Maize Genomics and Genetics 2025, Vol.16, No.4, 202-218 http://cropscipublisher.com/index.php/mgg 210 functions such as water and nutrient absorption, photosynthesis and mechanical support. The impact of high temperature on them varies. Generally speaking, leaves, as organs directly exposed to solar heat radiation, are the most sensitive to high temperatures: high temperatures cause the stomata on leaves to close, transpiration to decrease, and the temperature to rise, making them more prone to scorching and a decline in photosynthetic rate. The root system, being in the soil, has a relatively stable temperature and a relatively high tolerance for short-term high temperatures. However, continuous high temperatures can also inhibit root growth and cause poor root hair development, thereby affecting water and nutrient absorption (Zhang and Xu, 2024). A single-cell transcriptome study by Wang et al. (2025) revealed that different cell types in corn roots respond to high temperatures to varying degrees. Among them, cortical cells are the most sensitive to high temperatures. High temperatures can significantly alter the gene expression profile of cortical cells and inhibit their division and elongation. This indicates that even within the root, the responses of each tissue layer to heat stress are specific. As a nutrient transport and support organ, the stem has a relatively insignificant direct response to high temperatures, but high temperatures may indirectly affect the stem by interfering with hormone transport and structural development. For instance, research has found that under the combined effect of high temperature and drought, the vascular bundle tissue and cell wall structure of corn stalks change, and their mechanical strength decreases. Co-expression network analysis also indicated that there were differences in the gene modules activated by different tissues under heat stress: the upregulated genes enriched in leaf tissues were mostly related to photosynthesis and heat shock proteins, while the upregulated genes enriched in root systems were more related to aquaporin and osmotic regulation (Huo et al., 2023). In addition, the impact of high temperatures on reproductive organs (such as inflorescences) is particularly crucial. Although this paper focuses on the seedling stage, it should be pointed out that the filaments and male spikes are extremely sensitive to high temperatures. High temperatures may lead to a decline in pollen vitality and a reduction in pollen release in male spikes, as well as fertilization obstacles in female spikes, thereby resulting in a significant reduction in yield. Under high-temperature stress, different tissues of corn each have their vulnerable links and main response mechanisms. Therefore, when studying the heat tolerance of corn, tissue specificity needs to be taken into consideration. For instance, in breeding, improvements can be made to the organs most vulnerable to heat damage (such as spikelets and leaves) to enhance the overall heat resistance of the plant. Meanwhile, in the design of research experiments, the physiological and molecular indicators of roots, stems and leaves should also be separately tested to comprehensively assess the heat resistance characteristics of the material. 5.2 Temporal expression dynamics of genes under different time points of stress The response of corn to high temperatures is a dynamic process. Different durations of high-temperature stress will activate different gene expression programs. Usually, in the early stage of heat stress (within a few minutes to several hours), plants rapidly induce a batch of emergency response genes, such as the transcription of heat shock transcription factor and heat shock protein genes, which increases significantly within 15 to 30 minutes. This is a typical "instantaneous heat response". Subsequently, if the high-temperature stress persists, corn will enter a "sustained response" stage, and some metabolic pathway genes related to stress resistance (such as antioxidant enzymes, osmotic protective substance synthases, etc.) will maintain high levels of expression, while the HSP gene that initially rose sharply may gradually reach a stable state after several hours of high-temperature treatment. Li et al. (2024) conducted a time series transcriptome analysis on the maize B73 inbred line, comparing the expression at different time points such as 0.5 hours, 1 hour, 3 hours, and 6 hours of high-temperature treatment, and found that the set of genes upregulated at different times was different. Among them, the genes upregulated in the early stage (0.5 h-1 h) were mainly protective genes such as HSF and HSP; In the middle stage (about 3 hours), the upregulated genes expand to pathways such as antioxidation, hormone signaling, and glucose metabolism; In the late stage (6 hours and beyond), the expression of some genes related to cell repair and growth recovery is upregulated. This indicates that the heat tolerance response of corn is phased: first, it rapidly protects cells, then adjusts metabolic homeostasis, and finally attempts to resume growth. Another study also supports this dynamic rule: Jiang et al. (2020) observed through a time gradient experiment that the expression level of the HSP70 gene in corn seedlings soared within 15 minutes after the start of high temperature, reached a peak within 1 hour, and gradually decreased after 4 hours. However, the expression of the antioxidant enzyme gene was significantly
RkJQdWJsaXNoZXIy MjQ4ODYzNA==