Mucilaginous seeds – nanostructure encapsulated in hydrogel
In their research on the biological role of the seed mucus coat, dr hab. Agnieszka Kreitschitz from the Department of Plant Development Biology at the University of Wrocław and prof. Stanislav Gorb from the
University of Kiel (Germany), saw for the first time the exact structure of the mucus at the nanoscale and tested the hypothesis, which had been repeated for years, of the supporting role of mucus in the spread of seeds eaten by animals. Seed mucus as a natural hydrogel is used, among other things, in the food and pharmaceutical industries as well as in bioengineering and medicine.
Plants have accompanied man for thousands of years. They have been and continue to be used in a variety of ways. They serve as building material, raw material for textiles, but above all as a source of food. Due to their medicinal properties, various parts of plants are used in the pharmaceutical, cosmetic or medical industries. Some of the plant organs that can produce compounds that are useful to us include fruit and seeds. They can have different shapes, colours, vary in size, the presence of different compounds and their properties.
Mucilage
Fruits and seeds provide us with various nutrients, vitamins and trace elements on a daily basis and have a positive effect on our health. In recent years, seeds of various plant species have become quite popular on our market, including those that produce the so-called mucilage. We are sure that many people have already encountered mucilage seeds in their daily lives, for example by eating rolls sprinkled with flax seeds for breakfast, yoghurts, desserts, chia seed bars or by sowing cress as an Easter table decoration. Mucilage-producing seeds include the aforementioned chia/sage (Salvia hispanica), watercress (Lepidium sativum), basil (Ocimum basilicum), various species of plantain (Plantago ssp.) and flax (Linum usitatissimum). When hydrated, these seeds form a transparent (chia, plantain, flax) or sometimes slightly whitish (basil), gelatinous, viscous mass called mucilage (Fig. 1) (see also Figs. 2,3).
Mucilaginous seeds and fruits in nature are often produced by plants that inhabit dry areas (steppes, semi-deserts, deserts). Thanks to their ability to swell, they absorb and store water around the seed. The mucilage facilitates germination, protects against pathogens and is eaten by small insects (e.g. ants). The mucilage sticks the seeds to the substrate, protecting them from further transport on the one hand, and on the other hand, because it is sticky, mucilaginous seeds can also be carried and dispersed by animals.
A recent paper by dr hab. Agnieszka Kreitschitz (University of Wrocław) and Prof Stanislav Gorb (Kiel University, Germany), which was published in the Beilstein Journal of Nanotechnology, summarises the long-standing collaboration between the two scientists on mucilaginous seeds and fruits of various plant species. It focuses primarily on the structure of the mucilage envelope (a three-dimensional network of polysaccharides), its physical properties (friction and adhesion) and biological functions (assisting in seed dispersal).
Mucus – a nano-network with specific properties
The biological definition of mucus depicts it as a modified cell wall made up of polysaccharides such as cellulose, hemicelluloses and pectin. To the latter, mucus owes its ability to swell and store water (Fig. 1, 2 A, B). By using a special preparation method, so-called critical point drying, and scanning electron microscope (SEM) imaging, we were able to see the exact structure of the mucus at the nanoscale for the first time. The images show the shape and arrangement of the polysaccharide chains (Fig. 2 C), which form a delicate, loose, three-dimensional network held together by various bonds and interactions. This specific spatial structure of the mucus and its ability to store water classifies it as a natural hydrogel. Such systems find a variety of applications in many industries. Seed mucus is a readily available, natural product that is odourless, tasteless, colourless, non-toxic, edible and fully biodegradable. As a natural hydrogel, this makes it used, among other things, in the food industry as a thickening and stabilising ingredient for foods (yoghurts, ice creams) in the pharmaceutical industry as a binding agent in tablets, a moisturising agent in gels, or in bioengineering and medicine as a ‘scaffold’ in research into tissue regeneration and wound healing.
Slippery yet viscous mucus envelope
The most important property of mucus is its ability to bind and hold water, for which a three-dimensional network of polysaccharides is responsible. Friction and adhesion are the two main physical properties of mucus. Our studies have shown that they give it specific properties and depend on the amount of water accumulated in the mucus envelope. Fully hydrated mucus has a very low coefficient of friction which makes it slippery. Water loss, on the other hand, is associated with an increase in mucus adhesion and the ability to stick to different surfaces.
In our study of the biological role of the mucus envelope, we wanted to test the hypothesis, repeated for years, of the supporting role of mucus in the dispersal of seeds eaten by animals. For this purpose, we used pigeons from the private breeding of prof. Eberhard Haase (Kiel University, Germany). Our experiment showed that the mucus helps the seeds to pass through the pigeons’ digestive tract. The slippery coating protects the seeds from digestion, which also retain their ability to germinate after expulsion. Thus, they can make long-distance migrations (with the help of birds or other animals) and, expelled with the faeces, they can colonise new territories. Thus, the mucus facilitates seed dispersal and the water stored in the areola facilitates germination and seedling development, thus ensuring better colonisation success for plants that produce this type of fruit or seed.
The loss of water from the envelope causes the mucilage to stick more easily to various surfaces. Studies of the adhesive properties of mucilage on glass have shown us that it can attach seeds to a substrate with the strength of super glue. The ability of mucilage to collect and hold water and its adhesive properties make it an excellent mucoadhesive compound, which is used, for example, in the production of gels and moisturisers. Due to its strong adhesive properties, it is also used, for example, as a binding agent for the production of tablets. The literature reports many other applications for plant mucilages, including those derived from seeds and fruits.
The turn towards plant-based, organic raw materials, extracted in a natural, environmentally friendly way, has become very popular in recent years. Mucus research is also part of this trend. The more we know about the structure of mucus, its chemical composition and properties, the better we will be able to exploit its potential in various branches of industry, agriculture, medicine or pharmacy. We plan to continue our comprehensive mucus research, incorporating yet other unexplored species and analysis techniques.
Link to the article: Kreitschitz A., Gorb S., „Natural nanofibers embedded in the seed mucilage envelope: composite hydrogels with specific adhesive and frictional properties”. Beilstein J. Nanotechnol. 2024, 15, 1603–1618. https://doi.org/10.3762/bjnano.15.126.
Text: dr hab. Agnieszka Kreitschitz
Added by: E.K.
Date of publication: 21.02.2025
