A previously unknown cytoskeletal structure, now named the plasmalemmal reticulum (Gens et al. 2000, Protoplasma 212, 115–134), was found in cultured BY-2 tobacco cells during a search for a force-focusing mechanism that might enhance signal transduction by the cells’ mechanosensory Ca2+-selective cation channels (MCaCs). This polyhedral structure, which links cell wall, plasma membrane, and internal cytoplasm, prominently contains arabinogalactan protein (AGP). To check for reticulum-promoted Ca2+ elevation, the AGP-binding reagent (β-d-glucosyl)3 Yariv phenylglycoside has been applied to BY-2 cells expressing a free cameleon Ca2+ reporter. Ca2+ elevation was substantial and prolonged. Moreover it occurred in the nucleus as well as the cytoplasm. Cells treated with non-binding mannosyl Yariv reagent could not be discriminated from untreated controls or those treated with carrier solution alone. Supply of the MCaC inhibiter Gd3+ just before treatment with Yariv reagent prevented Ca2+ rise. These data strongly support the hypothesis that the plasmalemmal reticulum controls MCaC activity. The massive inward spread of Ca2+ suggested that entry of the ion through the channels initiated a wave of release from the ER, and YCX in the ER showed Ca2+ levels consistent with this premise. Cytosolic and nuclear Ca2+ often pulsed in control cells in near synchrony and at rates ranging from zero to five cycles per ∼20-min recording. (Pulsation was over-ridden by the applied amounts of glucosyl Yariv compound.) Suggestively but very crudely, oscillation rate was assessed as possibly correlating with stage of cell cycle. Because cell Ca2+ was lowered and pulsation was eliminated by Gd3+, MCaCs appear to participate in these endogenous fluctuations. The extent to which pulsing plays regulatory roles in relatively undifferentiated types of cells should be evaluated.
High level expression of transgenes by use of 5^|^#8242;-untranslated region of the Arabidopsis thaliana arabinogalactan-protein 21 gene in dicotyledons
