Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 155-165 http://genbreedpublisher.com/index.php/tgmb 161 RNA (SAUR) gene family and the auxin response factor (ARF) transcription factors. Researchers employed quantitative real-time polymerase chain reaction (qRT-PCR) to analyze the expression levels of these genes in various tissues, including leaves, stems, and fruits, under different auxin treatments. Additionally, genetic transformation techniques were used to overexpress or silence specific auxin-responsive genes to observe their effects on apple fruit development and size (Wang et al., 2020; Yue et al., 2020; Iqbal et al., 2022). 6.3 Findings and analysis The study identified 80 MdSAUR genes in apple, with several showing increased expression levels in response to exogenous auxin treatment. Notably, MdSAUR4, MdSAUR22, MdSAUR37, MdSAUR38, MdSAUR49, and MdSAUR54 were significantly up-regulated, indicating their potential roles in the auxin signaling pathway (Wang et al., 2020). Furthermore, the overexpression of MdARF5, an auxin response factor, was found to induce the expression of ethylene biosynthetic genes, thereby promoting ethylene production and fruit ripening in apple (Yue et al., 2020). Another key finding was the role of MdAux/IAA2, a transcriptional repressor, in regulating cell and fruit size. Overexpression of MdAux/IAA2 resulted in smaller fruit size and reduced cell size, while its silencing led to increased fruit weight and cell size (Iqbal et al., 2022). These results underscore the intricate interplay between auxin and other phytohormones, such as ethylene, in regulating apple fruit development. The findings also highlight the importance of specific auxin-responsive genes in modulating key physiological processes, providing valuable insights into the molecular mechanisms underlying auxin signal transduction in apple trees (Wang et al., 2020; Yue et al., 2020; Iqbal et al., 2022). 6.4 Implications for practical applications in fruit tree cultivation The insights gained from this study have significant implications for practical applications in fruit tree cultivation. Understanding the molecular mechanisms of auxin signal transduction can inform strategies to enhance fruit quality and yield in apple orchards. For instance, manipulating the expression of specific auxin-responsive genes, such as MdARF5 and MdAux/IAA2, could be used to control fruit ripening and size, thereby improving marketability and consumer preference (Yue et al., 2020; Iqbal et al., 2022). Moreover, the identification of key SAUR genes involved in auxin signaling provides potential targets for genetic engineering to optimize growth and development in apple trees. By fine-tuning the expression of these genes, it may be possible to achieve desired traits such as increased fruit size, enhanced ripening, and improved stress tolerance, ultimately contributing to more sustainable and productive apple cultivation practices (Wang et al., 2020; Yue et al., 2020; Iqbal et al., 2022). 7 Advances in Manipulating Auxin Signaling for Improved Branching 7.1 Genetic engineering approaches Genetic engineering has emerged as a powerful tool to manipulate auxin signaling pathways for improved branching in fruit trees. By targeting key components of the auxin signal transduction pathway, researchers have been able to modulate plant architecture effectively. For instance, the manipulation of auxin response factors (ARFs) and auxin/indole-3-acetic acid (Aux/IAA) proteins, which are crucial regulators of auxin signaling, has shown promising results in altering plant growth patterns (Li et al., 2022). Advances in whole-genome sequencing have facilitated the identification of numerous ARF genes in various crops, including tomato, rice, and maize, providing a rich resource for genetic manipulation (Li et al., 2022). Additionally, the use of CRISPR/Cas9 technology to edit genes involved in auxin biosynthesis, transport, and signaling has opened new avenues for precise control over auxin-mediated processes (Wang et al., 2018). These genetic engineering approaches hold significant potential for enhancing branching and overall plant architecture, thereby improving fruit yield and quality. 7.2 Use of auxin analogues and inhibitors The application of auxin analogues and inhibitors represents another strategy to manipulate auxin signaling for improved branching. Auxin analogues, such as synthetic auxins, can mimic the action of natural auxins and are used to regulate plant growth and development. For example, the use of auxin analogues has been shown to promote cell division and expansion, which are critical for branch formation (He and Yamamuro, 2022). On the other hand, auxin inhibitors can be employed to block specific steps in the auxin signaling pathway, thereby
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