Maize Genomics and Genetics 2025, Vol.16, No.4, 202-218 http://cropscipublisher.com/index.php/mgg 208 4 Heat Stress-Related Signaling Pathways and Molecular Networks 4.1 Initial responses of heat sensing and signal transduction Plants' perception of high temperatures begins with a series of physicochemical changes at the cellular level, including enhanced membrane lipid fluidity, alterations in protein conformation, and metabolic imbalances. The cell membrane of corn seedlings is regarded as one of the heat receptors. When the temperature rises sharply, the lipid bilayer of the biofilm changes from the gel phase to the liquid crystal phase, and the membrane fluidity increases. This change is captured by membrane-binding receptors, triggering the opening of Ca2+ ion channels and a transient increase in cytoplasmic Ca2+. Calcium ions, as second messengers, play an amplifying and propagating role in the initial stage of thermal signal transmission. Studies have shown that within minutes of the onset of high-temperature stress in corn leaves, the concentration of free Ca2+ in the cytoplasm significantly increases, activating downstream signaling components such as calcium-dependent protein kinase (CDPK) and calmodulin (CaM). In addition to Ca2+, the unfolded protein response (UPR) is also an important mechanism for heat perception. When the temperature is too high, causing denaturation and aggregation of proteins, the molecular chaperone BIP on the endoplasmic reticulum detects an increase in misfolded proteins and then dissociates from the endoplasmic reticulum reticulum receptor IRE1, triggering the dimerization activation of IRE1. IRE1, as a signaling enzyme, further cleases the XBP1 transcript, triggering the UPR cascade reaction. There is evidence in corn that UPR is involved in heat stress responses: the ZmBIP gene is rapidly upregulated under high-temperature conditions, and some molecular chaperone genes downstream of the IRE1 pathway are also induced (Wang et al., 2024). Meanwhile, high temperatures can also alter the phase transition and fluidity of cell membranes, affecting the activity of receptor kinases on the membranes. For instance, thermal stimulation may enrich or depolymerize through membrane microdomains, activating mechanosensitive channel proteins or thermal shock receptor proteins on the cell membrane of corn, thereby mediating ion inflow and signal transduction. The primary perception of high temperature in corn involves multiple mechanisms such as membrane structure changes, Ca2+ signals, and UPR responses. These initial events rapidly convert changes in environmental temperature into biochemical signals within cells, laying the foundation for the activation of the heat stress response network. Rapid thermal signal perception and transduction ensure that corn seedlings can initiate protective response reactions immediately when high temperatures arrive, thereby reducing damage. 4.2 Regulatory roles of Ca²⁺, ABA, MAPK and other signaling pathways During the process of heat stress signal transduction, the Ca2+ -mediated signaling pathway, the ABA-mediated hormone pathway, and the MAPK cascade are the three classic pathways for activating defense in plants such as corn. Firstly, Ca2+ plays a central role as a "second messenger". High-temperature induced cytoplasmic Ca2+ waves can be captured and activated by calcium-dependent protein kinases (CDPK). This type of CDPK can directly phosphorylate certain transcription factors or defense enzymes, thereby rapidly regulating gene expression. For instance, existing studies have shown that Ca2+-CaM signaling can activate ABA-induced antioxidant enzyme and nitric oxide (NO) synthesis, enhancing the antioxidant capacity and signal transduction efficiency of cells in the thermal response of corn. Secondly, the ABA pathway interacts and amplifies the thermal signal. After high temperature increases the ABA content, the downstream SnRK2 kinase cascade is activated through PYR/PYL receptors, thereby affecting the activity of a series of ABA response element (ABRE) binding proteins (such as ABI5, etc.). In corn, it has been reported that high temperatures can induce the joint participation of HSFA6b and ABI-like transcription factors: the MAPK pathway converges Ca2+ and ABA signals, phosphorylates transcription factors such as ABI and HSF, and enhances their transcriptional regulation of genes such as HSP and CyclinD, thereby improving the heat tolerance of corn. Thirdly, the MAPK cascading pathway serves as a convergence and amplification platform for various upstream signals. High temperature can activate MAPKKK through an unknown upstream kinase, and then successively activate MAPKK and MAPK through phosphoric acid. Some MAPK family members related to thermal response, such as ZmMPK3/6, were identified in corn, and their activity was enhanced under thermal stimulation (similar to the MPK3/6 of Arabidopsis thaliana involved in HSF regulation). The research by Gao et al. (2019) demonstrated that high-temperature stress can activate multiple transcription factors such as HSF and ABF, as well as the cell cycle regulatory factor CYCD5 through the MAPK cascade 1. Thereby promoting the heat tolerance of corn during the grain-filling period. MAPK can also regulate
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