Premise: Although many highly successful weed species use a ballistic seed dispersal mechanism, little is known about the mechanics of this process. Bittercress (Cardamine hirsuta) siliques are morphologically similar to Arabidopsis siliques, but they can project their seeds up to 5 m, while Arabidopsis seeds are dispersed by gravity. Comparison of these species should enable us to determine which structures might be responsible for ballistic seed dispersal.
Methods: Sections of Arabidopsis and bittercress siliques were immunolabeled with antibodies raised against a variety of polysaccharide epitopes.
Results: In bittercress, the second endocarp layer (enB) of the valve had strongly asymmetrical cell wall thickenings, whereas the analogous cells in Arabidopsis were reinforced symmetrically and to a lesser extent. Additionally, an accumulation of mucilaginous pectins was found between the first and second endocarp (enA and enB) layers in the bittercress valve that was not present in Arabidopsis. However, in both species, highly de-esterified homogalacturonan was lost in the dehiscence zone (at the carpel/replum interface) as the siliques matured, thus allowing for separation of the valve at maturity.
Conclusions: Ballistic seed dispersal in bittercress may involve the contraction of the outer pericarp tissue against the highly asymmetrically thickened enB cells, which are hypothesized to bend in one direction preferentially. The stress generated by the differential drying of the inner and outer layers of the valve is released suddenly as the adhesion between the cells of the dehiscence zone is lost, leading to a rapid coiling of the valve and dispersal of the seeds.