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.