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