Tree Genetics and Molecular Breeding 2025, Vol.15, No.3, 117-127 http://genbreedpublisher.com/index.php/tgmb 119 3.3 Molecular regulators involved in hormonal pathways In the process of regulating the growth of poplar roots, many important genes and regulatory modules also play a role. For instance, the PagFBL1-PagIAA28 module is a key component in auxin signals and can directly promote the growth of adventite roots (Shu et al., 2018). miR167a is a small RNA, and its target is ARF8. This combination is important for both adventing roots and lateral roots. When miR167a is expressed in large quantities, lateral roots will also grow more (Cai et al., 2019). Some miRNAs related to ABA, such as peu-miR-n68 and peu-miR477b, are also involved in the signal interaction between ABA and BR, GA, thereby regulating root growth (Lian et al., 2018). There is also the bHLH transcription factor PsPRE1, which can regulate the related genes of multiple hormones, including auxin, gibberellin and salicylic acid. This not only helps adventite root development, but also enhances the stress resistance of poplar (Du et al., 2023). The gene PtrXB38, which is regarded as an eQTL hotspot, also promotes adventitious root formation by regulating genes related to auxin transport and signaling pathways (Yao et al., 2023). 4 Key Genes and Transcription Factors in Root Development 4.1 WOX, LBD, ARF, and NAC families Some transcription factors have been found to be very important for root formation during the study of poplar root development. The WOX, LBD, ARF and NAC families are the most watched. Li et al. (2025) found that the expression of the ARF and LBD families would significantly decrease after exogenous addition of CLE13 signaling peptides, indicating that CLE signaling can regulate root growth and branching by influencing them. ARF plays a key role in the auxin signaling pathway and is directly involved in root differentiation and development (Yao et al., 2023). Nagle et al. (2022) hold that the NAC family is more extensively involved in cell differentiation, organ formation, and response to adverse conditions, exerting a considerable influence on the structure and function of roots. 4.2 Root meristem regulators and cell cycle genes The continuous growth of poplar roots requires that the meristem at the root tip remain active all the time. CLE signaling peptides and their receptors are crucial in this process. They can regulate the information exchange between cells and maintain the activity of meristem (Li et al., 2025). Nagle et al. (2022) discovered through GWAS and QTL studies that many genes related to cell division, structural regulation, and hormone signaling can affect the development of adventite roots. Yao et al. (2023) discovered that the gene PtrXB38 is a very important regulatory center. It can affect the transport and signal transmission of auxin, promote root formation and the activity level of meristem (Figure 1). 4.3 Genes controlling root elongation and branching The growth and branching of roots are crucial to the root structure and environmental adaptability of the entire tree. There is a gene called PsAAAP21, which belongs to the AAAP family. It can help form adventite roots and regulate the transport of amino acids such as tyrosine, methionine and arginine, thereby enhancing the growth ability of roots (Du et al., 2022). In addition, GWAS analysis also found that genes related to hormone signaling, cell division, and reactive oxygen species regulation would affect root elongation and branching (Nagle et al., 2022). Some genes, such as SWEET17, have variations that are also closely related to the developmental ability of roots (Zhang et al., 2024). These genes have different functions, but the regulatory network they form is very complex, jointly determining how poplar roots grow and how strong their ability to adapt to the environment is. 5 Functional Genomic Tools and Resources in Poplar 5.1 Genome sequencing and annotation databases In recent years, researchers have clarified the genomic structures of poplar varieties such as NL895 and 84K using PacBio high-fidelity sequencing and Hi-C technology (Qiu et al., 2019; Luo et al., 2024; Shi et al., 2024a). These new assembly results have distinguished the haplotypes of each chromosome and also enabled us to more accurately locate the gene positions and study the expression of different alleles. In addition to these genomic information, platforms like PoplarGene and PFGD have also been established, which include poplar gene function annotation, inter-gene interaction, expression level data, etc., and are very convenient to use.
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