Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 155-165 http://genbreedpublisher.com/index.php/tgmb 156 2 Auxin Biosynthesis and Transport 2.1 Pathways of auxin biosynthesis Auxin, primarily indole-3-acetic acid (IAA), is synthesized through multiple pathways, both tryptophan-dependent and tryptophan-independent. The tryptophan-dependent pathways involve several intermediate compounds such as indole-3-acetamide, indole-3-acetaldoxime, indole-3-pyruvic acid, and tryptamine. Key enzymes in these pathways include YUCCA (YUC) flavin monooxygenases and TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA) (Gomes and Scortecci, 2021; Liu et al., 2023). These enzymes catalyze the conversion of tryptophan to IAA through a series of biochemical reactions. The regulation of these biosynthetic pathways is crucial for maintaining appropriate auxin levels, which in turn influence various developmental processes in plants, including fruit tree branch differentiation. 2.2 Mechanisms of auxin transport within plant tissues Auxin transport is a highly regulated process that involves both influx and efflux carrier proteins. The polar nature of auxin transport is mediated by the AUXIN1/LIKE-AUX1 (AUX1/LAX) family of influx carriers and the PIN-FORMED (PIN) family of efflux carriers, along with P-GLYCOPROTEIN/ATP-BINDING CASSETTE B (PGP/ABCB) transporters (Swarup and Bhosale, 2019; Hammes et al., 2021). These transporters facilitate the directional movement of auxin from the site of synthesis to target tissues, ensuring the formation of auxin gradients essential for plant development. The PIN proteins, in particular, are known for their role in establishing auxin maxima and minima, which are critical for processes such as embryogenesis, organogenesis, and vascular patterning (Swarup and Bhosale, 2019; Hammes et al., 2021). 2.3 Regulation of auxin distribution in fruit trees The distribution of auxin within fruit trees is tightly regulated by both biosynthesis and transport mechanisms. Environmental cues such as light, temperature, and nutrient availability can influence auxin biosynthesis and transport, thereby affecting the overall growth and development of the plant (Mroue et al., 2018). For instance, the NRT1.1 nitrate transceptor in Arabidopsis has been shown to coordinate auxin biosynthesis and transport in response to nitrate availability, which in turn regulates root branching (Maghiaoui et al., 2020). Similarly, in fruit trees, the dynamic distribution of auxin is crucial for branch differentiation and fruit development. The interplay between auxin and other phytohormones, such as gibberellic acid (GA), further modulates these processes, highlighting the complexity of hormonal regulation in plant development (He and Yamamuro, 2022). 3 Auxin Signal Transduction Pathway Auxin, a key phytohormone, plays a crucial role in regulating plant growth and development through a complex signal transduction pathway. This pathway involves multiple components, including receptors, transcription factors, and downstream signaling cascades, which together modulate gene expression and interact with other hormonal signaling pathways. 3.1 Components of the auxin signaling pathway The auxin signaling pathway is primarily mediated by the interaction between auxin response factors (ARFs) and auxin/indole-3-acetic acid (Aux/IAA) proteins. ARFs are transcription factors that bind to auxin response elements in the promoters of auxin-responsive genes, while Aux/IAA proteins act as repressors of ARF activity. The pathway also involves the F-box proteins TIR1/AFBs, which function as auxin receptors and facilitate the degradation of Aux/IAA proteins, thereby releasing ARFs to activate gene expression (Gomes and Scortecci, 2021; Li et al., 2022). 3.2 The role of auxin receptors in signal perception Auxin perception is primarily mediated by the TIR1/AFB family of F-box proteins, which function as auxin receptors. These receptors form part of the SCF (SKP1-CUL1-F-box) ubiquitin ligase complex, which targets Aux/IAA proteins for ubiquitination and subsequent degradation. The binding of auxin to TIR1/AFB enhances the interaction between TIR1/AFB and Aux/IAA proteins, leading to the degradation of Aux/IAA and the activation of ARFs. This process is crucial for the regulation of gene expression in response to auxin (Figure 1) (Gomes and Scortecci, 2021; Yu et al., 2022).
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