Ultrastructural Studies of Gametophytic Apomicts in Guinea Grass (Panicum maximum) II. Characteristics of Aposporous Initial Cell-derived Embryo Sac

Abstract

The ultrastructural analysis using ultra-thin section and transmission electron microscope was carried out to characterize the features of the reproductive cells in aposporous initial cell (AIC)-derived embryo sac (AES), i.e. egg, synergid and polar cells, in ovule of facultative apomictic Panicum maximum. At anthesis, the first AES located in micropylar end consists of egg apparatus with 2 synergids and 1 egg cell, and the central cell with 1 nucleus. The other AESs contain less than four cells. The egg cell contains high dense of cytoplasm, and around the nucleus plastids and mitochondria are remarkable. There exist lipid bodies, few rough endoplasmic reticulum (rER), dictyosomes, and big vacuoles distributed in cytoplasm. The polar cell is occupied almost by one big vacuole, the lower dense cytoplasm than that of egg cell wraps 1 or 2 nuclei and is distributed around egg apparatus. Only in micropylar end of the AES the cell wall of the central cell with one nucleus exists with ingrowth of cell wall, indicating that this structure transports the nutrition for the embryo development. Synergid usually contains lowest dense of cytoplasm, and lipid bodies, plastid and mitochondria are distributed along the filiform apparatus in micropylar end. Filiform apparatus is located in the top place of the 2 synergids that is same to that of sexual embryo sac. Extreme rER and vacuoles distributed in chalazal end are the characteristics of the synergids. These vacuoles and rER might contain abundant inorganic substances that are absorbed as a part of nutrition for the developing embryo.