Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 155-165 http://genbreedpublisher.com/index.php/tgmb 158 4.2 Influence of auxin on apical dominance and lateral bud formation Apical dominance is a well-documented phenomenon where the presence of an active shoot apex inhibits the growth of lateral buds. This process is largely mediated by auxin, which is produced in the shoot apex and transported down the stem, suppressing lateral bud growth (Holalu et al., 2020). The suppression of bud growth is not solely due to auxin levels but also involves the enhanced expression of auxin-induced genes, which increase the sensitivity of buds to auxin (Holalu et al., 2020). Additionally, auxin's interaction with other hormones such as CKs and SLs plays a significant role in this process. For example, SLs have been shown to suppress branching after decapitation, although their effect is less pronounced when CKs or sugars are abundant (Cao et al., 2023). This intricate balance between auxin and other hormones ensures the proper regulation of apical dominance and lateral bud formation. 4.3 Role of auxin in the timing and patterning of branching The timing and patterning of branching in fruit trees are tightly regulated by auxin through its influence on gene expression and hormonal interactions. Auxin's role in branch patterning is evident in its ability to coordinate the transcriptional reprogramming of auxin-responsive genes, which are crucial for branch initiation and outgrowth (Thelander et al., 2022). In pear trees, for instance, early defoliation leads to auxin redistribution, promoting paradormancy release and out-of-season blooming, which negatively affects fruit production (Figure 2) (Wei et al., 2022). This redistribution of auxin involves changes in auxin metabolism, transport, and signal transduction pathways, highlighting the hormone’s role in the precise timing and patterning of branching (Wei et al., 2022). Moreover, auxin's interaction with other hormones such as gibberellins (GAs) further influences the timing of bud release and sustained growth (Cao et al., 2023). Figure 2 Inhibitory effects of a high auxin concentration on flower bud break after defoliation and expression patterns of hormone-related genes after an NAA treatment (Adopted from Wei et al., 2022) Image caption: A, Bud break percentage of current-year long shoots after a 300 mg/L NAA treatment. B, Representative images of bud break on day 20 on defoliated (left) and NAA-treated current-year long shoots (right). Scale bars, 5 cm. C and D, Scanning electron micrographs of flower buds of current-year long shoots on day 5 in the control group (C) and the NAA-treated group (D). pp, petal primordium; triangles, stamen primordium; and asterisks, pistil primordium. E and F, Expression patterns of genes related to auxin (E) and other hormones (F) after the NAA treatment. Error bars indicate the SEs of three biological replicates and asterisks indicate significant differences between control (defoliated) and NAA-treated branches (Student’s t test). *P < 0.05, **P < 0.01, ***P < 0.001 (Adopted from Wei et al., 2022)
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