Maize Genomics and Genetics 2025, Vol.16, No.4, 202-218 http://cropscipublisher.com/index.php/mgg 204 transport chain in organelles such as chloroplasts and mitochondria, leading to incomplete oxygen reduction and the generation of ROS such as peroxides and superoxide anions. An appropriate amount of ROS can act as a signaling molecule to trigger defense responses, but excessive ROS can cause membrane lipid peroxidation, enzyme inactivation and DNA damage. The ROS content of corn seedlings significantly increased at high temperatures, such as the levels of hydrogen peroxide (H2O2) and superoxide anion (O2 -), which rose markedly. The accumulation rate was relatively low in heat-resistant materials, but increased more rapidly in sensitive materials. To resist oxidative damage, corn activates the body's antioxidant defense system, including both enzymatic and non-enzymatic pathways. The enzymatic antioxidant system mainly includes superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbic acid peroxidase (APX), etc. The activities of these enzymes are rapidly upregulated at the initial stage of high temperature to eliminate excessive ROS. For example, the activities of SOD and POD in heat-tolerant varieties increased by more than 30% compared with the control after high-temperature stress, while the increase in the related enzyme activities of sensitive varieties was small or even decreased (Cao et al., 2021). Non-enzymatic antioxidants such as ascorbic acid, glutathione, proline and flavonoids also accumulate under high-temperature stress. They can directly eliminate free radicals and stabilize the membrane system structure. However, when the high temperature is too intense or lasts for too long, the antioxidant system may find it difficult to fully counteract the damage caused by ROS, resulting in a significant increase in the content of malondialdehyde (MDA), a lipid peroxidation product of the cell membrane, and an increase in the relative permeability of the cytoplasmic membrane. This is particularly evident in high-temperature sensitive corn materials: experiments observed that the MDA content in sensitive inbred line seedlings increased by more than twice after high-temperature treatment compared with the control, causing severe damage to membrane integrity, while the increase in MDA in heat-tolerant inbred lines was relatively small (Huo et al., 2023). High temperatures cause a large accumulation of ROS and induce the antioxidant defense system of corn to respond. The dynamic balance between the two determines the degree of oxidative damage suffered by the plants and is one of the important factors affecting the heat tolerance of corn. 2.3 Hormonal metabolism changes in response to heat stress Plant hormones play a key regulatory role in the response of corn to high-temperature stress. High temperatures can disrupt the hormonal balance within corn, causing changes in various hormone levels and metabolic pathways. Among them, abscisic acid (ABA) is regarded as an important response hormone under high-temperature stress. Studies have shown that high temperatures can induce a rapid increase in endogenous ABA content in corn seedlings, while growth-promoting hormones such as cytokinin (CTK) and auxin (IAA) relatively decrease. The accumulation of ABA helps to close stomata and reduce transpiration, thereby enhancing the plant's stress resistance, but it may also inhibit growth. As Cheikh and Jones' early research found, under high-temperature conditions, the ABA content in corn plants increases and the development of young corn panicles is hindered. Applying 6-benzyladenine (an artificial cytotin) to corn can partially counteract the negative effects of ABA and promote normal grain filling. In addition, calcium ions (Ca2+) and ABA work in synergy to play a role in heat signal transduction: exogenous application of both Ca2+ and ABA can enhance the activity of antioxidant enzymes in corn seedlings, reduce the level of membrane lipid peroxidation, and thereby improve heat tolerance. In addition to ABA, the roles of other hormones such as gibberellin (GA), salicylic acid (SA), jasmonic acid (JA), and brassinolide (BR) in the heat stress response of maize have also been reported. GA promotes the growth of corn at normal temperature, but at high temperatures, overly strong GA signals may intensify the contradiction between growth and stress resistance. On the contrary, moderately reducing the GA signal can decrease growth consumption and is beneficial for heat tolerance. Recent combined transcriptomic and metabolomic analyses have found that high-temperature stress significantly affects the expression levels of auxin and abolic acid related metabolic genes in corn seedlings. For instance, auxin polar transport proteins and signaling elements are suppressed at high temperatures, while the NCED gene, a key enzyme in ABA synthesis, is upreregulated, indicating that corn activates stress resistance by increasing the ABA/IAA ratio. It is worth mentioning that the transcription factor ZmGBF1 has been confirmed to directly bind and regulate the ZmxCXE2 gene of maize carboxylesterase, thereby increasing the metabolic level of gibberellin and enhancing the heat tolerance of plants. Cao et al (2021) revealed the close connection between hormonal pathways and transcriptional regulation in the
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