Electrotropism is a kind of tropism which results in growth or migration of an organism, usually a cell, in response to an exogenous electric field. Several types of cells such as nerve cells, muscle cells, fibroblasts, epithelial cells, green algae, spores, and pollen tubes, among others, have been already reported to respond by either growing or migrating in a preferential direction when exposed to an electric field.
Electrotropism is known to play a role in the control of growth in cells and the development of tissues. By imposing an exogenous electric field, or modifying an endogenous one, a cell or a group of cells can greatly redirect their growth. Pollen tubes, for instance, align their polar growth with respect to an exogenous electric field. It has been observed that cells respond to electric fields as small as 0.1 mV/cell diameter (Note that the average radius of a large cell is in the order of a few micrometers). Electric fields have also been shown to act as directional signals in the repair and regeneration of wounded tissue.
Electrotropism in pollen tubes
The pollen tube is an excellent model for the understanding of electrotropism and plant cell behavior in general. They are easily cultivated in vitro and have a very dynamic cytoskeleton that polymerizes at very high rates, providing the pollen tube with interesting growth properties. For instance, the pollen tube has an unusual kind of growth; it extends exclusively at its apex. Pollen tubes, as most biological systems, are influenced by electrical stimulus.
Under a constant electric field of 1 V/cm pollen tubes of Camellia japonica have been reported to grow towards the negative electrode. Tomato and tobacco pollen tubes grew towards the positive electrode for constant electric fields higher than 0.2 V/cm. Agapanthus umbelatus pollen tubes grow towards the nearest electrode when a constant electric field of 7.5 V/cm is applied. Another report states that pollen tubes do not change growth direction under AC electric fields.
Even though efforts have been made to clarify the mechanisms of intra- and extracellular electrical signaling in pollen tubes, the understanding of how pollen tubes react to electric fields and how the electric cue is related to the internal dynamics of pollen tube growth remains limited.
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