Changes in ultrastructure and subunit composition of protein body in rice endosperm during germination.

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Changes in Ultrastructure of protein bodies in subaleurone cells of rice endosperm during germination were studied by transmission electron microscopy. The subaleurone cells contained two different types of protein bodies: PB-I (spherical) and PB-II (crystalline). Both types of protein bodies were deconstructed during germination. But there was a considerable difference in digestibility between PB-I and PB-II. PB-II which did not have a dense core was easily digested from the central portion when germination began. At 6 days of germination, PB-II was almost deconstructed. On the other hand, PB-I which displayed concentric rings with a dense core was digested from the outside after 3 days of germination. At 9 days of germination, many kernels of the spherical protein bodies remained. Changes in subunit composition of protein bodies during germination were investigated by SDS-polyacrylamide gel electrophoresis. Protein body fractions were isolated from germinating grains at various stages by enzymatic digestion and two-phase system, then subjected to SDS-polyacrylamide gel. As germination proceeded, 15 (b1), 20 (d1), 24 (e), 35 (f1) and 37 (f3) k daltons subunits decreased. On the other hand, 16 (b2), 21 (d2) and 36 (f2) k daltons subunits remained at the later stage of germination. We think that PB-I contains b2, d2 and f2 subunits and is attacked only from the outside at middle and later stages of germination by de novo protease. On the contrary, PB-II contains b1 d1, e, f1 and f3 subunits is utilized at an early stage of germination. PB-II may possibly contain latent protease. The breakdown process of PB-I was by exo-type digestion, on the contrary, that of PB-II was by endo-type digestion.