Technical Abstract: Plants, as sessile organisms, employ multiple mechanisms to adapt to the seasonal and daily temperature fluctuations associated with their habitats. Here, we provide evidence that the FtsH11 protease of Arabidopsis contributes to the plants¿ overall tolerance to elevated temperatures. To identify the various mechanisms of thermotolerance in plants, we have isolated a series of Arabidopsis thaliana thermo-sensitive mutants (atts) that failed to acquire thermotolerance after a pre-conditioning at 38ºC. Two allelic mutants, atts244/atts405, were not only defective in acquired thermotolerance but also highly susceptible to moderately elevated temperatures. Growth and development of the mutant plants at all stages examined were arrested after they were exposed to temperatures above 30ºC, which were permissive temperatures in wild-type plants. The gene was identified through map-based cloning to encode a chloroplast targeted FtsH (Filamentous temperature sensitive) protease, AtFtsH11. The Arabidopsis genome contains 12 predicted FtsH protease genes. All the previously characterized AtFtsHs play roles in alleviation of light stress through degradation of unassembled thylakoid membrane proteins and photodamaged PSII D1 protein. Photosynthetic capability, as measured by chlorophyll content, chl a/b ratios and PSII quantum yield, reduced greatly in leaves of AtFtsH11 mutants when they were exposed to a moderately high temperature of 30oC. Under high light conditions, however, no significant difference in the changes in photosynthesis capacity was observed between AtFtsH11 mutants and wild-type plants. Our results demonstrate that AtFtsH11 plays a critical role in thermotolerance in Arabidopsis.