Changes in Ultrastructure and Subunit Composition of Protein Body in Rice Endosperm during Germination

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) kdaltons 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.