Manganese (Mn) is an essential catalytic metal in the Mn-cluster that oxidizes water to produce oxygen during photosynthesis.However,the transport protein(s) responsible for Mn2+ import into the chloroplast remains unknown.Here,we report the characterization ofArabidopsis CMT1 (Chloroplast Manganese Transporter 1),an evolutionarily conserved protein in the Uncharacterized Protein Family 0016 (UPF0016),that is required for manganese accumulation into the chloroplast.CMT1 is expressed primarily in green tissues,and its encoded product is localized in the inner envelope membrane of the chloroplast.Disruption of CMT1 in the T-DNA insertional mutant cmt1-1 resulted in stunted plant growth,defective thylakoid stacking,and severe reduction of photosystem Ⅱ complexes and photosynthetic activity.Consistent with reduced oxygen evolution capacity,the mutant chloroplasts contained less manganese than the wild-type ones.In support of its function as a Mn transporter,CMT1 protein supported the growth and enabled Mn1+ accumulation in the yeast cells of Mn2+-uptake deficient mutant (△smf1).Taken together,our results indicate that CMT1 functions as an inner envelope Mn transporter responsible for chloroplast Mn2+ uptake.