Embryogenesis of Polyembryonic Rice Ap Ⅲ: Structural and Histochemical Studies of Egg Apparatus Around Fertilization

摘要

The structural and histochemical changes of the egg apparatus in the polyembryonic rice (Oryza sativa L.), Ap Ⅲ with the highest frequence of additional embryos among the polyembryonic rice investigated, before and after fertilization were studied and compared with those of normal and other polyembryonic rices in a similar developmental period. A total of 2932 ovules were observed and each of them contained only a single embryo sac with a set of egg apparatus. Among 1655 embryo sacs, there were 1643 embryo sacs (99.27%) with one normal egg apparatus in each embryo sac, and only 12 embryo sacs (0.73%) from the remainder with 4-celled egg apparatus, i.e. two eggs and two synergids. Neither the numerous poly-egg apparatus and egg-like cells, nor the double set of embryo sacs each containing one egg apparatus and other abnormal egg apparatus in single ovary, which were reported by earlier investigators to have high frequency of embryo production in SB-1 and Ap Ⅳ, were observed. The egg cell was located at the subterminal site of the micropylar end of embryo sac. The cytoplasm of egg cell was rich in protein materials and polysaccharide grains, which did not disappear until the division of zygote. The prominent nucleus was closely surrounded by protein and polysaccharide grains, which did not disappear until the division of zygote. No cytological difference was found between egg cells from the normal and abnormal egg apparatus. The two synergids were fully developed and situated at the upper most part of the micropylar end of the mature embryo sac. In most embryo sacs, the synergids were flask-shaped with longer necks, and a widened cap-shaped top, in close contact with the micropyle. The synergids had a well developed filiform apparatus. The characteristic appearance of the filiform apparatus as well as the cap-neck region of synergids before and after pollen tube penetration were easily distinguishable from the egg cell. The structure, the stainability with Coomassie Brilliant Blue and PAS reaction, the process of accumulation, distribution and disapperance of the cytoplasmic protein materials and polysaccharide grains of the two synergids, the persistent and rarely the receptive synergids before and after pollen tube penetration, were closely similar to those of egg cell of the same developmental stage. In comparison with normal and other polyembryonic rice reported, the size of nucleus and nucleolus and their stainability also strongly resembled those of egg cell. Based on the results observed, the main conclusions are summarized as follows: (1) the additional embryos very frequently developed in the young and mature seed of polyembryonic rice Ap Ⅲ were produced by one or two synergids of normal egg apparatus, rarely by 4-celled egg apparatus; (2) during fertilization, the synergids, in addition to the natural specific function of introducing pollen tube and transferring sperms to egg cell and central cell, could be closely associated with the potentiality to breed one or two additional embryos; and (3) as compared with that of normal or other polyembryonic rice it is firstly disclosed that in a few embryo sacs of Ap Ⅲ, the cytoplasmic and nuclear structure, the active anabolism and catabolism of protein and polysaccharide materials and the delayed disorganization at the mid-basal region of the receptive and persistent synergid still remained unchanged before the division of zygote. Such salient features could be the predisposition for the origin of additional embryos in Ap Ⅲ.