Development and function of plasmodesmata in zygotes of Fucus distichus

Abstract

Abstract Brown algae have plasmodesmata, tiny tubular cytoplasmic channels connecting adjacent cells. The lumen of plasmodesmata is 10–20 nm wide, and it takes a simple form, without a desmotubule (the inner membrane structure consisting of endoplasmic reticulum in the plasmodesmata of green plants). In this study, we analyzed the ultrastructure and distribution of plasmodesmata during development of Fucus distichus zygotes. The first cytokinesis of zygotes in brown algae is not accompanied by plasmodesmata formation. As the germlings develop, plasmodesmata are found in all septal cell walls, including the first cell division plane. Plasmodesmata are formed de novo on the existing cell wall. Pit fields, which are clusters of plasmodesmata, were observed in germlings with differentiated cell layers. Apart from the normal plasmodesmata, these pit fields had branched plasmodesmata that appeared to arise from the lateral preexisting ones. Fluorescent tracers with different molecular sizes were microinjected to examine the size exclusion limit of molecules for transit through the plasmodesmata. Fluorescein isothiocyanate (FITC)-dextran of 3 kDa size was spread over the germlings, and 10 kDa FITC-dextran was tracked only in the rhizoid. The size exclusion limit was <10 kDa for the thallus but <40 kDa for the rhizoid.