Unusual trichome structure and composition in mericarps of catchweed bedstraw (Galium aparine)

Catchweed bedstraw is famous for its ability to adhere to other objects due to the presence of numerous trichomes surrounding the stem and mericarps and on the surfaces of the leaves. These trichomes serve as an efficient vector for the movement of the propagules via animals. In this study, we examined the structure and composition of the mericarp trichomes by microscopic and immunocytochemical techniques to determine the distribution of polysaccharides. Trichomes present around the mericarps are distinguished by a pronounced hooked tip, resembling in many ways those on Velcro (TM). In semi-thin sections, the hooked area of the trichome contains little or no lumen but rather appears to be solidly composed of cell wall material. This solid hook appears to be divided into a plug-like zone of material and a highly thickened primary wall. These trichomes are also compositionally unique. They contain very little xyloglucan, even though other tissues in the plant reacted strongly with antibodies that recognize these polysaccharides. The distribution of pectin epitopes on these hooked trichomes was extremely distinctive, with each of the antibodies recognizing domains along the surface of the primary wall and/or in the plug area. Despite the heavily thickened nature of the walls of these trichomes, xylans were not present. Thus, the unique plugged, thickened, and hooked tip of these trichomes appears to be the result of a specific combination and distribution of various pectic polysaccharide molecules. This unusual wall composition may facilitate the formation of highly curved structures that might be difficult to form with the more rigid xyloglucans and xylans.