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The effects of aluminum on cytoplasmic organization, the F-actin array and calcium homeostasis in Vaucheria Longicaulis var. Macounii Alessa, Lilian


The single-celled alga, Vaucheria longicaulis var.macounii Blum was used as a model organism to determine the effects of aluminum on cytoplasmic organization, the Factin array and calcium homeostasis. The translocation of chloroplasts and mitochondria is a microfilament-dependent process whereas that of nuclei translocation, is microtubuledependent. The addition of aluminum (80 μM) resulted in the inhibition of microfilamentbased organelle translocation (within 60 seconds). Aluminum also caused the disorganization of cortical, cytoplasmic strands, which were visible using differential interference contrast (DIC) microscopy. These strands were shown to represent the endoplasmic reticulum in this species. Since the endoplasmic reticulum is supported by an intact F-actin array in many types of plant cells, this suggested that A l caused changes in the F-actin array in cells of V. longicaulis. Consistent with this, fluorescein labeled (FITC) phalloidin was used to show that exposure to A l resulted in the fragmentation of the F-actin array and the formation of F-actin aggregates which appeared to self-assemble in vivo. The hypothesis that aluminum causes cytoplasmic disorganization and fragmentation of the Factin array via the perturbation of calcium homeostasis was tested. The calcium-sensitive tetracarboxylate dye, fluo-3, was used to image free cytosolic calcium in this system. Treatment with aluminum caused an increase in cytosolic calcium levels within two minutes of exposure. It was noted that the response of cytosolic calcium to aluminum treatment varied between the youngest, apical region and the older, vacuolated region of the cell. Consistent with this, aluminum toxicity was particulary acute in the cell tip. The aluminum-induced rise in cytosolic calcium correlates with aluminum-induced effects on cytoplasmic streaming and the F-actih array using calcium modulators in an aluminum-free system. The use of ionomycin resulted in changes in the F-actin array which were similar, but not exactly the same as those observed during aluminum treatment. Thapsigargin and TMB-8, used to stimulate and inhibit release of calcium from intracellular stores, respectively, were used to determine whether the source of the aluminum-induced rise in cytosolic calcium was internal. Data obtained using these latter calcium modulators were inconclusive.

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