How a rare cycad's wax crystals conjure blue without pigment

(Press-News.org) The endangered South African cycad Encephalartos horridus may resemble a relic from the Jurassic age, but the species itself evolved long after dinosaurs disappeared. Still, it carries a biochemical legacy inherited from its distant ancestors—plants that once thrived alongside Jurassic fauna. A team led by Hiroshima University (HU) researchers found that its spiky, silvery-blue leaves owe their color not to pigment, but to a wax-based optical effect produced by a lipid compound that may date back to the dawn of land plants.

In a study published in the Journal of Experimental Botany, researchers revealed that the coating of epicuticular wax on E. horridus leaves forms tubular crystals that reflect light from ultraviolet (UV) to blue wavelengths, giving the plant its bluish sheen. The paper will also appear on the front cover of the journal's upcoming Volume 76, Issue 12.

Nonacosan-10-ol, the key wax compound, is found across diverse plant lineages—including gymnosperms such as ginkgos, conifers, and cycads, and has even been detected in certain mosses—suggesting the ability to produce it emerged early in the evolution of land plants. However, only a few species can organize it into specialized wax structures that produce structural color, vivid hues generated not by pigment, but by microscopic architectures that scatter light. It’s the same optical effect behind the iridescent wings of morpho butterflies and the vibrant plumage of blue jays—both of which appear blue despite lacking the pigment.

The team found, however, that this unique color in E. horridus doesn't come from the wax alone. It also depends on how the wax interacts with the dark green, chlorophyll-rich tissues underneath.

“The blue color of Encephalartos horridus leaves comes not from pigments but from a clever natural trick. Tiny wax crystals on the surface create what’s called ‘structural coloration,’” explained study corresponding author Takashi Nobusawa, assistant professor at HU’s Graduate School of Integrated Sciences for Life.

“The leaf surface is coated with ultra-thin wax crystals about one ten-thousandth of a millimeter wide. Peeling off the leaf’s surface layer makes the blue disappear. But placing it back on a dark surface brings the blue back, as if by magic.”

UV defense and pollinator lure? To understand why the leaves of E. horridus appear bluish, researchers ran Monte Carlo multi-layered (MCML) simulations to model how light interacts with the wax crystals about 0.1 micrometers in diameter, thousands of times smaller than a typical grain of sand. The simulations revealed that when the wax layer sits against a dark background, it minimizes unwanted reflection, intensifying the blue hue. But if there is an air gap between the wax and the underlying tissue, reflectivity increases, causing a grayish cast. Replacing the air with water restores the original color by letting more light reach the chlorophyll-rich cells beneath the wax.

Although the superhydrophobic properties of nonacosan-10-ol have been well-documented, its connection to efficient UV reflection remains less understood. Shielding against UV rays is important for survival in desert environments, where the radiation can harm plant cells. However, the researchers suspect there's more to it. The glaucous sheen could also be a visual cue for insect pollinators like a neon sign pointing toward the plant’s reproductive organs. Insects can see UV light, which is invisible to the human eye, and many also have heightened sensitivity to blue wavelengths.

Lost to time Although E. horridus is known to accumulate the secondary alcohol nonacosan-10-ol in its epicuticular wax, how this compound is biosynthesized remains a mystery. By contrast, wax biosynthesis has been extensively studied in Arabidopsis thaliana and other model plants in the angiosperm group (flowering plants), which evolved much later. In Arabidopsis, nonacosan-10-ol is not detected; instead, nonacosan-14-ol and nonacosan-15-ol are produced as secondary alcohols by a characterized pathway.

To investigate how the E. horridus produces its distinctive wax compound, the team focused on KCS (keto-acyl-CoA synthases) enzymes, which they suspected to be responsible for nonacosan-10-ol biosynthesis. However, introducing these enzymes into a model plant did not result in production of the compound—suggesting that additional, as-yet-unknown pathways are likely involved.

“Why do the leaves of Encephalartos horridus, an endangered South African cycad, appear strikingly blue even though they contain no blue pigments? The question itself is scientifically fascinating—it uncovers a natural optical strategy far more refined than we might expect from plants. Understanding this mechanism not only deepens our grasp of plant adaptation in extreme environments but could also inspire nature-based technologies,” Nobusawa said.

“The next step is to figure out how the plant makes the special wax compound, nonacosan-10-ol, and to uncover the genes and enzymes behind it. In the long run, the goal is to understand how this adaptation evolved and to use these insights to develop new materials inspired by nature.”

Other members of the research team were Makoto Kusaba also from HU’s Graduate School of Integrated Sciences for Life, Takashi Okamoto from Kyushu Institute of Technology, and Michiharu Nakano from Kochi University.

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About Hiroshima University

Since its foundation in 1949, Hiroshima University has striven to become one of the most prominent and comprehensive universities in Japan for the promotion and development of scholarship and education. Consisting of 12 schools for undergraduate level and 5 graduate schools, ranging from natural sciences to humanities and social sciences, the university has grown into one of the most distinguished comprehensive research universities in Japan. English website: https://www.hiroshima-u.ac.jp/en

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