The Arabidopsis thaliana kas3 mutant was isolated based on the hypersensitivity of PSII to low temperature using a Chl fluorescence imaging technique. Chl content was lower in kas3 seedlings cultured at 23 degrees C than in the wild type, but PSII activity was only mildly affected. However, after the chilling treatment at 4 degrees C for 7 d, PSII activity was severely impaired in kas3. PSII was more sensitive to light at 4 degrees C in the presence of lincomycin, suggesting that the kas3 mutation accelerates at least the PSII photodamage. The kas3 mutation causes an amino acid alteration in 3-ketoacyl-ACP synthase III (KasIII), leading to the partial loss of the de novo synthesis pathway for fatty acids in plastids. Consequently, the total fatty acid level was reduced to 75% of the wild-type level in kas3 at 23 degrees C and was further reduced to 60% at 4 degrees C. The composition of fatty acids was also slightly affected in kas3 at both 4 and 23 degrees C. Consistent with the results of the electron transport analysis, the chilling treatment also destabilized PsaA and cytochrome (Cyt) f and D1 in kas3. An analysis of double mutants with pgr1 conditionally defective in Cyt b(6)f activity and with var2 defective in FtsH protease suggested that the kas3 mutation has pleiotropic effects on chloroplast function, probably impacting both the Cyt b(6)f activity and translation in chloroplasts at 23 degrees C. The full activity of KasIII is required for the biogenesis of the intact electron transport machinery in thylakoid membranes and is especially important for the process of responding to low temperature.