The transition metal manganese (Mn) is indispensable for photoautotrophic growth since photosystem Ⅱ (PSⅡ) employs an inorganic Mn4CaO5 cluster for water splitting.Here,we show that the Arabidopsis membrane protein CHLOROPLAST MANGANESE TRANSPORTER1 (CMT1) is involved in chloroplast Mn homeostasis.CMT1 is the closest homolog of the previously characterized thylakoid Mn transporter PHOTOSYNTHESIS-AFFECTED MUTANT71 (PAM71).In contrast to PAM71,CMT1 resides at the chloro plast envelope and is ubiquitously expressed.Nonetheless,like PAM71,the expression of CMT1 can also alleviate the Mn-sensitive phenotype of yeast mutant △pmr1.The cmt1 mutant is severely suppressed in growth,chloroplast ultrastructure,and PSⅡ activity owing to a decrease in the amounts of pigments and thylakoid membrane proteins.The importance of CMT1 for chloroplast Mn homeostasis is demonstrated by the significant reduction in chloroplast Mn concentrations in cmt1-1,which exhibited reduced Mn binding in PSⅡ complexes.Moreover,CMT1 expression is downregulated in Mn-surplus conditions.The pam71 cmt1-1double mutant resembles the cmt1-1 single mutant rather than pam71 in most respects.Taken together,our results suggest that CMT1 mediates Mn2+ uptake into the chloroplast stroma,and that CMT1 and PAM71 function sequentially in Mn delivery to PSⅡ across the chloroplast envelope and the thylakoid membrane.