Function of isoamylase‐type starch debranching enzymes ISA 1 and ISA 2 in the Z ea mays leaf

Summary Conserved isoamylase‐type starch debranching enzymes ( ISA s), including the catalytic ISA 1 and noncatalytic ISA 2, are major starch biosynthesis determinants. A rabidopsis thaliana leaves require ISA 1 and ISA 2 for physiological function, whereas endosperm starch is near normal with only ISA 1. ISA functions were characterized in maize ( Z ea mays ) leaves to determine whether species‐specific distinctions in ISA 1 primary structure, or metabolic differences in tissues, are responsible for the differing ISA 2 requirement. Genetic methods provided lines lacking ISA 1 or ISA 2. Biochemical analyses characterized ISA activities in mutant tissues. Starch content, granule morphology, and amylopectin fine structure were determined. Three ISA activity forms were observed in leaves, two ISA 1/ ISA 2 heteromultimers and one ISA 1 homomultimer. ISA 1 homomultimer activity existed in mutants lacking ISA 2. Mutants without ISA 2 differed in leaf starch content, granule morphology, and amylopectin structure compared with nonmutants or lines lacking both ISA 1 and ISA 2. The data imply that both the ISA 1 homomultimer and ISA 1/ ISA 2 heteromultimer function in the maize leaf. The ISA 1 homomultimer is present and functions in the maize leaf. Evolutionary divergence between monocots and dicots probably explains the ability of ISA 1 to function as a homomultimer in maize leaves, in contrast to other species where the ISA 1/ ISA 2 heteromultimer is the only active form.