The sessile nature of plants requires them to rapidly change physiological processes upon biotic and abiotic stress. Dynamic regulation of functions such as nutrient uptake, root or shoot growth, and stomata regulation can prove vital for survival. The plant plasma membrane (PM) H+-ATPase is in the center of these functions. During my PhD, I have studied modulation of H+-ATPase activity in response to metabolites from plant-associated fungi (biotic stress) and investigated the role of the H+-ATPase kinase, PSY1R, in response to blue light (abiotic stress). I have tested the effect of metabolites from plant-associated fungi: Alternata, Fusarium, Trichoderma, and Stemphylium on activity of the plant PM H+-ATPase. The screening process led to identification of metabolites from two fungi that changed plant H+-ATPase activity: Stemphylium loti and Trichoderma harzianum. A continuous cycle of chemical purification and testing in biological assays led to the identification of tenuazonic acid (TeA) and peptaibols, from S. loti and T. harzianum, respectively.The metabolites extracted from S. loti severely inhibited H+-ATPase activity. The identified compound, TeA, inhibited both ATP hydrolysis and H+ pumping of the H+-ATPase in isolated PM fractions, and similarly, leaves pretreated with TeA were found to have reduced H+-ATPase activity. Furthermore,TeA was able to induce apoplastic alkalization in Arabidopsis thaliana expressing the pH biosensor, Apo-pHusion. TeA inhibition was much less pronounced in the yeast H+-ATPase and in plant H+-ATPases with mutated regulatory domains, suggesting a specific role of the regulatory domain in TeA-induced inhibition.T. harzianum metabolites were found to increase H+-ATPase activity. Further purification was complicated due to the presence of a large, heterogenous group of non-proteogenic peptides named peptaibols. We isolated two peptaibols from T. harzianum, trichorzins PA II and VI (tPAII and tPA VI), which were shown to increase ATP hydrolysis of the H+-ATPase in a vanadate-sensitive manner. The effect of tPA II and VI was compared to that of the model peptaibol, alamethicin. Further biochemical characterization led to the hypothesis that peptaibols increase plant H+-ATPase activity by integrating into the PM, thus relieving the lateral pressure of the membrane. The leucine-rich repeat receptor-like kinase, PSY1R, activates the plant PM H+-ATPase through direct interaction. Here, PSY1R was found to associate with the blue-light receptors, PHOT1 and 2. I studied blue light responses regulated by PHOT1 and 2, and found that phototropism was significantly impaired in PSY1R knock-out mutants (psy1r). PSY1R has previously been associated with light-regulated responses, however more evidence is needed to elucidate the exact connection.