Abstract
Although phytohormones are known to be important signal molecules involved in wood formation, their roles are still largely unclear. Here, Populus simonii × P. nigra seedlings were treated with different concentrations of exogenous phytohormones, indole-3-acetic acid (IAA), gibberellin (GA3), and brassinosteroid (BR), and the effects of phytohormones on growth were investigated. Next, 27 genes with known roles in wood formation were selected for qPCR analysis to determine tissue-specificity and timing of responses to phytohormone treatments. Compared to the control, most IAA, GA3, and BR concentrations significantly increased seedling height. Meanwhile, IAA induced significant seedling stem diameter and cellulose content increases that peaked at 3 and 30 mg·L−1, respectively. Significant increase in cellulose content was also observed in seedlings treated with 100 mg·L−1 GA3. Neither stem diameter nor cellulose content of seedlings were affected by BR treatment significantly, although slight effects were observed. Anatomical measurements demonstrated improved xylem, but not phloem, development in IAA- and BR-treated seedlings. Most gene expression patterns induced by IAA, GA3, and BR differed among tissues. Many IAA response genes were also regulated by GA3, while BR-induced transcription was weaker and slower in Populus than for IAA and GA3. These results reveal the roles played by phytohormones in plant growth and lay the foundation for exploring molecular regulatory mechanisms of wood formation in Populus.
Highlights
Wood, or secondary xylem, is a water-conductive and supportive vascular tissue that is predominantly found in trees
We investigated the effects of indole-3-acetic acid (IAA), GA3, and BR on growth of Xiaohei poplars (Populus simonii × P. nigra), including effects on plant height, stem diameter, cellulose content and xylem development
Because phosphorylation dynamics of PIN proteins have been shown to be affected by protein phosphatase 2A (PP2A) and PINOID kinase, which act antagonistically to mediate their apical-basal polar delivery [61,62], we looked for similar effects in the present study
Summary
Secondary xylem, is a water-conductive and supportive vascular tissue that is predominantly found in trees. Wood has uses in construction, pulp, and paper production and in the future will likely play a major role in production of biofuels—a renewable, cost-effective alternative to fossil fuels [1] The formation of both primary and secondary xylem involves a cascade of interesting processes, including arrangement of primary vascular tissue into bundles, cell proliferation within primary bundles or in secondary vascular cambium, initiation of xylem differentiation, regulation of cell expansion, deposition of a secondary cell wall, and programmed cell death [2]. Elucidation of mechanisms underlying the coordinated activation of secondary wall biosynthetic genes will undoubtedly augment our understanding with regard to the molecular regulation of wood formation [8]
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