Septal Plugs in a Green Alga

Abstract

Septal plugs, resembling those found in red algae, occur in the transverse wall between all cells in a newly discovered marine green alga, Pilinia earleae Gallagher & Humm.3 No plasmodesmata traverse the cross-wall, and the septal plug blocks cytoplasmic continuity between cells. The septal plug consists of an electron-translucent core bordered at each end by two electron-opaque caps. Cytochemical procedures demonstrate that the plug consists of protein and polysaccharide, but lacks peroxidase. The outer cap is highly proteinaceous while the inner cap is composed primarily of polysaccharide. The plug core is not routinely stained by Coomassie Blue but it is pronase sensitive and probably proteinaceous. Historically, septal plugs have been considered unique to the red algae and the fungi, but ultrastructural and biochemical data provide no support for derivation of the septal plug in this green alga from a symbiotic relationship. The discovery of septal plugs in a green alga makes the hypothesis of an independent origin of this structure in a number of plant groups more likely.