Involved in starch catabolism. ISA3 removes different branches than ISA1-ISA2, namely short chains that prevent amylopectin crystallization. May be the debranching enzyme required to assist beta-amylases for starch degradation in leaves at night. Hydrolysis of (1->6)-alpha-D-glucosidic branch linkages in glycogen, amylopectin and their beta-limit dextrins. Glycan degradation; starch degradation. Localizes to granule-like structures inside the chloroplast. Strong increase of the starch level in leaves at the end of both day and night periods, but no modification in water-soluble polysaccharides content. No alteration of the amylase-to-amylopectin ratio. Double mutant shows that ISA3 and PU1 have redundant function for starch degradation. Belongs to the glycosyl hydrolase 13 family.