Possesses endoamylolytic activity in vitro, but seems not required for breakdown of transitory starch in leaves. May be involved in the determination of the final structure of glucans by shortening long linear phospho-oligosaccharides in the chloroplast stroma. Can act on both soluble and insoluble glucan substrates to release small linear and branched malto-oligosaccharides (PubMed:24089528). Works synergistically with beta-amylase toward efficient starch degradation (PubMed:24089528). Has activity against p-nitrophenyl maltoheptaoside (BPNP-G7), amylopectin and beta-limit dextrin (PubMed:24089528). Involved in stress-induced starch degradation (PubMed:27436713). Endohydrolysis of (1->4)-alpha-D-glucosidic linkages in polysaccharides containing three or more (1->4)-alpha-linked D-glucose units. Redox-regulated, with the highest activity under reducing conditions. The midpoint redox potential is -329 mV. The disulfide bridge between Cys-499 and Cys-587 inhibits catalysis. Inhibited by CuCl(2) and H(2)O(2). Optimum pH is 7.5 with p-nitrophenyl maltoheptaoside or amylopectin as substrate. Expressed in developing siliques. Circadian-regulation. Expression increases during the light phase and decreases during the dark phase. Up-regulated during osmotic stress and by abscisic acid (PubMed:27436713). The N-terminal domain is not required for endoamylolytic activity. No visible phenotype under normal growth conditions. Belongs to the glycosyl hydrolase 13 family.