Spatial memory during the tropism of maize (Zea mays L.) coleoptiles

Photo- or gravitropic stimulation of graminean coleoptiles involves the formation of putative tropistic transverse polarities. It had been postulated that these polarities can be extended by stabilization to developmentally active polarities. Such polarities are known from unicellular spores and zygotes of lower plants and regeneration experiments in dicotyledonous plants. In coleoptiles, photo- or gravitropic stimulation results in stability to counterstimulation of equal strength (with only transient bending in the direction of the second stimulus), as a result of a directional memory, if the time interval between both stimuli exceeds 90 min. This directional memory develops from a labile precursor, which is present from at least 20 min after induction. Once it is stable, spatial memory is conserved for many hours. The formation of spatial memory involves at least one step not present in the common tropistic transduction chain. The spatial expression of memory as curvature is restricted to three distinct responses: (i) curving in the direction of the first stimulus (for time intervals exceeding 90 min); (ii) curving in the direction of the second stimulus (for time intervals shorter than 65 min); and (iii) zero-curvature (for time intervals between 65 and 90 min). This can be interpreted in terms of a stable transverse polarity, which is not identical with the putative tropistic transverse polarity, but might be an extension of it.