Structural change in the endoplasmic reticulum during the in situ development and in vitro fertilisation of wheat egg cells

Abstract

The mechanism by which the sperm activates the egg cell of higher plants is largely unknown. Ca2+—implicated as a second messenger in the response of various plant cells to a wide range of stimuli—is a potential candidate for encoding the information brought by the sperm cell into the angiosperm egg. In higher plant cells the dominant calcium store appears to be the central vacuole; however, in the receptive wheat egg cell few vacuoles can be observed, thus it seems likely that the principal cell organelle performing a pivotal role in regulation of intracellular Ca2+ is the endoplasmic reticulum (ER). To examine this hypothesis, microinjection of the ER-specific fluorescent dicarbocyanine dye, 1,1′-dihexadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate [DiIC16(3)], and low-light level CCD technology were used. Following the injection of an oil drop saturated with DiI, structural changes occurring in the ER during the in planta maturation of the female gamete were visualised. The ER was identified by chlorotetracycline labelling to be the main calcium store in the wheat egg cell. Structural changes occurring in the ER during the in planta maturation of the wheat female gamete led to a polarised ER network in the receptive egg cell. Within 3 min of in vitro sperm-egg fusion, a rapid, transient loss of continuity of the ER occurred, which underlines the significance in fertilisation of the ER structure in the female gamete of wheat.