Tree Genetics and Molecular Breeding 2025, Vol.15, No.3, 117-127 http://genbreedpublisher.com/index.php/tgmb 118 particularly important for poplar asexual reproduction and response to environmental changes (Li et al., 2018; Xiao et al., 2020). The main roots and lateral roots grow naturally after the seeds germinate. The development of lateral roots is influenced by various factors, such as calcium signals, plant hormones and the external environment (Cai et al., 2019; An et al., 2023). Adventite roots can also grow flexibly under adverse conditions such as salt and diseases, helping trees survive better (Li et al., 2024; Zhang et al., 2024). 2.2 Anatomical traits and developmental stages In the initial stage, both the main root and lateral roots of poplar trees have clear root tip meridians, root crowns, cortex and vascular tissues. The formation of adventitious roots is often accompanied by cell division and differentiation, and is also regulated by hormone signals. Genes such as the WOX family, GLR3.3 and CML are highly expressed in the root apex region and can regulate root growth (Ahkami, 2023; Zhang et al., 2024). If the expression of these genes is artificially altered, the vascular tissue of the roots will be more developed and the xylem cells will increase, which can make the roots thicker and have more branches (Liu et al., 2022). The structure and growth of roots can also change with environmental variations under different conditions such as drought, nutrient deficiency or disease stress. 2.3 Natural variation among Populus species Poplar trees of different species and strains vary greatly in the morphology of their roots and their adaptability. Studies have found that poplar trees of different genotypes show significant differences in the number, length and root weight of advancements, and these traits are closely related to genomic variations such as SNPS and Indel (Zhang et al., 2024). For instance, the advanced-roots of the varieties P. × canadensis ‘Guariento’ and P. deltoides ‘Zhongcheng5’ grow particularly well and are very suitable for breeding and rapid propagation (Zhang et al., 2024). In addition, the performance of roots of different varieties varies when facing changes in water, soil structure and nutrients. For example, traits such as the distribution pattern of roots, the thickness of fine roots and the root length per unit weight will all be different (Frymark-Szymkowiak and Kieliszewska-Rokicka, 2023; Frymark-Szymkowiak et al., 2023). These natural differences provide excellent materials for studying the functional genomics and ecological adaptability of poplar root systems. 3 Hormonal Regulation of Root Development 3.1 Auxin biosynthesis, transport, and signaling Auxin plays a crucial role in the development of poplar roots, especially in the formation of adventite roots and lateral roots. Studies have found that genes such as PagFBL1 (TIR1 receptor) and Aux/IAA28 are highly expressed when poplar trees start to grow adventitious roots, which helps promote root formation and increase biomass (Shu et al., 2018). When PagFBL1 is overexpressed, more adventitious roots grow. On the contrary, if it is expressed less, the roots grow slowly, indicating that the auxin signal must be activated for the roots to develop normally. Furthermore, some auxin responsive-related genes, such as ARF, IAA14 and SAUR, are also activated at the beginning of advanced-root formation, thereby further promoting root growth (Cai et al., 2019; Zhang et al., 2023). Auxin transport direction (polar transport) is also important in lateral root formation, and changes in related genes can affect the shape and quantity of roots (Yao et al., 2023). 3.2 Crosstalk with cytokinin, ethylene, and abscisic acid In addition to auxin, the roots of poplar trees are also jointly regulated by a variety of plant hormones. For instance, Cytokinin inhibits the formation of adventite roots through factors such as PtRR13. If this signal is too strong, roots will not grow out. However, after pruning, its level will decline, which instead helps hormones such as auxin and ethylene to function better, thereby promoting root growth. The genes related to Ethylene are also activated at the beginning of adventitious root development, indicating that it is also involved in the regulatory process of roots (Zhang et al., 2023). Abscisic acid (ABA) directly affects root growth and regulates root development together with other hormones such as brassinolide (BR) and gibberellin (GA) through some small RNAs (miRNAs) (Lian et al., 2018). Gibberellin mainly exerts a countereffect by inhibiting the initiation process of lateral roots, that is to say, it reduces the number of lateral roots (Du et al., 2023).
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