PalWRKY40 negatively modulates Pi content in P. alba var. pyramidalis. A, Phenotype of PalWRKY40 overexpressing poplars exposed to low-Pi (10 μM Pi) treatment for one month. B and C, Pi and anthocyanin contents of PalWRKY40 overexpressing poplars before and after treatment. Error bars represent SD (n ≥ 6). Different letters above bars represent statistically significant differences between groups (P < 0.05) as determined by one-way ANOVA Duncan’s test. D, Overexpression of PalWRKY40 increased tolerance of Pi toxicity. PalWRKY40-OE and WT plants grown for two months in high Pi conditions generated by watering with 10-mM KH₂PO₄ solution every 5 days. Plants watered with ddH₂O acted as control. Scale bar = 7 cm. E and F, Pi content and plant height of PalWRKY40 overexpressing and WT poplars after 2 months of treatment. Error bars represent SD values (n ≥ 4). Different letters above bars represent statistically significant differences between groups (P < 0.05) as determined by one-way ANOVA Duncan’s test.