Polarity and endoplasmic microtubules in food‐conducting cells of mosses: an experimental study

Abstract

SUMMARYThe effects of decapitation and of drugs affecting microtubules, microfilaments and the endomembrane system, on the ultrastructure of food‐conducting cells have been studied in the sporophyte of the mosses Polytrichum juniperinum Hedw. and Mnium hornum Hedw. In both species decapitation caused a marked reduction of cellular polarity. Moreover in Mnium but not in Polytrichum decapitation induced the disappearance of endoplasmic microtubules (MTs), the loss of longitudinal alignment of organdies and the accumulation of abundant starch. Starch accumulated in the cortical parenchyma cells in Polytrichum. In both species extensive MT depolymerization was induced by 100 μM oryzalin, whereas 10 μM oryzalin was less effective and 2·5 mM colchicine had no apparent effect. Treatment with 100 μM oryzalin also caused disruption of organdie alignment, alterations in the shape and position of the nucleus and a marked reduction in, or disappearance of, endoplasmic reticulum, trans Golgi network and membrane‐bounded tubules, that were abundant in the controls. Treatment with 10 μM monensin partially mimicked the effects of oryzalin, causing a reduction of the endomembrane system and disappearance of most endoplasmic MTs. Neither oryzalin nor monensin suppressed cellular polarity. Cells treated with D2O or the actin microfilament inhibitor cytochalasin D showed no structural alterations. The data indicate that: (a) endoplasmic MTs have a major role in organelle alignment and shaping in food‐conducting cells of mosses; (b) cellular polarity depends on the presence of a source/sink gradient and is not directly associated with MTs; (c) interactions between MTS and organelles, notably the ER and other components of the endomembrane system, have mutually stabilizing effects.