Tree Genetics and Molecular Breeding 2024, Vol.14, No.2, 57-68 http://genbreedpublisher.com/index.php/tgmb 60 4 Salt Resistance Mechanisms in Trees 4.1 Genetic modifiers of salt tolerance: ion transport and osmotic regulation Salt tolerance in trees involves complex genetic and physiological mechanisms that help mitigate the detrimental effects of high salinity. One of the primary strategies is the regulation of ion transport and osmotic balance. For instance, the WRKY transcription factor SlWRKY8 in tomato has been shown to enhance salt tolerance by increasing the levels of osmotic substances like proline and activating stress-responsive genes such as SlAREB, SlDREB2A, andSlRD29 (Gao et al., 2019). Similarly, the sweetpotato β-amylase gene IbBAM1.1 enhances salt stress resistance by regulating reactive oxygen species (ROS) homeostasis and osmotic balance, promoting the accumulation of osmoprotectants like maltose and proline (Zhu et al., 2021). In rice, high performance photosynthesis and better osmotic adjustment have been associated with salt tolerance, as observed in near isogenic lines carrying drought tolerance QTL (Nounjan et al., 2018). These genetic modifiers play crucial roles in maintaining ion homeostasis and osmotic regulation, which are essential for salt tolerance in trees. 4.2 Case studies: successful salt resistance breeding and genetic engineering Several case studies highlight the success of breeding and genetic engineering in enhancing salt resistance in trees. For example, the overexpression of the NAC transcription factor PeNAC036 in Populus euphratica has been shown to increase tolerance to salt and drought by upregulating stress-responsive genes such as COR47, RD29B, ERD11, RD22, and DREB2A (Lu et al., 2018). Another study demonstrated that silencing the Raf-like MAPKKK gene GhRaf19 in cotton enhanced tolerance to salt stress by reducing ROS accumulation and increasing the expression of ROS-related genes (Jia et al., 2016). These case studies illustrate the potential of genetic engineering and selective breeding in developing salt-resistant tree varieties. 4.3 Challenges in enhancing salt resistance through genetic approaches Despite the promising results from genetic studies, several challenges remain in enhancing salt resistance in trees through genetic approaches. One major challenge is the complexity of stress responses, as trees often face multiple abiotic stresses simultaneously, which can complicate the effectiveness of single-gene modifications. Additionally, field studies are scarce, and the positive effects observed under laboratory conditions may not always translate to real-world environments where trees are exposed to variable and prolonged stress conditions (Yuan et al., 2020). Another challenge is the potential trade-off between growth and stress resistance, as enhancing one trait may negatively impact the other. Therefore, a better understanding of the intricate networks that regulate stress responses and the development of stress-inducible promoters may be necessary to optimize the balance between growth and defense (Golldack et al., 2011). 5 Cold Resistance Mechanisms in Trees 5.1 Cold acclimation processes influenced by genetic factors Cold acclimation in trees involves a series of genetic and physiological changes that enhance their ability to withstand low temperatures. Genetic factors play a crucial role in these processes. For instance, the expression of dehydration-responsive element binding protein (DREB) genes, such as OsDREB1C, OsDREB1E, and OsDREB1G, has been shown to be key regulators of cold acclimation and freezing tolerance in rice, which can be extrapolated to trees (Wang et al., 2022). Additionally, the upregulation of cold-responsive genes like RD29B and MbCBF2 in transgenic plants overexpressing PeCPK10 from Populus euphratica indicates the importance of these genes in cold stress response (Figure 2) (Chen et al., 2013). Furthermore, transcriptome analyses in apple trees have identified differentially expressed genes (DEGs) that are enriched in metabolic processes and signal transduction components, which are crucial for cold acclimation.
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