PtrWRKY75-overexpressing transgenic poplar showed improved drought tolerance under long-term drought stress
To further explore the function of PtrWRKY75 in long-term drought stress, the OE lines and WT were subjected to 35 days of soil RWC at 70% (control) or 20% (drought). On day 35, the OE lines showed better growth than that of WT (Figure 6A). Compared with WT, the OE lines had a higher leaf RWC, indicative of stronger drought tolerance (Figure 6B). The OE lines and WT showed no significant differences in photosynthetic rate in the control (70% soil RWC), but the OE lines had a higher photosynthetic rate under drought conditions (Figure 6C).
To investigate the difference in growth between OE lines and WT, we monitored the plant shoot elongation rate and biomass under well-watered and drought conditions. The shoot elongation rate was significantly higher in OE lines than in WT under 20% RWC, but not significantly different between OE lines and WT under well-watered conditions (Figure 6D). After long-term drought treatment, consistent with the photosynthetic rate, the biomass accumulation was higher in the two OE lines (by 35.003% and 46.32%, respectively) than in WT under low soil RWC conditions, but not significantly different between the OE lines and WT under well-watered conditions (Figure 6E).
The chlorophyll a , chlorophyll b , and total chlorophyll contents were not significantly different between OE lines and WT under 70% soil RWC, but were significantly higher in the OE lines than in WT under long-term drought conditions (Figure S3A–C). This result indicated that the OE plants showed a better ability to absorb light energy compared with WT, and thus maintained a higher photosynthetic rate under long-term drought conditions. Compared with WT, the OE lines also showed higher maximal photosystem II (PSII) quantum yield (F v/F m) after long-term drought treatment (Figure S3D).Therefore, overexpression ofPtrWRKY7 5 was beneficial for plant growth under long-term drought conditions.