Proc. Natl Acad. Sci. USA 113, E3193–E3202 (2016)

English ivy — a creeping plant — is known to secrete a yellowish sticky substance when it climbs vertical surfaces. This glue-like secretion, which contains spherical nanoparticles, helps the plant to attach onto surfaces as it climbs. Researchers in the US have now shown that these nanoparticles are composed mainly of arabinogalactan proteins, which are important in promoting strong adhesion.

Mingjun Zhang and co-workers — at the Ohio State University, the University of Georgia and the University of Tennessee — isolated the sticky substance from ivy rootlets and characterized it using various microscopy and chemical methods. The nanoparticles are about 70 nm in diameter and have a negatively charged surface at pH 7.0. Tests using a phenylglycoside dye confirmed the presence of arabinogalactan proteins — a hydroxyproline-rich glycoprotein typically present in the extracellular matrix of plant cells and other botanic adhesives. The nanoparticles showed low intrinsic viscosity in solution and this gives the adhesive a favourable wetting behaviour. Furthermore, due to their size, the nanoparticles are thought to penetrate easily into any surface irregularities, further promoting intimate interactions with the substrate onto which the plant clings. Calcium ions contribute to the curing of the adhesive by promoting electrostatic binding between the nanoparticles and pectin. When the nanoparticles, pectin and calcium ions were combined, the bioadhesive characteristics were reproduced, further validating the adhesion mechanism.