Triticeae Genomics and Genetics, 2025, Vol.16, No.3, 138-147 http://cropscipublisher.com/index.php/tgg 139 in cell type-specific regulation and offers new genetic resources for breeding high-yield and high-quality wheat varieties. This study aims to construct a high-resolution spatiotemporal transcriptome map, clarify the regulatory mechanism of grain filling at the cellular level, and promote targeted genetic improvement for global food security. 2 Physiological and Developmental Characteristics of Wheat Grains during the Grain Filling Stage 2.1 Developmental stages and characteristics of grains during the grain filling stage Once wheat enters the grain-filling stage, the development pace of the grains begins to accelerate significantly. However, this process is not accomplished all at once but is carried out in stages. Starting from flowering, the grains roughly go through three filling stages: early, middle and late. Each stage has its own "main activity" - in the early stage, it is cell division and differentiation; in the middle stage, it focuses on dry matter accumulation, such as a large amount of starch and protein piling up in the endosperm; in the late stage, it gradually turns to maturity and water loss (Guan et al., 2022). But this rhythm is not static. The environmental and genetic background can change at any time. When common stresses such as high temperature and shading occur, photosynthesis is affected, and the homoides decrease. As a result, the size of the grains, starch content, and final weight often have to be reduced (Chunduri et al., 2021; Mirosavljevic et al., 2021; Hou et al., 2024; Li et al., 2025). 2.2 Functional differences among various tissues Ultimately, a wheat grain is not a "unified combat unit", but is composed of different functional organizations that "divide labor and cooperate". Endosperm is the main warehouse, which also contains multiple parts such as the aleurone layer, sub-aleurone layer and endosperm transfer cells. Its main task is to store starch and protein for the germination of seedlings or for the nutrition of human food. The aleurone layer acts as a "transport hub", rich in various enzymes, and it is the key to mobilizing nutrients at critical moments. The embryo should not be overlooked either. It is the starting point of future plants, with active metabolism and unique protein expression. Many proteomics and metabolomics studies have shown that each of these tissues has its own specific expression rhythm and metabolic pathways (Zhang et al., 2021; 2023). To put it bluntly, it's about who should do what and when they should do it, which is already planned within the body. 2.3 Dynamics of carbon and nitrogen metabolism during grain filling To ensure that the grains are fully filled, water alone is not enough; the key lies in how carbon and nitrogen "flow" and "transform". The carbon source is mainly sucrose, and nitrogen enters the grains in the form of amino acids, which are eventually converted into starch and storage proteins (Wang et al., 2021). But this transformation is not achieved overnight; there is a series of enzymes "holding the stage" in between. Just as sucrose synthase and starch synthase are responsible for carbon metabolism, GOGAT and GDH play a key role in nitrogen assimilation. The activity and expression of these enzymes are closely related to the type of tissue and the time point of development. Different "switches" are turned on at different times. But then again, once there are problems with the environment, such as insufficient nitrogen or excessively high temperatures, these metabolic activities will also be disrupted. Eventually, it is often the case that starch rises but protein drops, and the composition of the grains changes accordingly. Throughout the entire filling period, endosperm is the most crucial "metabolic main battlefield", where the synergistic operation between carbon and nitrogen plays the greatest role, and has a particularly direct impact on yield and quality. 3 Gene Expression Dynamics during the Wheat Grain Filling Stage 3.1 Global spatiotemporal patterns of gene expression During the grain filling process of wheat, the changes in gene expression are not merely a "start - enhance - end" process. In fact, there is a complex, cell type-specific regulatory system behind it, and this system is constantly changing over time and space. Research in spatial transcriptomics has found that gene activity is not evenly distributed throughout the grains. As development progresses, the expression activity around the embryo and endosperm is the highest, while the expression levels of peripheral structures such as the seed coat and fruit coat
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