Chloroplastic alpha-glucanotransferase involved in maltotriose metabolism. Probably uses maltotriose as substrate to transfer a maltosyl unit from one molecule to another, resulting in glucose and maltopentaose. The latter can then be further metabolized to maltose and maltotriose by beta-amylase. Required for normal starch degradation in leaves. Transfers a segment of a (1->4)-alpha-D-glucan to a new position in an acceptor, which may be glucose or a (1->4)-alpha-D-glucan. Increased amounts of maltotriose and leaf starch. Belongs to the disproportionating enzyme family.