Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 155-165 http://genbreedpublisher.com/index.php/tgmb 157 Figure 1 Subcellular localization and activity patterns of candidate auxin receptors (Adopted from Yu et al., 2022) Image caption: During auxin-driven cell cycle progression, the nuclear S-PHASE KINASE-ASSOCIATED PROTEIN 2a (SKP2a) binds to indole-3-acetic acid (IAA) to enhance its interaction with G1-to-S checkpoint regulators, E2FC, and its dimerization partner DPB, leading to their degradation via the 26S proteasome and accelerating cell division. ETT/ARF3 (AUXIN RESPONSE FACTOR 3), which lacks the Phox/Bem1 (PB1) domain necessary for interaction with Aux/IAAs, follows a distinct auxin signaling pathway: in low auxin conditions, ETT/ARF3 represses auxin-responsive genes by recruiting TOPLESS (TPL)/TPL-RELATED PROTEINs (TPRs) and HISTONE DEACETYLASE 19 (HDA19). Elevated auxin levels disrupt this complex, enabling transcription. Auxin also serves as a “molecular glue” in canonical signaling, facilitating the interaction between TRANSPORT INHIBITOR RESPONSE 1 (TIR1) and AUXIN SIGNALING F-boxes (AFBs) with Aux/IAAs, leading to their degradation and subsequent activation of ARF transcription factors. The role of AUXIN BINDING PROTEIN 1 (ABP1) in auxin signaling, potentially involving its secretion and interaction with TRANSMEMBRANE KINASE 1 (TMK1), remains speculative (Adapted from Yu et al., 2022) 3.3 Downstream signaling cascades and gene expression Upon the degradation of Aux/IAA proteins, ARFs are released to bind to auxin response elements in the promoters of target genes, thereby modulating their expression. This regulation of gene expression is essential for various developmental processes, including cell division, elongation, and differentiation. Recent studies have also highlighted the role of microRNAs in fine-tuning the auxin signaling pathway, adding another layer of complexity to the regulation of gene expression (Gallei et al., 2019; Gomes and Scortecci, 2021). 3.4 Crosstalk with other hormonal signaling pathways Auxin signaling does not operate in isolation but interacts with other hormonal pathways, such as gibberellins (GA), to coordinate plant growth and development. For instance, the interaction between ARF and DELLA proteins mediates crosstalk between auxin and GA signaling pathways, which is crucial for processes like fruit initiation and development. This crosstalk ensures that the hormonal signals are integrated to produce a coherent response to environmental and developmental cues (Zhou et al., 2020; He and Yamamuro, 2022; Li et al., 2022). 4 Auxin and Branch Differentiation in Fruit Trees 4.1 Mechanisms of auxin-induced branching Auxin plays a pivotal role in the regulation of branch differentiation in fruit trees by influencing various hormonal and genetic pathways. The hormone auxin, primarily synthesized in the shoot apex, is transported basipetally and regulates the growth and development of lateral buds. This transport mechanism is crucial for maintaining apical dominance, where the main shoot apex suppresses the growth of lateral buds (Holalu et al., 2020). Additionally, auxin interacts with other hormones such as cytokinins (CKs) and strigolactones (SLs) to modulate bud outgrowth. For instance, auxin depletion after decapitation can lead to an increase in CK levels, promoting bud growth, while SLs can inhibit this process (Cao et al., 2023). Furthermore, auxin's role in bud outgrowth is also influenced by sugars, which act in concert with CKs but antagonistically to SLs, highlighting a complex network of hormonal interactions (Kotov et al., 2021).
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