The Journal of Cell Biology 66: 556-567 (Sept 1, 1975)
From the Biology Department, Purdue University, Lafayette, Indiana 47907. Dr. Weisenseel's present
address is Botanisches Institut, Universität Erlangen-Nürnberg, 852 Erlangen, West Germany.


Using a newly developed vibrating electrode, we have explored the electric fields around lily pollen germinating in vitro. From these field measurements, we infer that each wetted pollen drives a steady current of a few hundred picoamperes through itself. Considered as a flow of positive ions, this current enters an ungerminated grain's prospective growth site and leaves its opposite end. After a grain germinates and forms a tube, this current enters most of the growing tube and leaves the whole grain. The current densities over both of these extended surface regions are relatively uniform, and the boundary zone, near the tube's base, is relatively narrow. This current continues as long as the tube grows, and even continues when elongation, as well as cytoplasmic streaming, are blocked by 1 g/ml of cytochalasin B.

After an otherwise indistinguishable minority of tubes have grown to lengths of a millimeter or more, their current comes to include an endless train of discrete and characteristic current pulses as well as a steady component. These pulses are about 30 s long, never overlap, recur every 60-100 s, and seem to enter a region more restricted to the growing tip than the steady current's sink.

In most ways, the current through growing lily pollen resembles that known to flow through fucoid eggs.

Some time ago, and with the use of a rather specialized technique, it was shown that developing fucoid eggs drive large electrical currents through themselves (7, 8). With the recent emergence of a vibrating electrode system for measuring extracellular currents (9), it has become possible to investigate the currents which may traverse a variety of developing cells. As a first object for this survey, we have chosen germinating lily pollen. Germinating pollen grains and fucoid eggs undergo a somewhat similar development. Both of them form a long process by tip growth. But they are cells from completely unrelated organisms, and they grow in a radically different ionic milieu and at very different rates. Thus, lily pollen can grow optimally in a solution of low millimolar salt concentration, and the only necessary inorganic ions of such salts are K+ and Ca++, while fucoid eggs grow in seawater, a half-molar salt solution bearing six major inorganic ions. Moreover, lily pollen tubes elongate at 6-10 µm/min, and their cytoplasm streams at about 50 µm/min, while fucoid rhizoids elongate at 0.03-0.05 µm/min and show no detectable streaming at all.

In view of these developmental similarities coupled with physiological and essentially genetic differences, we wondered if there would be an endogenous transcellular current, and if so, to what degree the currents through pollen would resemble those through fucoid eggs, for a sufficient similarity might suggest a causal and general role for these currents in initiating and maintaining tip growth. Indeed, we have advanced the more specific thesis that one important mechanism of developmental localization lies in local cation (particularly Ca++ ) entry, which could constitute part of a current loop, and the resultant establishment of a cytoplasmic field which pulls critical constituents towards the cation entry region (10). Here, we will present some information about the net electrical current and also briefly discuss the calcium theory of localization.