**5. Conclusions**

Genome-wide identification and evolutionary, gene structure, and expression analyses under drought stress of poplar *MORF* genes provide a deep insight into the gene family. Moreover, analyses of *MORF* genes based on qRT-PCR validation of poplar tissues provide functional information. Our results provide an important resource for advancing our understanding of the roles of *MORF* genes in poplar.

**Supplementary Materials:** Supplementary materials can be found at http://www.mdpi.com/1422-0067/20/6/ 1425/s1.

**Author Contributions:** D.W. and S.M. designed and conducted all of the experiments. X.X., C.L. and W.Y. conceived the project. W.S., Y.B. and Y.L. helped perform the experiments. D.W. contributed to analyze the experimental results. D.W., S.M., C.L. and X.X. wrote the paper.

**Funding:** This research was sponsored by grants from the National Key Program on Transgenic Research (2018ZX08021001) and the National Natural Science Foundation of China (31770649, 31570308, and 31600484).

**Acknowledgments:** We thank Jinbu Jia for bioinformatics analysis, experimental design, and useful discussion.

**Conflicts of Interest:** The authors declare no conflict of interest.
