Abstract
Brassinosteroids (BRs) are known to be essential regulators for wood formation in herbaceous plants and poplar, but their roles in secondary growth and xylem development are still not well-defined, especially in pines. Here, we treated Pinus massoniana seedlings with different concentrations of exogenous BRs, and assayed the effects on plant growth, xylem development, endogenous phytohormone contents and gene expression within stems. Application of exogenous BR resulted in improving development of xylem more than phloem, and promoting xylem development in a dosage-dependent manner in a certain concentration rage. Endogenous hormone determination showed that BR may interact with other phytohormones in regulating xylem development. RNA-seq analysis revealed that some conventional phenylpropanoid biosynthesis- or lignin synthesis-related genes were downregulated, but the lignin content was elevated, suggesting that new lignin synthesis pathways or other cell wall components should be activated by BR treatment in P. massoniana. The results presented here reveal the foundational role of BRs in regulating plant secondary growth, and provide the basis for understanding molecular mechanisms of xylem development in P. massoniana.
Highlights
Wood or secondary xylem is a vascular tissue for water conduction, and for mechanical support
To determine whether exogenous BR affect normal growth and xylem development of P. massoniana, seedlings were treated with spray-applied BR for four months with different concentrations
Our results indicated that exogenous BR appeared to affect development of xylem more than phloem, and promoted xylem development in a dosage-dependent manner in a certain concentration rage
Summary
Wood or secondary xylem is a vascular tissue for water conduction, and for mechanical support. Wood formation involves a cascade of complex and dynamic biological processes, including cell expansion, secondary cell wall (SCW) deposition and programmed cell death (PCD) [1]. SCW deposition produces the wood biomass, which is mainly composed of lignin, cellulose and hemicellulose. Understanding the mechanisms of secondary xylem development could raise valuable information to further increase the production of wood biomass. One of the important insights is that phytohormones play key roles in the development of xylem [2,3]. The signal transduction pathways for some phytohormones are quite well-documented, such as brassinosteroid (BR), gibberellin (GA) and auxin, whereas the molecular mechanisms of their regulating xylem development are not fully revealed [3,4]
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