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Title: The mechanism for explosive seed dispersal in Cardamine hirsuta (Brassicceae)

Author
item Vaughn, Kevin
item BOWLING, ANDREW - Dow Agrosciences
item RUEL, KATIA - Center For Research On Plant Macromolecules

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2011
Publication Date: 7/6/2011
Citation: Vaughn, K.C., Bowling, A.J., Ruel, K.J. 2011. The mechanism for explosive seed dispersal in Cardamine hirsuta (Brassicceae). American Journal of Botany. 98(8):1-10.

Interpretive Summary: Many of the most pernicious weed seeds have the capacity to discharge their seeds ballistically, sending them many meters from the parent plant. Little is known as to how these weed species achieve this, however. In this study, we investigated the seed capsules of bittercress, a weed that can discharge its seeds up 5 meters. What we found is that there is a layer of mucilage in the capsule walls that, as it dries, shrinks and puts tension on the capsule. When the tension reaches a certain point, the capsule explodes, discharging the seeds far away from the parent plant.

Technical Abstract: Many highly successful weed species have the ability to disperse their propagules through the ballistic discharge of their seeds either at ripening or when disturbed by an herbivore. However, very little is known of how ballistic seed dispersal is achieved. Bittercress (Cardamine hirsuta L.) is a weed species that makes siliques that are morphologically similar to those of Arabidopsis but has the ability to project its seeds as far as 5 m, compared to the relatively passive seed dispersal of Arabidopsis. Light microscopic sections through siliques of these two species were very similar. The separation zones at the carpel-replum interfaces are virtually identical in the two species and both species lose highly de-esterified homogalacturonans from the middle lamellae as the silique matures, allowing for dehiscence. However, the enB layer (second endocarp layer) of the carpel in the bittercress was highly enriched in xylans and lignin in an inverted V pattern, whereas the Arabidopsis silique has similar labeling but the lignin and xylan were spread more thinly and uniformly distributed on the walls. In addition, a large accumulation of mucilaginous pectins occurs in the space between the degenerate enA layer (first endocarp layer) and the enB in the bittercress carpel that is also not present in Arabidopsis. We propose that the mechanism of ballistic seed dispersal in bittercress involves the contraction of the mucilage layer, which produces a stress on the adjacent highly thickened asymmetric enB cells. When the dehiscence zone is sufficiently weakened, the stress is released by the violent opening of the carpels, dispersing the seeds and leaving the carpel highly coiled.