Ultrastructure of Synergid in Its Degenerative Process in Sugar Beet ( Beta vulgaris )

Abstract

An experiment was conducted by using TEM to study the degenerative process of synergid in sugar beet, so as to provide more information for reproductive biology of angiosperm. The results were as follows: two synergids were similar in flower bud stage with large number of organelles. Both of them had developed filiform apparatus(FA) at micropyle end and lacked cell wall at chalazal end, showing obvious polarity. Then electron density in one synergid increased. On the other hand, vacuole membrane disappeared, membrane of mitochondrium, plastid and nuclear became illegible, which suggested cell degeneration began.But endoplasmic reticulum and Golgi bodies were still active. Complete degeneration of this synergid occurred before pollination.Number of organelles including mitochondrium, plastids and ribosomes gradually increased in the other synergid(persistent synergid). Mitochondria contained many tubular cristae and obvious DNA fibril. Plastids were irregular in shape and usually contained starch gains and thylakoid membranes. The metabolism of persistent synergid gradually enhanced until fertilization(about13 h after pollination). It began to degenerate when zygote had alveolate cell wall at the chalazal end and endosperm occurred,while complete degeneration and disappearance of FA took place at late stage of zygote. The results indicated that degeneration of one synergid in sugar beet must be triggered by other stimulation than pollination and pollen tube growth. As a transfer cell, persistent synergid may absorb and transport nutrition for the development of embryo sac.