![Difference in carbohydrate composition of Lallemantia royleana (BENTH.) seed mucilage between two studies [54,55].](https://www.researchgate.net/publication/363878586/figure/fig1/AS:11431281086796297@1664336240128/Difference-in-carbohydrate-composition-of-Lallemantia-royleana-BENTH-seed-mucilage_Q320.jpg)
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Citation: Kuˇcka, M.; Ražná, K.;
Harenˇcár, L’.; Kolaroviˇcová, T. Plant
Seed Mucilage—Great Potential for
Sticky Matter. Nutraceuticals 2022,2,
253–269. https://doi.org/10.3390/
nutraceuticals2040019
Academic Editor: IvanCruz-Chamorro
Received: 7 June 2022
Accepted: 26 August 2022
Published: 26 September 2022
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Review
Plant Seed Mucilage—Great Potential for Sticky Matter
Matúš Kuˇcka 1, Katarína Ražná1, * , L’ubomír Harenˇcár1and Terézia Kolaroviˇcová2
1Institute of Plant and Environmental Sciences, Faculty of Agrobiology and Food Resources,
Slovak University of Agriculture, Tr. A. Hlinku 2, 94976 Nitra, Slovakia
2Faculty of Agrobiology and Food Resources, Slovak University of Agriculture, Tr. A. Hlinku 2,
94976 Nitra, Slovakia
*Correspondence: katarina.razna@uniag.sk
Abstract:
Some seeds of flowering plants can differentiate their seed coat epidermis into the spe-
cialized cell layer producing a hydrophilic mucilage with several ecological functions, such as seed
hydration, protection, spatial fixation, stimulation of metabolic activity and development of seed.
Due to the species- and genotype-dependent variabilities in the chemical composition of mucilage,
mucilage does not display the same functional properties and its role depends on the respective
species and environment. Mucilaginous substances, depending on their composition, exhibit many
preventive and curative effects for human and animal health, which has significant potential in the
agricultural, food, cosmetic and pharmaceutical industries. This paper summarizes the ecological,
biological, and functional properties of mucilaginous plant substances and highlights their significant
nutritional potential in terms of the development of functional foods, and nutraceuticals and dietary
supplements. A paragraph describing the gene regulation of seed mucilage synthesis is included, and
some recommendations for the direction of further research on mucilaginous substances are outlined.
Keywords:
mucilages; ecological functions; human and animal health-promoting properties; application
in agriculture; genes; nutritional components
1. Introduction
Some plants are characterized by producing a large quantity of various above- and
below-ground secretions called mucilages or exudates. These can be secreted by roots,
leaves, stems, or seeds, and perform different functions depending on the plant species [
1
].
Myxodiaspores are plants with the ability to initiate the differentiation of seed coat epi-
dermis into the specialized cell layer upon fertilization, which synthesizes hydrophilic
mucilage in the Golgi apparatus. Subsequently, the mucilage is secreted into the apoplastic
compartment via secretory vesicles [
2
,
3
]. The mucilage forms a shell around the seed in the
form of a gel-like transparent capsule, which represents a kind of modified cell wall with all
typical polysaccharides, i.e., celluloses, pectins and hemicelluloses. Examples of plants with
seeds that produce mucilage include Arabidopsis thaliana L.,
Ocimum basilicum L.
,Lepidium
sativum L., Salvia sclarea L., Artemisia annua L., Linum usitatissimum L. and Artemisia leucodes
Schrenk [4,5].
2. Methodology
We used the keywords (seed mucilage) to query the PubMed
®
database https://
pubmed.ncbi.nlm.nih.gov/ (accessed on 24 May 2022), and the query returned a total of
528 search results. Since 1999, we have been observing a linear increase in the number
of articles on this topic, with a few exceptions. The first article on seed mucilage was
written in 1932, and the highest number of articles on seed mucilage was published in 2021
(70), which only confirms the current trend of increasing interest in this functional food
ingredient. In our research, we tried to link the already established knowledge on plant
seed mucilage with new information. In total, 92 articles related to plant seed mucilage
Nutraceuticals 2022,2, 253–269. https://doi.org/10.3390/nutraceuticals2040019 https://www.mdpi.com/journal/nutraceuticals
Nutraceuticals 2022,2254
were used, with 41 of these being less than 5 years old. Three articles were written in 2022,
thirteen in 2021, five in 2020, nine in 2019 and eleven in 2018.
