Legume Genomics and Genetics 2025, Vol.16, No.5, 204-214 http://cropscipublisher.com/index.php/lgg 208 (Mergner et al., 2020). This type of expression dynamics is highly consistent with the stage of rapid cell volume growth and organ formation, and also indirectly indicates that the participation of expansion proteins in the development process is actually quite high-especially in morphological formation and structural shaping. 5 Expansin Gene Regulation under Abiotic and Biotic Stress 5.1 Expression modulation in response to drought, salinity, cold, and heat stress In the face of various abiotic stresses, plants do not just sit and wait to die. Expansin genes show significant expression changes under such environmental stress, but these changes often vary depending on the species, tissue and type of stress. In wild soybeans, the GsEXLB14 gene is activated under stress. Overexpression can promote root growth and enhance the plant's stress resistance at the same time (Figure 2). Not only soybeans, but similar examples can also be found in other plants. Once the expression of NtEXPA4 in tobacco and BrEXLB1 in Brassica plants is upregulated, proline accumulation increases and the root system becomes longer, resulting in enhanced drought and salt tolerance (Muthusamy et al., 2020). However, it is not always so direct in all cases. For instance, under cold and hot stress, the expression of blotin will also increase, but the underlying mechanism of action may be more complex. Like TaEXPB7-B in wheat, it is activated under the combined influence of low temperature and ABA (abscisic acid), which helps to enhance cold resistance and maintain growth (Feng et al., 2019). From these circumstances, it can be seen that the expression regulation of the expansion protein gene is intricately related to osmosis regulation, cell wall structure stability, and even antioxidant mechanisms (Kuluev et al., 2016; Chen et al., 2019). 5.2 Induction by pathogen infection and involvement in defense signaling As soon as a plant encounters a pathogen, its defense system begins to be activated, and the expansin gene is often involved. But things are not that simple. Sometimes their roles are even a bit "contradictory". In wild peanuts, a gene called AdEXLB8, when introduced into tobacco, can significantly enhance resistance to pathogenic bacteria and nematodes, and also improve drought tolerance. This is mainly achieved by activating jasmonic acid and ABA signaling pathways, thereby enhancing antioxidant defense capabilities (Brasileiro et al., 2021). However, this positive effect is not static. For instance, in tobacco, although NtEXPA4 can make plants more drought-resistant and salt-tolerant, it appears more "vulnerable" when facing powdery mildew or bacterial infection-infection becomes easier (AbuQamar et al., 2013; Chen et al., 2018). This indicates that there may be some kind of "tug-of-war" between immunity and growth for expansins, and it is not always the best of both worlds. 5.3 Crosstalk with plant hormones (ABA, auxin, ethylene) in stress adaptability When plants are under stress, hormone signaling pathways are almost impossible to be absent. The expression changes of the expansin gene are largely regulated by abscisic acid, auxin and ethylene. These three hormones play different roles in stress responses, but all can influence the behavior of dilator proteins. Abolic acid, which has long been regarded as the "big boss" of drought and salt resistance responses, can induce the expression of genes such as TaEXPA2 (wheat) and OfEXLA1 (osmanthus), thereby helping plants enhance their adaptability (Dong et al., 2023). Auxin related dilating proteins, on the other hand, are more inclined to promote root growth and cell expansion, representing a more "growth regulation" pathway. The role of ethylene is slightly complex. It is involved in regulating cell wall remodeling. For example, the upregulation of GsEXLB14 when there is insufficient water can help maintain the extended state of cells (Han et al., 2012). The three do not operate independently but interweave and collaborate to jointly determine whether the plant maintains its state, accelerates growth, or adjusts its shape under stress. This coordination ability is essentially a manifestation of plant adaptability and also indicates that the regulation of expansion proteins is far more complex than it appears on the surface. 6 Functional Characterization of Expansin Genes 6.1 Gene knockout and overexpression studies in model and crop legumes What exactly can expansive protein do? The most direct way is to "turn them on" or "turn them off" to see the effect. Through the research on gene knockout and overexpression, many problems have begun to become clear
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