Tree Genetics and Molecular Breeding 2024, Vol.14, No.5, 218-228 http://genbreedpublisher.com/index.php/tgmb 219 2 Identification and Functional Analysis of Cambium-Specific Expression Genes 2.1 Methods for identifying cambium-specific genes The identification of cambium-specific genes has been facilitated by various advanced techniques. One notable method involves the use of laser capture microdissection combined with transcriptome profiling, which allows for the precise isolation and analysis of cambium tissues. This approach was effectively utilized to identify genes induced during different phases of cambium formation in Arabidopsis thaliana (Agustí et al., 2011). Additionally, the use of in vitro systems to induce cambium formation in isolated stem fragments has proven valuable in characterizing transcriptome remodeling in a tissue- and stage-specific manner (Agustí et al., 2011). In coniferous trees, such as Pinus sylvestris, the expression profiles of cambium-specific genes were studied by collecting trunk tissue samples at different stages of ontogenesis and cambial growth. This method enabled the identification of spatial and temporal expression patterns of key regulatory genes (Galibina et al., 2023). Furthermore, comprehensive analyses of transcriptome profiles, DNA methylome, and miRNAs during the transition from dormancy to activation in Populus tomentosa have provided insights into the molecular mechanisms underlying cambium activity (Chen et al., 2021). 2.2 Functional roles of identified genes The functional roles of identified cambium-specific genes are diverse and critical for the regulation of cambium activity and secondary growth. For instance, the receptor-like kinases REDUCED IN LATERAL GROWTH1 (RUL1) and MORE LATERAL GROWTH1 (MOL1) have been identified as opposing regulators of cambium activity in Arabidopsis, with RUL1 inhibiting and MOL1 promoting lateral growth (Agustí et al., 2011). In Populus, two MADS-box genes, VCM1 andVCM2, were found to modulate auxin homeostasis, thereby regulating vascular cambium proliferation and secondary growth (Zheng et al., 2020; Haas et al., 2022). The WUSCHEL-RELATED HOMEOBOX (WOX) gene family, particularly WOX4, plays a crucial role in maintaining and proliferating stem cells in the cambium. In Scots pine, the expression of WOX4 was highest during active cell proliferation in the cambial zone, indicating its importance in cambial activity (Galibina et al., 2023). Similarly, in Populus, WOX4-like genes were shown to control cell division activity in the vascular cambium, with their expression regulated byCLE41-related genes (Kucukoglu et al., 2017). 2.3 Comparative analysis of cambium-specific genes across different tree species Comparative analysis of cambium-specific genes across different tree species reveals both conserved and species-specific regulatory mechanisms. For example, the CLE41/44-PXY-WOX signaling module, which regulates cambial growth in Scots pine, is also conserved in Populus, where it controls cell division activity in the vascular cambium (Kucukoglu et al., 2017; Galibina et al., 2023). This suggests an evolutionarily conserved program for the regulation of vascular cambium activity between angiosperm and gymnosperm tree species (Wang, 2020). In addition, the identification of stress-response transcription factors that control cambium activity in radish and their comparison with Arabidopsis data highlights the conservation of gene-regulatory networks that integrate environmental sensing and growth (Hoang et al., 2020; Furuya et al., 2021). The role of miRNAs and DNA methylation in regulating cambium activity in Populus further underscores the complexity and diversity of epigenomic regulation across different species (Chen et al., 2021). These studies provide valuable insights into the molecular mechanisms of cambium formation and activity maintenance, highlighting the collaborative regulation of tree stem cells in growth, development, and environmental adaptation. 3 Regulatory Mechanisms of Hormones in Cambium Formation and Activity Maintenance 3.1 Overview of hormone regulation in plant stem cells Hormones play a crucial role in the regulation of plant stem cells, particularly in the cambium, which is responsible for secondary growth. The cambium’s activity is influenced by a complex interplay of hormonal signals, including auxin, cytokinin, gibberellin, and strigolactones, among others. These hormones coordinate to regulate the balance between cell proliferation and differentiation, ensuring the proper formation and maintenance of the cambium (Groover and Robischon, 2006; Turley and Etchells, 2021; Ben-Targem et al., 2021; Hu et al., 2021).
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