Plant aquaporins (AQPs) were initially characterised as a family of membrane-localised proteins facilitating water transport. However some plant AQPs exhibit diverse functionalities beyond water transport (Tyerman et al., 2021). The aquaporin AtPIP2;1, an abundant plasma membrane intrinsic protein in Arabidopsis thaliana, facilitates transport of univalent cations and water (Qiu et al. 2020; Byrt et al., 2017). Despite this, the mechanisms governing the selectivity of AtPIP2;1 for cations and water remain incompletely understood. We have shown that the regulation of water and cation transport via AtPIP2;1 is finely controlled by phosphorylation/dephosphorylation at four conserved serine residues within loop B, D, and the C-terminal domain. Concurrent modifications at these specific sites can serve as a 'selectivity switch,' modulating the preference between cations and water. When heterologously expressed in Xenopus laevis oocytes, AtPIP2;1 demonstrated selectivity of K+ > Rb+ > Cs+ > Na+ > Li+ > TEA > choline > NMDG. Significantly reduced K+ efflux from roots was observed in atpip2;1 mutants compared to wild-type controls, demonstrating AtPIP2;1's dual transport capacity in plants. These findings shed light on the intricate regulation of plant water transport and offer potential avenues for enhancing water use efficiency under varying water stress conditions.