Cytosolic/nuclear molecular chaperones of the heat shock protein families HSP90 and HSC70 are conserved and essential proteins in eukaryotes. These proteins have essentially been implicated in the innate immunity and abiotic stress tolerance in higher plants. Here, we demonstrate that both chaperones are recruited in Arabidopsis (Arabidopsis thaliana) for stomatal closure induced by several environmental signals. Plants overexpressing HSC70-1 or with reduced HSP90.2 activity are compromised in the dark-, CO(2)-, flagellin 22 peptide-, and abscisic acid (ABA)-induced stomatal closure. HSC70-1 and HSP90 proteins are needed to establish basal expression levels of several ABA-responsive genes, suggesting that these chaperones might also be involved in ABA signaling events. Plants overexpressing HSC70-1 or with reduced HSP90.2 activity are hypersensitive to ABA in seed germination assays, suggesting that several chaperone complexes with distinct substrates might tune tissue-specific responses to ABA and the other biotic and abiotic stimuli studied. This study demonstrates that the HSC70/HSP90 machinery is important for stomatal closure and serves essential functions in plants to integrate signals from their biotic and abiotic environments.