Microtubule arrays in regeneratingMougeotia protoplasts may be oriented by electric fields

Summary Initially non-polar protoplasts of the green algaMougeotia will regenerate to re-establish their original cylindrical cell shape. The orientation of the growth axis of regenerating protoplasts held in agarose was independent of both the direction of incident white light and gravity. Protoplasts elongated parallel to applied DC electric fields of approx. 0.2 Vcm^–1 (1 mV/protoplast) and greater, with an increasing percentage oriented with increasing field strength. At the maximum field strength used (10 mV/cell), 53% of protoplasts were oriented within +- 10° of the 0/180° axis of the field. In untreated controls, the orientation of elongation was random. Protoplast survival was unaffected by field treatment. Some protoplasts (up to 37% in 10 mV/cell fields) formed outgrowths towards the cathode and occasionally towards the anode. Regenerating protoplasts in fields displayed the normal sequence of microtubule reorganization. This means that the positioning of the ordered symmetrical array of microtubules centred on two foci that appears within 3 to 4 h, and the subsequent organization of microtubules by 8 to 12 h into a band that intersects both foci and which is transverse to the axis of elongation (Galway and Hardham 1986), may be controlled by externally applied electric fields. In the region of this microtubule band, the applied field causes the plasma membrane to be stretched parallel to the field (Bryant and Wolfe 1987). We suggest that microtubules may become oriented perpendicular to the direction of field-induced membrane stretching, and that membrane stretching may be one of the orienting mechanisms for membrane-linked microtubules in elongating plant cells.Abbreviations PBS phosphate buffered saline - PMM protoplast maintenance medium - DMM dilute maintenance medium - MES 2(N-morpholino)ethanesulfonic acid - TRIS tris(hydroxymethyl)aminomethane - ANOVA analysis of variance