Starch degradation in cells is closely associated with cereal seed germination, photosynthesis in leaves, carbohydrate storage in tuberous roots, and fleshy fruit development. α_Amylase is considered as one of the key enzymes catalyzing starch breakdown, but up to date its role in starch breakdown in living cells remains unclear because the enzyme was often shown extrachloroplastic in living cells. The present experiment showed that α_amylase activity was progressively increasing concomitantly with the decreasing starch concentrations during the development of apple ( Malus domestica Borkh cv. Starkrimson) fruit. The apparent amount of α_amylase assessed by Western blotting also increased during the fruit development, which is consistent with the seasonal changes in the enzyme activity. The enzyme subcellular_localization studies via immunogold electron_ microscopy technique showed that α_amylase visualized by gold particles was predominantly located in plastids, but the gold particles were scarcely found in other subcellular compartments. A high density of the enzyme was observed at the periphery of starch granules during the middle and late developmental stages. These data proved that the enzyme is compartmented in its functional sites in the living cells of the fruit. The predominantly plastid_distributed pattern of α_amylase in cells was shown unchanged throughout the fruit development. The density of gold particles (α_amylase) in plastids was increasing during the fruit development, which is consistent with the results of Western blotting. So it is considered that α_amylase is involved in starch hydrolysis in plastids of the fruit cells.