3. Ecological Functions of Mucilage
Mucilaginous substances have several ecological functions for plants (Figure 1), in-
cluding seed hydration and protection from desiccation and spatial fixation in the soil,
which affects their topochory, epizoochory, endozoochory and hydrochory. In addition,
they maintain the metabolic activity of the seed and encourage its development. Mucilage
contains substances that serve as a source of energy for the seeds and microorganisms in
the soil. The exact role of mucilage seems to depend on the species and environmental
context [
3
,
6
]. Eragostris pilosa (L.) BEAUV. seeds produce mucilage that allows them to
survive in dry habitats. Their mucilage consists of pectins that form uniform layers on the
inner surface of the cell walls, which are bounded by a thin layer of cellulose preventing
them from being released into the cell lumen. In the presence of water, these pectins are
hydrated and cause the mucilage cells to swell up. Subsequently, they start to detach. The
aforementioned ability of Eragostris creates suitable conditions for germination [
7
]. Simi-
larly, even the seeds of Henophyton deserti COSS. & DUR . are drought resistant. Mucilage
represents 30% of the seed mass in this species. It can increase the weight of seeds by up
to 550%. It has been shown that the mucilage of H. deserti works as a physical barrier in
the regulation of the diffusion of water and oxygen into the inner seed coat. With this
mechanism, it can prevent germination from occurring in unsuitable conditions. It was
proved experimentally that higher concentrations of PEG inhibit mucilage hydration, but
salt concentration has no effect on it. Mucilage reduces both the percentage and rate of
seed germination, especially at 10
◦
C, and at high concentrations of NaCl and PEG [
8
]. The
ability of mucilage to reduce germination under mild osmotic stress and subsequently to
assist germination once this stress is relieved has also been confirmed in Nepeta micrantha
BUNGE [
9
]. In addition to drought, plant survival on the desert dunes also depends on the
burial depth in the sand. In the experiments conducted with the Artemisia sphaerocephala
KRASCH. seeds, it was found that mucilage significantly increased seed emergence at a
0.5 and 10 mm burial depth under low irrigation, at a 0 and 5 mm burial depth under
medium irrigation, and at a 0 and 10 mm burial depth under high irrigation. Seed mucilage
also reduced seed mortality at shallow sand burial depths [
10
]. In addition, seed mucilage
increased the surface dislocation force, allowing the seeds to anchor in highly erosive soils.
When mucilage seeds from 52 plant species varying in their characteristics were tested,
it was found that the largest effect on the resistance to water flow during erosion is due
to the mucilage mass. Moreover, resistance to flow was largely dependent on the water
flow speed and the rate of seed germination [
11
]. When mucilage is released from the
seed, various particles of sand and dirt adhere to the seeds and remain on the seed surface
after drying. This leads to the formation of a physical barrier that protects the seeds from
predators (e.g., ants) [
12
]. Mucilaginous substances also affect seed germination. In optimal
laboratory conditions, the difference between mucilaginous seeds (s1) and seeds with the
mucilage removed (s2) was only in the germination rate (s1: 97% germination after 26 h; s2:
63% germination after 26 h). When exposed to salt stress, the s1 seeds germinated up to
48% more than the s2 seeds [
13
]. This may also be due to the presence of some enzymes
in the mucilage that may assist in breaking the radicle envelope of the seeds, whereas
demucilaged seeds do not contain such apoplast enzymes. Examples of such enzymes
include pectinases,
β
-D-xylosidases and
α
-L-arabinofuranosidases, which are found in the
mucilage of flaxseed [5,14].
