Tree Genetics and Molecular Breeding 2024, Vol.14, No.3, 155-165 http://genbreedpublisher.com/index.php/tgmb 159 Wei et al. (2022) investigates how high auxin concentrations influence flower bud break after defoliation. Using 1-naphthaleneacetic acid (NAA) treatment on mature shoots, the research found that high auxin levels significantly inhibit bud break, as evidenced by reduced bud development and suppressed flower bud primordia formation. The study also reveals that high auxin concentrations disrupt the expression of genes related to auxin metabolism, transport, and signaling, which are typically upregulated following defoliation. Notably, the inhibition of PpyPIN1b protein expression is identified as a key factor in preventing auxin efflux from buds, leading to the suppression of bud break. These findings underscore the role of auxin in regulating developmental processes and suggest that manipulating auxin levels could control flowering in horticultural practices. 4.4 Comparative analysis of auxin-mediated branching in different fruit tree species Auxin-mediated branching mechanisms exhibit both conserved and species-specific features across different fruit tree species. In pear trees, early defoliation-induced auxin redistribution accelerates bud paradormancy release, whereas in longan, auxin signal transduction is crucial for the growth and maturity of autumn shoots, influencing floral bud induction (Liang et al., 2022; Wei et al., 2022). Additionally, in Chinese fir, auxin plays a key role in inhibiting bud outgrowth by modulating the biosynthesis of secondary messengers such as CKs, GAs, and abscisic acid (ABA), thereby inducing paradormancy of axillary buds (Yang et al., 2022). These examples illustrate the diverse yet interconnected roles of auxin in regulating branching across different fruit tree species, emphasizing the hormone's central role in plant architecture and development. 5 Environmental Factors Influencing Auxin-Mediated Branching 5.1 Light and photoperiod effects on auxin distribution Light is a critical environmental factor that influences auxin distribution and, consequently, the branching patterns in fruit trees. Light-regulated auxin signaling plays a significant role in various developmental processes, including root and shoot development. Phytochromes (PHYs), cryptochromes (CRYs), phototropins (PHOTs), and phytochrome-interacting factors (PIFs) are some of the light-response components that regulate auxin-mediated growth. These components affect the primary root, lateral root, adventitious root, and root hair development through the auxin signaling transduction pathway (Yun et al., 2023). The interaction between light and auxin signaling is complex and varies among different plant species, such as barley and wheat, highlighting the importance of light in modulating auxin levels and distribution, which in turn affects branching patterns. 5.2 Impact of nutrient availability on auxin-induced branching Nutrient availability is another crucial factor that influences auxin-induced branching in fruit trees. Auxin production and distribution are tightly regulated by the plant's nutritional status. Nutrients such as nitrogen, phosphorus, and potassium can affect auxin biosynthesis and transport, thereby influencing the plant's growth and branching patterns. The integration of nutrient signals with auxin signaling pathways ensures that the plant can adapt its growth and development to the availability of essential nutrients. This fine-tuning mechanism allows plants to optimize their branching architecture for better resource acquisition and overall fitness (Mroue et al., 2018). 5.3 The role of temperature and water stress on auxin signaling Temperature and water stress are significant environmental factors that impact auxin signaling and, consequently, the branching of fruit trees. Plants have evolved complex signaling networks to integrate these environmental cues and modulate their growth and development accordingly. Auxin plays a pivotal role as an integrator of these signals, helping plants to adapt to varying temperature and water conditions. For instance, temperature fluctuations can affect auxin biosynthesis, metabolism, and transport, leading to changes in auxin gradients that influence branching patterns. Similarly, water stress can alter auxin distribution, affecting cell expansion, division, and tissue specification, which are critical for branch formation and growth (Mroue et al., 2018). Understanding the interplay between temperature, water stress, and auxin signaling is essential for developing strategies to improve the resilience and productivity of fruit trees under changing environmental conditions.
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