Rice Genomics and Genetics 2025, Vol.16, No.1, 50-60 http://cropscipublisher.com/index.php/rgg 54 4.2 Role of circadian rhythms in integrating environmental signals Sometimes you will find that different rice varieties have different sensitivities to their internal clocks. But in any case, this circadian rhythm is like a time manager, aligning physiological activities with the changes of day and night and seasons. For example, it will refer to light and temperature signals together to adjust the rhythm of gene expression so that the rice plants are in the best growth state as much as possible (Venkat and Muneer, 2022). Speaking of key roles, we have to mention the "giant gene" (GI), which actually regulates PIF in the background, and PIF is the core machine for integrating light and temperature signals (Nohales et al., 2019). What's more interesting is that the circadian rhythm can also influence the "deployment" of chromatin in the cell nucleus, thereby determining which genes should be activated and when (Zhang et al., 2023). In the final analysis, this seemingly complex feedback loop and regulatory network allows rice to "predict" the sunrise and sunset every day in advance and to flexibly respond to seasonal changes (Creux and Harmer, 2019). 4.3 Epigenetic regulation in temperature-light adaptation Sometimes, you will hear that the "switch" in rice depends not only on genes, but also on epigenetic "lubricants" to regulate the effects of light and temperature - don't think they are always rigid. Studies have found that histone modifications (such as H3K9 acetylation) do not simply remain unchanged, but fluctuate with the circadian rhythm. This oscillation happens to be consistent with the rhythm of gene expression, as if the two are "cooperating" to integrate environmental signals (Zhang et al., 2023). But don't forget that not all transcription factors are online on time every day-those transcription factors regulated by circadian rhythms have to have a dynamic party with chromatin to truly make epigenetic mechanisms come in handy in responding to climate change. What's more interesting is that core clock genes occasionally "link" with some transcription factors related to light and temperature signals, forming a seemingly messy but precise transcriptional regulatory network, highlighting the complex interaction between epigenetic regulation and environmental signal integration. 5 Case Study: Molecular Basis of Temperature and Light Adaptation in the Tropical Japonica Rice Cultivar 5.1 Selection criteria for the case study Speaking of this case, it is tropical japonica rice that was chosen-the reason is simple. This kind of rice has its own unique "coping strategy" in the face of changes in temperature and light in tropical regions, and this is the key to its good survival and stable yield. Although it has undergone many genetic improvements, it has been able to gain a foothold under high temperature and unstable light conditions. It is these characteristics that make it an excellent "experimental subject" for studying the molecular mechanisms behind temperature and light adaptation. In fact, tropical japonica rice shows a lot of molecular and physiological adjustments that help it resist cold and heat stimulation. It is particularly worth mentioning that the activity of some specific genes and proteins makes it resistant to cold and heat, and its sensitivity to photoperiod has been improved, which further affects the yield. 5.2 Analysis of key findings In tropical japonica rice, heat shock proteins (HSPs) play an indispensable role. They are actually like "repairmen" in cells, helping to stabilize those easily deformed proteins under high temperature stress, and even helping them to refold and return to normal. Don't underestimate this group of proteins. When the temperature soars, their number will increase rapidly, providing a protective umbrella for plants, helping them survive heat stress and ensuring that the basic functions of cells are not destroyed (Li et al., 2021; Sales et al., 2021). Sometimes you may feel that tropical japonica rice is like a "time adjustment master" - it changes its sensitivity to the photoperiod, so that it can head and bear fruit on time even under conditions of flickering light. Let's talk about an exception first: not all tropical japonica rice relies on changing Hd1 to adjust, some varieties have other secrets. But the mainstream approach is to make the Heading date 1 (Hd1) gene inoperable. This deletion allele is not very popular in temperate japonica rice, but it comes in handy in the tropics. Under short-day conditions, this "broken" Hd1 prevents rice from flowering early, helping growers move the flowering time to a more appropriate
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