Legume Genomics and Genetics 2025, Vol.16, No.5, 204-214 http://cropscipublisher.com/index.php/lgg 205 This study focuses on the diversity and expression changes of the expansin gene family in leguminous plants. We will systematically sort out their classification methods, evolutionary features and structural compositions, and analyze them in combination with their expression patterns during plant growth, development and under adverse conditions. Of course, leguminous elements such as root development, nodulation, and adaptation to drought or salt stress will be the focus of our attention. With the rapid development of genomics and transcriptomics, our understanding of the regulatory mechanisms and functions of these genes is gradually deepening, which provides new directions and research foundations for the molecular improvement of leguminous crops in the future. 2 Structural Diversity of the Expansin Gene Family in Legumes 2.1 Classification of expansins: α-expansins, β-expansins, expansin-like A, and expansin-like B The blotin family in leguminous plants is not A collection of a single type. In fact, it is divided into four categories according to phylogenetic relationships: α-expansin (EXPA), β -expansin (EXPB), expansin-like A (EXLA), and expansin-like B (EXLB) (Figure 1). Among them, the α and β types have the strongest presence and the most research. Functionally, they mainly involve the relaxation of cell walls and the process of plant development. In contrast, Sample A and Sample B are much quieter. Currently, our understanding of them mainly remains at the level of gene sequences, and there is a lack of experimental evidence to prove whether they truly possess cell wall activity. Take soybeans as an example, this point is quite obvious. Researchers have identified 75 expansin genes and classified them into four subfamilies, with EXPA taking the majority, followed by EXLB, then EXPB and EXLA (Wang et al., 2024a). This distribution also indirectly confirms the focus of research and function of each sub-family. 2.2 Gene duplication, divergence, and chromosomal distribution across legume genomes Why is the expansin gene family so large in leguminous plants? One major reason is gene replication. Especially tandem replication and fragment replication, these two mechanisms provide "channels" for the increase of family members. Such replication events are particularly frequent in soybeans and their wild relatives, playing a promoting role in the expansin gene families. But these genes are not evenly distributed on the chromosomes. Some chromosomes, due to the concentrated burst of tandem replication, have formed distinct gene clusters, and this concentration also brings potential functional or regulatory connections. Meanwhile, the evolution of different genes is not static-they are influenced by natural selection. Some have experienced positive selection pressure, resulting in the gradual formation of differentiated functional characteristics in some subfamilies (Zhu et al., 2014). 2.3 Conserved motifs and structural domains: insights from sequence alignment and phylogenetics Sequence analysis of the expansin family reveals a delicate balance: on the one hand, conservation, and on the other hand, differentiation. Structurally, most inflated proteins contain two key domains: DPBB_1 (a double psi β barrel domain) and CBM63 (a carbohydrate-binding module). These structures are not randomly combined but highly conservative, especially within the same subfamily. Take soybeans as an example. EXPA members often have a set of eight conserved motifs arranged in a similar order, while other subfamilies, such as EXPB, EXLA and EXLB, although they also have their own motif combinations, the patterns are significantly different. Some motifs even only appear in specific subfamilies. This "belonging only to oneself" marker not only indicates the evolutionary differentiation among them, but may also predict the functional specialization tendency (Feng et al., 2022). In other words, although they all belong to the same superfamily, the differences between different subclasses are real, reflecting their respective "personalities" and the divergence in the process of evolution. 3 Evolutionary Insights into Expansin Genes in Legumes 3.1 Phylogenetic relationships of expansin genes among legume species and other angiosperms The story of expansins actually dates back to an even earlier stage of green algae-their rudimentary forms had already emerged. As plants gradually "come ashore" and move towards terrestrial ecosystems, these genes have not been idle either, and have begun to continuously differentiate and evolve. EXPA was the first to appear among the four subfamilies, followed by EXPB, EXLA and EXLB. However, there are also many differences among
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