Rhizoid Formation in Megagametophytes of Marsilea in Response to Growth Substances

Abstract

In an earlier paper by Bloom (1962), dealing with rhizoid formation in unfertilized megagametophytes of Marsilea, it was suggested that rhizoid formation is a geotropic response involving an endogenous auxin. It was suggested that the auxin becomes concentrated in the ventral region of the gametophyte plant as a result of gravitational force. As a result of a series of experiments conducted in our laboratories, further evidence has been gathered to confirm the roles of auxins and gravitational forces in determining rhizoid formation. Maravolo and Voth (1966) have reported that gemmalings of Marchantia normally form ventral rhizoids on new tissue, but that when exogenous auxins are applied and reach a threshold concentration on the dorsal surface, rhizoids form there as well. The number of rhizoids formed was reported to be proportional to the concentration of the exogenous auxin. Miller and Miller (1964) reported effects of auxins on cell enlargement in fern gametophytes of Onoclea sensibilis, but did not report any observations of unusual effects on rhizoid formation. Kelley and Postlethwait (1962) studied the effects of the plant growth substance 2-chloroethyltrimethyl-ammonium chloride on gametophytes of Pteridium aquilinum and noted an effect on the time sequence of rhizoid production, but did not report any unusual effect on the position of the rhizoids formed. If auxins are indeed involved in rhizoid formation in the megagametophytes of Marsilea and if a higher concentration of auxin at the ventral sites is a response to gravitational force, certain predictions can be made concerning the effect of the application of exogenous auxins to the megagametophytes. In diverse processes auxins are generally stimulatory at certain concentrations and inhibitory at higher concentrations (Leopold, 1964). It could be predicted that rhizoid formation would be inhibited at high concentrations of exogenous auxins and stimulated at low concentrations. An additive effect could be expected if exogenous auxins are applied at very low concentrations. To test the above hypotheses the following experiments were conducted. Mineral nutrient solutions were prepared using Hoagland's No. 2 solution as given by Hoagland and Arnon (1938), with appropriate micronutrients added. Separate solutions were prepared by adding indole-3-acetic acid, napthalene acetic acid, or gibberellic acid in amounts to provide concentrations by weight of one part in 10,000 (10-4), 100,000 (10-5), 1,000,000 (10-6), and 10,000,000 (10-7). Plain agar was added at a concentration of 1.5% to each solution and the solutions were autoclaved and poured into sterile 100 X 15 mm plastic Petri dishes. Sporocarps of Marsilea mucronata were hydrated in deionized water. As soon as the megaspores were free they were transferred to the surface of the agar plates. Organic materials other than agar were not included in the media since the mega-