Abstract:

Objective: Auxin plays an important regulatory role in plant growth and development via its signal transduction. Auxin receptor is one of the most critical components in auxin signaling pathway. Therefore, it is of great significance to study the roles of auxin receptor family gene (FBL) in the growth and development of poplar to reveal the mechanism of auxin in the secondary growth of woody plants. In this study, the sequence structure, tissue specific expression and subcellular localization of PagFBL3 in poplar ‘84K’ (Populus alba × P. glandulosa) were investigated to exlore the effects of PagFBL3 on stem secondary growth by creating transgenic poplar overexpressing PagFBL3, so as to provide support in further elucidating the mechanism of action of auxin receptors in secondary stem growth. Method: The bioinformatics methods and related software were used to analyze the phylogenetic relationship, sequence similarity and biochemical characteristics. PagFBL3 gene was cloned and identified from 84K poplar, and its expression levels in roots, young leaves, mature leaves, young stems, vascular cambium, xylem and phloem were detected by real-time quantitative PCR. The fusion expression vector 35S::PagFBL3-GFP was constructed using the Gateway technique, and the subcellular localization of PagFBL3 was analyzed by transient transformation of tobacco leaf epidermal cells. At the same time, the coding region of PagFBL3 was recombined into the PMDC32 vector to construct the plant overexpression vector 35S::PagFBL3. The transgenic poplar was created by Agrobacterium-mediated transformation method. The effects of overexpressing PagFBL3 gene on growth and development of transgenic poplar stem were analyzed by phenotype and tissue sections assays. Result: PagFBL3 gene was able to encode 571 amino acid residues and its encoded protein was localized in the nucleus. PagFBL3 gene was expressed in roots, stems and leaves, and was highly expressed in mature leaves and secondary stems. Based on resistance screening and molecular identification, 11 transgenic poplar lines overexpressing PagFBL3 were generated, and two transgenic lines with high expression levels were selected for further functional analysis. Compared with non-transgenic 84K poplar plants, overexpression of PagFBL3 gene promoted xylem development, significantly increased xylem width of transgenic poplars, and improved stem radial growth. The two transgenic lines showed 6.7% and 8.5% increase in ground diameter, and 17.3% and 19.9% increase in the xylem width of the fifteen internodes, respectively. In addition, overexpression of PagFBL3 gene promoted plant height of transgenic poplar. The two transgenic lines showed 6.3% and 6.9% increase in stem height, respectively. Conclusion: PagFBL3 gene is mainly expressed in mature leaves and secondary stems and plays a role in regulating the radial growth of poplar by affecting xylem development and improving xylem width. This study provides a reference for further revealing the molecular mechanism of PagFBL3 gene involved in the stem growth and development of poplars.

Key words: Populus alba × P. glandulosa, auxin receptor, PagFBL3, overexpression, xylem development.