Study On Growth And Endosperm Cell Development Of Sorghum, Rice And Maize

Cereal, also known as grain, is the main source of carbohydrates for human kind. Endosperm, wrapped inside cereal caryopsis, is the most important tissue utilized by people, which development poses great effect on final quality and yield of the crops. After fertilization, filial grain organs are surrounded by the maternal nucellus embedded within the integuments and pericarp. Growth and differentiation of endosperm cells, from after fertilization to full maturity of caryopsis, must be coordinated with maternal tissue development. Therefore, it is of deep significance to clarify development pattern of both endosperm and maternal tissue.In this dissertation, three crops, rice, sorghum and maize were used as the object of study, with their caryopsis development days precisely recorded and caryopsis development closely observed. Starch accumulation and physiological activities of caryopsis were observed by I2-KI and TTC staining. Structural changes of endosperm cells and its maternal tissue related were observed through semi-thin sections under light microscopy by applying Spurr resin tissue embedding. Ultrastructure of cells from different tissue (starchy endosperm, aleurone layer, nucellus, integuments and pericarp) and the amyloplasts wherein in developing caryopsis were carried out through TEM. Final results were as follow: 1. Sorghum endosperm cell development and its relationship with the maternal tissue(1) The caryopsis needs about 35 days to reach its full mature stage. (2) It takes long as 15days for the nucellar epidermis to be fully degraded. (3) The outer peripheral endosperm cells start to accumulate lipid bodies as early as 7dap and, turn into aleurone cells in 11 dap. Only 1 layer of aleurone cells formed in full ripe caryopsis. Besides commonly seen aleurone granules and globoids, some single amyloplasts are also found on ripe aleurone cells, circa.3μm in diameter. (4) Growth and development of endosperm cells also varies according to different locations, amyloplasts in cells surrounding embryo tend to have a slow growth rate and are loosely packed than other areas, making them floury endosperm compared to others corneous endosperms when finally matured. (5) Amyloplasts constitution in subaleurone cells are quite composite, in a manner that mingles both aleurone and starchy endosperm cells. (6) Amyloplast formation in starchy endosperm is unique, which has an "occurrence center" during initiation. Starch "grow" inside a tube-like plastid, and when mature, falls apart, leaving the mature amyloplast with an uneven oval shape. (7) Starch/amyloplasts in mesocarp may not be exhausted approaching mature stage, and on the contrary, there seems to be a secondary growth in both quantity and diameter.2. Rice endosperm cell development and its relationship with the maternal tissue(1) Rice peripheral endosperm cells have the character of primary cells which form the endosperm through repeated periclinal divisions. (2) Nucellus tissue lasts approximately 4days before it is finally degraded. (3) Lipid bodies are occasionally seen scattered in cytoplasm of some dorsal peripheral endosperm cells as early as 3DAP. Aleurone forms circa.6-9dap. Dorsal aleurone often consists of over 3 cell layers while only 1-2 layers of aleurone cells formed in full ripe caryopsis. Besides commonly seen aleurone granules and globoids, some single amyloplasts are also found on ripe aleurone cells, approx.2μm in diameter. (4) Amyloplasts constitution in subaleurone cells are quite composite, in a manner that mingles both aleurone and starchy endosperm cells. (5) Growth and development of endosperm cells also varies according to different locations, amyloplasts in ventral endosperm cells tend to have a slow growth rate and are loosely packed than dorsal ones when finally matured. (6) Two types of protein bodies, pb I and pbⅡare observed in endosperm cells around 9DAP. (7) During middle and late development stages of endosperm, nuclei of endosperm cells distort and degenerate when cells accumulate starch, however, nuclei of aleurone cells do not degenerate during the development. (8) Starch granule/amyloplasts in mesocarp cells are gradually exhausted approaching mature stage(starts from 12DAP), left the empty cells undergo PCD procedure centrifugally, which finally causing drastic shrinkage of the pericarp, such phenomenon may account for the provision of extra space for the expanding endosperm/embryo. (9) Endosperm amyloplasts proliferation pattern varies diversely, including:① through envelope dilating, invaginating, and budding to form new individuals, ② envelop constriction,③ envelop dilating and invaginating to form new amyloplast.3. Maize endosperm cell development and its relationship with the maternal tissue(1) The volume of primary endosperm tissue is small and locates itself at the base of the embryo sac, with its antipodal cells inserted into the nucellus. (2) It takes long as 12days for the nucellus to be fully degraded. (3) Peripheral endosperm cells sre seen to accumulate lipid bodies as early as 4DAP and, turn into aleurone cells in approximately 12DAP. Only 1 or 2 layer of aleurone cells formed in full ripe caryopsis. Besides commonly seen aleurone granules and globoids, some single amyloplasts are also found on ripe aleurone cells, circa.2μm in diameter. (4) Subaleurone cells possess the feature of both aleurone and starchy endosperm in which are saturated with masses of protein bodies during later developing stages. At least two protein bodies are observed under TEM, of which the younger smaller ones have a electron dense appearance and could fuse with other individuals. (5) Growth and development of endosperm cells also varies according to different locations, amyloplasts in cells situated inside tend to have a slower growth rate and are loosely packed than its sorrounding area, making them floury endosperm compared to others corneous endosperms when finally matured. (6) Amyloplasts stored in pericarp cells mostly are compound ones, and are contributed as carbon source during cell expansion. (7) Starch granule/amyloplasts in mesocarp cells are gradually exhausted approaching mature stage, left the empty cells undergo PCD procedure centrifugally, which finally causing drastic shrinkage of the pericarp, such phenomenon may account for the provision of extra space for the expanding endosperm/embryo. (8) Traces of integument development are elusive(due to the complexity of the unevenly distributing organizing pattern of maize kernel episperm), but what is certain is that it starts to degenerate at circa.12DAP, and fully degraded at 21 d, which synchronizes degradation of necellar epidermis cells. Testa is finally formed through these degraded cell debris.4. Comparison of endosperm cell development of four different rice varietiesFour stages occur in caryopses development of all the rice varieties tested:formation stage, milky stage, dough stage, and full maturity stage. Compared to development stages of the endosperm, formation stage runs in parallel to the coenocyte and cellurization stage, which are the initial stage of endosperm formation; milky stage correspond to the differentiation stage of endosperm cells, while the last two stages, dough and full maturity stage, are equivalent to that of the endosperm maturity stage. The shape of two rice sub subspecies, japonica and indica, varies significantly. Of the four rice varieties tested, two indica varieties showed a relatively faster growth rate than two japonica cultivars; Wuyunuol6 and Yangfunuo 4 had almost identical growth rate of dry matter accumulation. Nuclei of starch endosperm cells will degenerate with the extension of amyloplasts, while amyloplasts are still enlarging and growing. Amyloplasts in endosperm cells are compound granules, and their development pattern possibly affect certain qualities of endosperm. Amyloplast in endosperm cells is spherical or elliptic in shape during early developmental stage, but turns to polyhydric during late development stage. Two glutinous rice cultivars tend to have a loosely stacked endosperm and as a result, cause the appearance to be opaque. Aleurone cell volume of two indica cultivars are bigger than japonica ones, Mature aleurone cells are rich in minerals like P、K、Mg、S. Cultivar Yangfunuo 4 has some single amyloplast detected in aleurone cells and, showed a higher level of O element content.