The main goal of this project was to further characterize two MRP-type ABC transporters involved in stomata regulation. In a previous work we demonstrated that in knock-out mutants for AtMRP5 stomata function is impaired and that guard cells only partially respond to Ca2+ and ABA. In this project we localized AtMRP5 to the plasma membrane and could show that the AtMRP5-GFP construct was able to restore the wild-type phenotype. Using the patch clamp technique we were able to demonstrate that the anion channel, but not the potassium channels are affected by the absence of AtMRP5. AtMRP4 exhibits an opposite phenotype to AtMRP5, since deletion mutants for this ABC transporter exhibit an increased transpiration also exhibit a faster stomata opening. Transport analysis with methotrexate revealed that AtMRP4 expressed in yeast is able to efficiently transport this toxic folate analog. In order to see whether there is some interaction between AtMRP4 and other AtMRPs we have generated the atmrp5 x atmrp4 and atmrp4 x atmrp14 double knock-outs to investigate the resulting phenotype.