Tree Genetics and Molecular Breeding 2024, Vol.14, No.5, 218-228 http://genbreedpublisher.com/index.php/tgmb 222 gradients across the cambium area are crucial for regulating the differentiation of cambial derivatives into xylem and phloem (Zheng et al., 2020). Moreover, the interaction between auxin and gibberellin signaling pathways has been shown to promote xylem expansion and maintain cambium homeostasis (Ben-Targem et al., 2021). The dual regulation of xylem formation by the PaC3H17-PaMYB199 module in Populus further exemplifies the collaborative regulation of these gene networks (Tang et al., 2019). 5.3 Case studies of collaborative regulation in specific tree species In Populus, the MADS-box genes VCM1 andVCM2 have been shown to regulate cambium activity and secondary growth by modulating auxin homeostasis (Figure 1). Knock-down of these genes enhances cambium proliferation and xylem differentiation, while their overexpression suppresses these processes (Zheng et al., 2020). Another study in Populus identified the PaC3H17-PaMYB199 module, which regulates xylem formation through an auxin-mediated pathway. This module controls cambial cell proliferation and secondary cell wall thickening, highlighting the complex regulatory networks involved in wood formation (Figure 2) (Tang et al., 2019). Figure 1VCM1and VCM2expression and phenotypes of their transgenic lines (Adopted from Zheng et al., 2020) Image caption: (A and B) qRT-PCR analysis of VCM1 (A) andVCM2 (B) expression in different tissues. Values are means ± SD (n = 3 biological replicates). SMT, shoot meristem tissue; YL, young leaf; IN, internode; YT, young roots. (C and D) GUS activity in transgenic Populus driven by the VCM1 promoter (C) and the VCM2 promoter (D). Ca, cambium; Xy, xylem; Ph, phloem. Scale bar corresponds to 100 μm. (E) Phenotypes of the wild-type (WT), VCM1 and VCM2 knock-down lines, (DR4 and DR15) and VCM1 overexpression lines (UR13 and UR25). Scale bar corresponds to 10 cm. (F) qRT-PCR analysis of VCM1 and VCM2 expression in transgenics. Values are means ± SD (n = 3 biological replicates). (G–I) Cross-section of the 13th internode stained with toluidine blue (above) and enlarged rectangle area (below). (G) WT; (H) VCM1 and VCM2 knock-down lines; (I), VCM1 overexpression lines. Xy, xylem; Ph, phloem; Ca, cambium. Scale bars correspond to 200 μm (50 μm in enlargements). (J) Number of cambium cell layers (n> 50). (K) Number of xylem cell layers (n > 50). Plants used for analysis were grown in a phytotron for 3 months. Values are means ± SD. Statistical significance of differences was calculated based on two-tailed, two-sample Student's t-test (**P < 0.01, *P < 0.05) (Adopted from Zheng et al., 2020)
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