Analysis of the salt tolerance of the birch lines overexpressing BpTCP20²⁵⁹ (K²⁵⁹). A) The expression levels of BpTCP20²⁵⁹ in 10 K²⁵⁹ lines. Gene expression data were transformed by log2, and the value of the wild-type (WT) was set as 0: >0 indicates the expression increased compared with that in the WT plants. B) Analysis of the growth of OE (overexpressing BpTCP20), K²⁵⁹ (overexpressing BpTCP20²⁵⁹), and WT plants under control or salt stress conditions. Scale bar = 5 cm. C and D) Comparison of the plant height C) and fresh weight D) of all lines. E) Stomata aperture analysis. Scale bar = 20 μm. F) The ratio of the length of the stomata aperture. G) The water loss rate analysis. H–J) Analysis of Evans blue staining H), scale bar = 1 cm, electrolyte leakage rates I), MDA contents J) of the OE, K²⁵⁹ and WT. OE: birch plants overexpressing BpTCP20; K²⁵⁹: birch plants overexpressing BpTCP20²⁵⁹. WT: wild-type birch. Control: the plants were grown under normal growth conditions. NaCl: treated with 200 mM NaCl. Rehydration: after salt stress, the plants were watered well for 15 d. The error bar represents the SD of three biological replicates. * indicates statistically significant differences at P < 0.05, ** indicates statistically significant differences at P < 0.01 compared with the WT plants.