Ectopic over-expression of four PtoWOXs promotes regeneration of AR. (A–B, E, and F) Regenerated ARs in transgenic poplar plants ectopically expressing (A) PtoWUSa, (B) PtoWOX4a, (E) PtoWOX5a, and (F) PtoWOX11/12a. Wild type and over-expressing lines are on the left and right of each panel, respectively. (C–D, G, and H) Statistical result of ARs in different transgenic lines. No. of AR: adventitious root numbers; bars in A, B, E, and F represent 1 cm. ** in C, D, G, and H indicates significant difference using LSD test at P ≤0.01.