Effects of exogenous 24-epibrassinolide and brassinazole on negative gravitropism and tension wood formation in hybrid poplar (Populus deltoids × Populus nigra)
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Exogenous 24-epibrassinolide (BL) and brassinazole (BRZ) have regulatory roles in G-fiber cell wall development and secondary xylem cell wall carbohydrate biosynthesis during tension wood formation in hybrid poplar.
Brassinosteroids (BRs) play important roles in regulating gravitropism and vasculature development. Here, we report the effect of brassinosteroids on negative gravitropism and G-fiber cell wall development of the stem in woody angiosperms. We applied exogenous 24-epibrassinolide (BL) or its biosynthesis inhibitor brassinazole (BRZ) to slanted hybrid poplar trees (Populus deltoids × Populus nigra) and measured the morphology of gravitropic stems, anatomy and chemistry of secondary cell wall. We furthermore analyzed the expression levels of auxin transport and cellulose biosynthetic genes after 24-epibrassinolide (BL) or brassinazole (BRZ) application. The BL-treated seedlings showed no negative gravitropism bending, whereas application of BRZ dramatically enhanced negative gravitropic bending. BL treatment stimulated secondary xylem fiber elongation and G-fiber formation on the upper side of stems but delayed G-fiber maturation. BRZ inhibited xylem fiber elongation but induced the production of more mature G-fibers on the upper side of stems. Wood chemistry analyses and immunolocalization demonstrated that BL and BRZ treatments increased the cellulose content and modified the deposition of cell wall carbohydrates including arabinose, galactose and rhamnose in the secondary xylem. The expression of cellulose biosynthetic genes, especially those related to cellulose microfibril deposition (PtFLA12 and PtCOBL4) was significantly upregulated in BL- and BRZ-treated TW stems compared with control stems. The significant differences of G-fibers development and negative gravitropism bending between 24-epibrassinolide (BL) and brassinazole (BRZ) application suggest that brassinosteroids are important for secondary xylem development during tension wood formation. Our findings provide potential insights into the mechanism by which BRs regulate G-fiber cell wall development to accomplish negative gravitropism in TW formation.
KeywordsBrassinosteroids Gelatinous layer Cellulose biosynthesis PtFLA12 PtCOBL4 Immunofluorescence Cell wall carbohydrates
This work was supported by the National Key Research and Development Program of China (No. 2017YFD0600201), the China Postdoctoral Science Foundation (No. 2016M601995), and the Anhui Province Postdoctoral Science Foundation (No. 2017B165). We are very grateful to Mr K. Liu for suggestions in identification of tension wood experimental design and manuscript writing. We would also like to thank Robbie Lewis for his assistance with language editing.
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