Embryology of Brassica campestris: the entrance and discharge of the pollen tube in the synergid and the formation of the zygote

Abstract

The postanthesis synergids and zygote of Brassica campestris cv. Candle were examined using techniques of light, fluorescence, and electron microscopy. The pollen tube enters the degenerate synergid by way of the filiform apparatus. A degeneration of one of the two synergids occurs after anthesis and is independent of pollination. The first sign of synergid degeneration is a more intense staining of one of the synergids, followed by a loss of organelle membrane integrity. There is a disappearance of the plasma membrane and dictyosome cisternae; however, profiles of degenerate synergid mitochondria, plastids, and dilated endoplasmic reticulum remain along with dictyosome vesicles that contain periodic acid – thiocarbo-hydrazide – silver proteinate positive substances. The zygote, shortly after fertilization, is reduced in size and lacks the large micropylar vacuole characteristic of the mature unfertilized egg cell. Plastids and mitochondria are concentrated around the centrally located nucleus of the zygote, and dictyosomes, active in vesicle production, are located in the lateral and chalazal regions of the cell, adjacent to the cell wall. The lateral cell walls are periodic acid – Schiff's and Calcofluor positive, while the ampulliform chalazal tip of the cell is weakly periodic acid – Schiff s positive and Calcofluor negative. Microtubules, with the long axis perpendicular to the long axis of the zygote, are abundant in the ampulliform chalazal tip of the cell. Following fertilization the central cell becomes highly vacuolate. There is continuity between the zygote – central cell plasma membrane, the central cell vacuole tonoplast, and membranes of the central cell endoplasmic reticulum. Central cell wall projections, of the transfer cell type, are located in the lateral regions of the megagametophyte adjacent to the developing zygote cell and are positioned adjacent to the region of inner and outer integument starch. Key words: Brassica, ultrastructure, synergid, megagametophyte, pollen tube, zygote.