PP7 is the first protein Ser/Thr phosphatase to be found to interact with calmodulin (CaM) in plants. The T-DNA insertion AtPP7 knockout line and AtPP7 overexpression lines were employed to study the specific function of AtPP7. The AtPP7 knockout impaired the thermotolerance of Arabidopsis seedlings while the overexpression of AtPP7 resulted in plants with increased thermotolerance. Results from real-time polymerase chain reaction (PCR) showed that the expression of AtHSP70 and AtHSP101 genes was up-regulated in AtPP7 overexpression lines after heat shock (HS) at 37 degrees C for 1 h. Protein gel blot analysis showed that HSP70 protein levels increased in AtPP7 overexpression lines after HS at 37 degrees C for 2 h. The expression of the AtPP7 gene was also induced by HS at 37 degrees C in wild-type Arabidopsis. Using a yeast two-hybrid screen, we showed an interaction between AtPP7 and CaM. In addition, we found that AtPP7 interacts with an HS transcription factor (HSF), suggesting a possible role for AtPP7 in regulating the expression of heat shock protein (HSP) genes.