Nutraceuticals 2022,2255
Nutraceuticals 2022, 2, FOR PEER REVIEW 3
Figure 1. Ecological functions of mucilage (Kučka, elaborated based on [3–14].
4. Effects of Mucilage on Human and Animal Health
Depending on the composition, mucilaginous substances can exhibit antihypercho-
lesterolemic, laxative and anticarcinogenic effects, and also have an effect on glucose me-
tabolism. These effects help to prevent, or at least reduce, the risk of various major diseases
such as diabetes, lupus nephritis, arteriosclerosis and hormone-dependent cancers [15–
18]. Cordia dichotoma G. FORST. seed mucilage has been investigated for its antihypercho-
lesterolemic effects. The study used rats, which were on a high-lipid diet, resulting in a
significant increase in total cholesterol and low-density lipoprotein cholesterol, as well as
in a significant decrease in antioxidant enzymes in the liver (glutathione reductase, gluta-
thione peroxidase, glutathione-S-transferase, catalase and superoxide dismutase). Treat-
ment with the C. dichotoma mucilage at a 0.5 and 1g per kg not only improved the lipid
profile, but it also improved the liver and kidney function, even in the rats on a normal
diet. Additionally, the antioxidant system in the liver was also improved [15]. The muci-
lage from Abelmoschus esculentus (L.) MOENCH, in addition to its antihypercholesterolemic
effects, also had an effect on glucose levels when abnormal changes in body weight, water
consumption, feed consumption and blood glucose levels occurred after 3 weeks of mu-
cilage administration to alloxan-induced diabetic mice. At baseline, all mice had fasting
blood glucose levels of approximately 4.1 mmol·L−1. After the induction of alloxan, the
blood glucose concentration increased to 12.3 ± 0.8 mmol·L−1 in one group and to 13.1 ±
0.8 mmol·L−1 in the other group. After the administration of 150 mg per kg of mucilage to
the first group, the blood glucose level decreased to 7.1 ± 0.4 mmol·L−1 after three weeks,
and in the second group the level decreased to 6.7 ± 0.4 mmol·L−1 [18] after the administra-
tion of 200 mg per kg of mucilage. The laxative activities of flaxseed mucilage and oil have
also been investigated. Flaxseed mucilage had laxative effects at doses of 1 and 2.5 g·kg-1
with the resulting percentage increase of 65.06 ± 6.5% and 89.33 ± 4.04% in wet feces. The
spasmogenic effect of flaxseed mucilage was completely blocked in the presence of atro-
pine and partially blocked (63.9%) in the presence of pyrilamine. The laxative effect of
both flaxseed mucilage and oil is probably mediated by the stimulation of cholinergic and
histaminergic receptors, with a more pronounced cholinergic component in flaxseed mu-
cilage [19]. Mucilage also exhibits anti-inflammatory and antioxidant effects, and the mu-
cilage from fenugreek seeds showed a beneficial effect against rat arthritis when induced
by intradermal injection of complete Freund’s adjuvant. The maximum rate of edema in-
hibition was observed at a mucilage dose of 75 mg·kg-1 on the 21st day of adjuvant
Figure 1. Ecological functions of mucilage (Kuˇcka, elaborated based on [3–14].
4. Effects of Mucilage on Human and Animal Health
Depending on the composition, mucilaginous substances can exhibit antihyperc-
holesterolemic, laxative and anticarcinogenic effects, and also have an effect on glucose
metabolism. These effects help to prevent, or at least reduce, the risk of various ma-
jor diseases such as diabetes, lupus nephritis, arteriosclerosis and hormone-dependent
cancers [
15
–
18
]. Cordia dichotoma G. FORST. seed mucilage has been investigated for its
antihypercholesterolemic effects. The study used rats, which were on a high-lipid diet,
resulting in a significant increase in total cholesterol and low-density lipoprotein choles-
terol, as well as in a significant decrease in antioxidant enzymes in the liver (glutathione
reductase, glutathione peroxidase, glutathione-S-transferase, catalase and superoxide dis-
mutase). Treatment with the C. dichotoma mucilage at a 0.5 and 1g per kg not only improved
the lipid profile, but it also improved the liver and kidney function, even in the rats on
a normal diet. Additionally, the antioxidant system in the liver was also improved [
15
].
