Figure 1. Xylemin promotes xylem differentiation by the inhibition of thermospermine synthesis.

(a) Biosynthetic pathway of polyamines in plants. (b) Effects of xylemin on the levels of spermine (Spm) and thermospermine (Tspm). The 7-day-old wild-type seedlings of Arabidopsis were transferred to the liquid medium with 10 μM spermidine (-Xylemin) or with 10 μM spermidine plus 30 μM xylemin (+Xylemin) and grown for 24 hours. The levels of polyamines were analyzed by HPLC. Data are displayed as averages ± SD (n = 3). Asterisk indicates significant differences from the value in the mock treatment (Student t-test, P < 0.01). (c) Effect of various concentrations of xylemin on xylem differentiation in hypocotyls. The wild type seedlings were germinated and grown for 7 days in the liquid medium with different concentrations of xylemin. Scale bar: 50 μm. (d) Shoot morphology and xylem differentiation in inflorescence stems of wild-type (WT) and acl5-1 plants treated with or without xylemin. Xylemin solution at the concentration of 100 μM was daily applied to each shoot apex. Xylem is indicated as the area enclosed by yellow dashed line. IF: interfascicular fiber. Scale bars: 1 cm (upper panels); 100 μm (middle and lower panels). (e) Response of gene expression to xylemin. Total RNA was isolated from the wild type seedlings grown for 7 days in the liquid medium and treated for 2 or 24 hours in the medium supplemented with 50 μM xylemin. All transcript levels are relative to those in mock-treated seedlings. Data are displayed as averages ± SD (n = 3). Different letters indicate Tukey’s HSD groupings (P < 0.05). (f) Effect of various concentrations of xylemin on expression of ACL5 and SPMS. Total RNA was isolated from the wild type seedlings grown for 7 days in the liquid medium with different concentrations of xylemin. All transcript levels are relative to mock controls. Data are displayed as averages ± SD (n = 3). Different letters indicate Tukey’s HSD groupings (P < 0.05).