Cytokinin activation and redistribution of plasma-membrane ion channels in Funaria

Abstract

I have investigated changes in electrical current across the plasma membrane that occur during cytokinin-induced bud formation in Funaria hygrometrica Hedw., using a non-intrusive vibrating microelectrode. Before cytokinin treatment the target caulonema cells have maximal inward current at the nuclear region. After cytokinin treatment inward current increases twofold along the length of the cell. Within minutes, however, current decreases at both the nuclear zone and the proximal end while increasing at the distal end of target cells, preceding and predicting the presumptive division site. Inward current at the distal end falls to resting levels after establishment of a bulging growth zone, and remains low around developing buds. This current is blocked by gadolinium nitrate, a Ca(2+)-uptake inhibitor, indicating a Ca(2+) component of the current. The polarity of the target cells can be disrupted by microfilament inhibitors and cytokinin-induced buds form over the nucleus, halfway along the length of the cell. I suggest that cytokinin activates plasma-membrane ion channels which are subsequently redistributed to the distal ends of target cells by a microfilament-dependent process. Cytokinin-induced concentration of ion channels over presumptive bud sites may be envisioned to exert spatial control of cytoplasmic ion concentrations and stimulate bud formation by establishing a new growth zone, directing nuclear migration, and stimulating cell division.