Structural and chemical changes in developing and mature Rice Grains

Abstract

The pattern of distribution of major storage chemicals and their route of entry into the caryopsis were investigated in IR50 rice and compared with several other cultivars and 17 wild species of Oryza. Fluorescence microscopy was a major tool in this investigation. During development the endosperm and embryo are isolated from the rest of the maternal tissue by a prominent cuticular layer that surrounds the nucellar epidermis (NE). However, nutrients are transported to the endosperm through a single ovular vascular bundle (OV) on the ventral side of the ovary. A pigment strand and a nucellar projection mediate this transport process. Between 1-7 days after fertilization (DAF) nutrients enter the endosperm through the still persistent nucellar tissue. Subsequently solute from the OV moves circumferentially into the NE and centripetally from the NE into the endosperm. About 2 layers of cross-cells in the inner pericarp are photosynthetically active and accumulate starch. Starch occurs abundantly in the pericarp during early stages of grain filling. Starch begins to accumulate within the endosperm about 5 DAF and by 14 DAF starch in the pericarp is completely depleted, presumably transported as sucrose into the endosperm. Lipids are stored mostly in the aleurone cells. Proteins occur as aleurone grains and as discrete particles of three different sizes in the endosperm. 80% of protein occurs in the subaleurone layers. The starch grains are smaller in the aleurone and subaleurone layers and increase in size towards the centre of the caryopsis. The dead endosperm cells contain remnants of nuclear material, readily revealed by DAPI. The rice embryo is rich in lipids and proteins and accumulates smaller amounts of starch. The aleurone and embryo also store phytin. Calcium, potassium and iron in phytin can be easily localized. The above pattern of distribution of major storage reserves is remarkably alike in all cultivars and species examined. The same pattern is also observed in grains stored for 12 years and in grains induced to enlarge in size through application of brassin and benzylaminopurine. A comparison of grain-filling in C3 and C4 cereals suggests that rice has structural features intermediate between these two types of cereals. This study on temporal changes and location of cereal components can help in attempts to improve grain quality of rice.