)
(Zhou et al. , 2022). However, the beneficial roles of endogenous
MT in apple thermotolerance remain unclear. Therefore, this study
investigated MdASMT9 -overexpressing (OE) apple plants under
extreme heat stress. Results showed that MdASMT9 overexpression
positively impacted the apple plant’s response to heat stress by
improving thermotolerance through scavenging harmful ROS and maintaining
higher photosynthetic capacity. MdASMT9 overexpression also
increased soluble sugar and amino acid levels under heat stress.
Furthermore, MdASMT9 overexpression increased stomatal aperture
through promoting MdWRKY33-mediated transcriptional inhibition ofMdNCED1 and MdNCED3 . Moreover, exogenous MT and
overexpression of MdASMT9 enhanced autophagic activity through
promoting MdWRKY33-mediated transcriptional enhancement ofMdATG18a under heat stress.
MATERIALS AND METHODS