Nanoballs give English ivy one of nature’s strongest glues

http://www.sciencemag.org/news/2016/05/nanoballs-give-english-ivy-one-nature-s-strongest-glues

By Jessica McDonaldMay. 23, 2016 , 3:00 PM

Clearing your house of English ivy—even after the plant has died—can be tough, if not impossible. Patches of brick and plaster have been known to come off buildings before the green-leaved vine surrenders its grip. More than 130 years ago, Charles Darwin discovered that ivy’s sticking power is thanks to a thin yellow glue secreted from its roots. But since then, little has been known about how the adhesive works. Now, after an 8-year investigation, scientists report the mechanism today in the Proceedings of the National Academy of Sciences. The glue’s secret ingredients are tiny balls of sugar-coated proteins. These nanoparticles are highly uniform, allowing them to spread out and work their way into nooks and crannies of surfaces. Once the adhesive’s water evaporates, the nanoballs concentrate, and with the help of other materials, including calcium and pectin, the glue hardens. The research team thinks mimicking the approach could yield some new high-strength adhesives—and might even work in tissue engineering to stick cells to scaffolds when building artificial organs. The nanoparticles also have potential as safer targeted drug delivery systems. Unlike many of the current nanomaterials used to ferry chemotherapies into cells, the ivy nanoparticles don’t contain metal, which can be toxic.

•                                Early Edition
• > Yujian Huang, doi: 10.1073/pnas.1600406113

Nanospherical arabinogalactan proteins are a key component of the high-strength adhesive secreted by English ivy

1. Yujian Huang^a,^b,^c,
2. Yongzhong Wang^a,^b,^c,
3. Li Tan^d,^e,
4. Leming Sun^a,^b,^c,
5. Jennifer Petrosino^c,
6. Mei-Zhen Cui^f,
7. Feng Hao^f, and
8. Mingjun Zhang^a,^b,^c,¹

Author Affiliations

1. Edited by Peter Ladurner, University of Innsbruck, Innsbruck, Austria, and accepted by Editorial Board Member Maarten J. Chrispeels April 29, 2016 (received for review January 12, 2016)

1. Abstract
2. Full Text
3. Authors & Info
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Significance

Despite the significant progress that has been made in exploring the molecular basis for multiple adhesive events in the animal kingdom, the exceptional adhesion behaviors of climbing plants, such as English ivy, are still poorly understood. In this study, the spheroidal nanoparticles observed in the mucilage exuded by the English ivy were identified to be predominantly composed of arabinogalactan proteins (AGPs). The roles of these AGP-rich nanoparticles in favoring the generation of strong adhesion strength are elucidated. The Ca²⁺-driven electrostatic interactions among uronic acids within AGPs and pectin upon curing could be exploited as guidelines in the design and fabrication of novel synthetic adhesives, and the ivy-derived adhesive composite is capable of serving as a template for inspiring the development of diverse adhesive biomaterials.

Abstract

Over 130 y have passed since Charles Darwin first discovered that the adventitious roots of English ivy (Hedera helix) exude a yellowish mucilage that promotes the capacity of this plant to climb vertical surfaces. Unfortunately, little progress has been made in elucidating the adhesion mechanisms underlying this high-strength adhesive. In the previous studies, spherical nanoparticles were observed in the viscous exudate. Here we show that these nanoparticles are predominantly composed of arabinogalactan proteins (AGPs), a superfamily of hydroxyproline-rich glycoproteins present in the extracellular spaces of plant cells. The spheroidal shape of the AGP-rich ivy nanoparticles results in a low viscosity of the ivy adhesive, and thus a favorable wetting behavior on the surface of substrates. Meanwhile, calcium-driven electrostatic interactions among carboxyl groups of the AGPs and the pectic acids give rise to the cross-linking of the exuded adhesive substances, favor subsequent curing (hardening) via formation of an adhesive film, and eventually promote the generation of mechanical interlocking between the adventitious roots of English ivy and the surface of substrates. Inspired by these molecular events, a reconstructed ivy-mimetic adhesive composite was developed by integrating purified AGP-rich ivy nanoparticles with pectic polysaccharides and calcium ions. Information gained from the subsequent tensile tests, in turn, substantiated the proposed adhesion mechanisms underlying the ivy-derived adhesive. Given that AGPs and pectic polysaccharides are also observed in bioadhesives exuded by other climbing plants, the adhesion mechanisms revealed by English ivy may forward the progress toward understanding the general principles underlying diverse botanic adhesives.

• ivy nanoparticle
• ivy adhesive
• arabinogalactan protein
• adhesion mechanism
• reconstructed adhesive

Footnotes

• ¹To whom correspondence should be addressed. Email: zhang.4882@osu.edu.

• Author contributions: Y.H. and M.Z. designed research; Y.H., Y.W., L.T., L.S., M.-Z.C., and F.H. performed research; L.T. and M.Z. contributed new reagents/analytic tools; Y.H., Y.W., L.T., L.S., J.P., and M.Z. analyzed data; and Y.H., J.P., and M.Z. wrote the paper.
• The authors declare no conflict of interest.
• This article is a PNAS Direct Submission. P.L. is a guest editor invited by the Editorial Board.
• Data deposition: The sequence of iagp has been deposited in the GenBank database (accession no. KM820289).
• This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1600406113/-/DCSupplemental.