**7. Conclusions**

ABC transporters, MATE transporters, MSTs, SUTs, and amino acid transporters are involved in the transport of ions, toxic compounds, sugars, hormones, and amino acids. The transport of these molecules is important in the growth, development, and stress adaptations of plants. Since these transporters are driven by proton motive force, the activities of these transporters are highly regulated by cellular pH, which is usually influenced by both abiotic and biotic stresses. Besides directly participating in the transport activity by providing the proton motive force, pH also influences the charges of the substrates to be transported. In addition, the pH itself could possibly affect the transporter function. These factors together affect the transport efficiency. The role of the proton in molecular transports deserves more attention than it has received thus far. In-depth structural and mechanical studies will be needed to delineate the effects of pH, proton gradient, and electrical potential gradient on the transporter functions.

**Author Contributions:** Y.-S.K. put together the first complete draft and prepared the final version. H.-M.L. planned and coordinated the writing and prepared the final version. Y.-S.K., S.-S.C., M.-S.N., G.C. and H.-M.L. contributed to the search of the literature and writing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by grants from the Hong Kong Research Grants Council: General Research Fund (14143916) and Area of Excellence Scheme (AoE/M-403/16).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Jee-Yan Chu copy-edited the manuscript. Any opinions, findings, conclusions or recommendations expressed in this publication do not reflect the views of the Government of the Hong Kong Special Administrative Region or the Innovation and Technology Commission.

**Conflicts of Interest:** Authors declare no conflict of interest.
