Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 155-165 http://genbreedpublisher.com/index.php/tgmb 160 6 Case Study 6.1 Case study: auxin signal transduction in apple trees Auxin, a pivotal phytohormone, plays a crucial role in the growth and development of apple trees (Malus domestica). The auxin signaling pathway involves various components, including auxin response factors (ARFs) and small auxin-up RNA (SAUR) genes, which regulate diverse physiological processes such as fruit ripening, cell division, and differentiation. Recent studies have highlighted the complexity and significance of auxin signal transduction in apple trees, providing insights into its molecular mechanisms and potential applications in horticulture (Figure 3) (Gallei et al., 2019; Wang et al., 2020; Yue et al., 2020). Figure 3 Auxin-induced ethylene production and expression of ethylene-related genes in apple (Adopted from Yue et al., 2020) Image caption: (a–e) Apple fruits were harvested at 145 days after full bloom (DAFB) in 2018 and treated with naphthaleneacetic acid (NAA), 1-methylcyclopropene (1-MCP), or a combination of 1-MCP followed by NAA (1-MCP + NAA) (a). Fruits were stored at room temperature for 20 days and sampled every 5 days. Untreated fruits served as controls. Scale bars represent 1 cm. Ethylene production was measured (b), and the expression levels of ethylene-related genes MdACS3a (c), MdACS1 (d), and MdACO1 (e) were analyzed using quantitative reverse transcription PCR (qRT-PCR). (f–j) Apple fruits were harvested at 115 DAFB, treated with NAA (f), stored at room temperature for 20 days, and sampled every 5 days. Scale bars represent 1 cm. Ethylene production was measured (g), and the expression levels of MdACS3a (h), MdACS1 (i), and MdACO1 (j) were analyzed. Control fruits did not receive any treatment. Three independent biological experiments were conducted for each treatment group. Data are presented as means ± SE. Asterisks indicate significant differences determined by Student’s t-test (**, P < 0.01); ns, not significant (Adapted from Yue et al., 2020) Yue et al. (2020) examines the effect of auxin, specifically naphthaleneacetic acid (NAA), on ethylene production and the expression of ethylene-related genes in apple fruit. The research demonstrates that NAA treatment significantly enhances ethylene production and accelerates the expression of key ethylene biosynthesis genes (MdACS3a, MdACS1, and MdACO1) during apple ripening. This effect is observed even when fruit are treated with 1-methylcyclopropene (1-MCP), a compound known to inhibit ethylene production, indicating that auxin can override ethylene inhibition. The study highlights the critical role of auxin in regulating ethylene synthesis and suggests that manipulating auxin levels could be a potential strategy for controlling apple ripening and improving post-harvest fruit quality. 6.2 Study design To investigate the role of auxin signal transduction in apple trees, a comprehensive study was conducted involving the identification and expression analysis of key auxin-responsive genes. The study focused on the small auxin-up
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