Changes in actin organization in the living egg apparatus of Torenia fournieri during fertilization

Abstract 

Changes in actin organization in the living egg apparatus of Torenia fournieri from anthesis to post-fertilization have been investigated using microinjection and confocal microscopy. Our results revealed that the actin cytoskeleton displays dramatic changes in the egg apparatus and appears to coordinate the events of synergid degeneration, pollen tube arrival and gametic fusion during fertilization. Synergid degeneration occurs after anthesis and is accompanied by actin fragmentation and degradation. The actin cytoskeleton becomes organized with numerous aggregates in the chalazal end of the degenerating synergid, and some of the actin infiltrates into the intercellular gap between synergids, egg and central cell, forming a distinct actin band. An actin cap is present near the filiform apparatus after anthesis and disappears after pollen tube arrival. In the egg cell, actin filaments initially organize into a network and after pollination become fragmented into numerous patches in the cortex. These structures, along with the actin in the degenerating synergid and intercellular spaces form two distinct actin coronas during fertilization. The actin coronas vanish after gametic fusion. This is the first report of changes in actin organization in the living egg apparatus. The reorganization of the actin cytoskeleton in the egg apparatus and the presence of the actin coronas during fertilization suggest these events may be a necessary prelude to reception of the pollen tube and fusion of the male and female gametes.