Secondary Formation of Plasmodesmata in Cultured Cells - Structural and Functional Aspects

Abstract

Almost immediately after the first description of protoplasmic connections between plant cells (Tangl 1879) the question arose, whether these connections are of cytokinetic origin or if they also could be a result of secondary formation. Strasburger (1901) first described secondary plasmodesmata between partners of a graft. Since then, they have been reported in chimeras (Buder 1911, Hume, 1913, Binding et al. 1987), between parasitic higher plants and their hosts (Dörr 1968, 1969, Tainter 1971, Dell et al. 1982), as well as in grafts (Funck 1929, Jeffree and Yeoman 1983, Kollmann et al. 1985), including the final ultrastructural proof by Kollmann and Glockmann (1985). On the other hand, secondary plasmodesmata have also been proposed to be present in the intact, regularly growing plant. Secondary cell contacts have been reported for the postgenital fusion of carpels (Boeke 1971) and between pollen mother cells (Cheng et al. 1987). Schnepf and Sych (1983), as well as Seagull (1983) observed an increase of plasmodesmata during cell elongation. Closely related to the problem of secondary cell contacts, half plasmodesmata have been described in several studies (Dörr 1968, Boeke 1971, Burgess 1972, Kollmann et al. 1985); presumably these structures are also of secondary origin (cf. Jones 1976, Kollmann and Glockmann 1989). As a major point of interest, the mechanism of secondary plasmodesmata formation has also been discussed over a long period (Strasburger 1901). The course of regular plasmodesmata formation during cell division has been demonstrated to be closely related to cell plate formation (Hepler 1982, cf. Jones 1976). However, for secondary plasmodesmata formation, the situation is less clear. Almost hypothetically, a mechanism of directed enzymic wall degradation has been postulated (Jones 1976, cf. Cheng et al. 1987).