The mucilage from Abelmoschus esculentus (L.) MOENCH, in addition to its antihypercholes-
terolemic effects, also had an effect on glucose levels when abnormal changes in body
weight, water consumption, feed consumption and blood glucose levels occurred after
3 weeks of mucilage administration to alloxan-induced diabetic mice. At baseline, all mice
had fasting blood glucose levels of approximately 4.1 mmol
·
L
−1
. After the induction of
alloxan, the blood glucose concentration increased to 12.3
±
0.8 mmol
·
L
−1
in one group
and to 13.1
±
0.8 mmol
·
L
−1
in the other group. After the administration of 150 mg per kg
of mucilage to the first group, the blood glucose level decreased to 7.1
±
0.4 mmol
·
L
−1
after three weeks, and in the second group the level decreased to 6.7
±
0.4 mmol
·
L
−1
[
18
]
after the administration of 200 mg per kg of mucilage. The laxative activities of flaxseed
mucilage and oil have also been investigated. Flaxseed mucilage had laxative effects at
doses of 1 and 2.5 g
·
kg
−1
with the resulting percentage increase of 65.06
±
6.5% and
89.33
±
4.04% in wet feces. The spasmogenic effect of flaxseed mucilage was completely
blocked in the presence of atropine and partially blocked (63.9%) in the presence of pyril-
amine. The laxative effect of both flaxseed mucilage and oil is probably mediated by the
stimulation of cholinergic and histaminergic receptors, with a more pronounced cholinergic
component in flaxseed mucilage [
19
]. Mucilage also exhibits anti-inflammatory and an-
tioxidant effects, and the mucilage from fenugreek seeds showed a beneficial effect against
Nutraceuticals 2022,2256
rat arthritis when induced by intradermal injection of complete Freund’s adjuvant. The
maximum rate of edema inhibition was observed at a mucilage dose of 75 mg
·
kg
−1
on the
21st day of adjuvant arthritis. After the treatment with mucilage from fenugreek seeds, the
activity of inflammatory enzymes (cyclooxygenase-2 and myeloperoxidase) as well as the
concentrations of thiobarbituric acid reactive substance decreased. On the other hand, there
was an increase in the activity of antioxidant enzymes (catalase, superoxide dismutase,
glutathione peroxidase), the levels of glutathione and vitamin C and lipid peroxidation.
Additionally, the erythrocyte sedimentation rate and total white blood cell count increased
significantly [
20
]. In addition, the prebiotic effect of chia mucilage, which is mainly due
to the neutral mucilage polysaccharides, has been demonstrated. Compared to the low
molecular weight prebiotics, the growth of some groups of intestinal bacteria, such as
Enterococcus and Lactobacillus, is more delayed on mucilage but it lasts longer. The effects
of chia mucilage at three different concentrations (0.3, 0.5 and 0.8%) on the growth and
metabolic activity of human gut microbiota using the Simgi
®
dynamic gastrointestinal
model have also been investigated. The researchers found that all mucilage concentrations
significantly affected all bacterial groups of the gut microbiota, but the 0.3% concentration
of chia mucilage had the most significant effect on the increase in total aerobes in the
transverse colon and descending colon. Increases were also observed for lactic acid bacteria,
Enterococcus spp. and Staphylococcus spp., and in contrast, no significant changes were
observed for Enterobacteriaceae,Clostridium spp. and Bifidobacterium spp. By providing a
substrate for the microorganisms, the chia mucilage also affects the resulting fermentation
products, such as short-chain fatty acids (SCFAs). In the experiment, different values of
SCFAs (acetic, propionic and butyric acid) were observed at different concentrations of
chia mucilage, and the dependence of SCFA production on different parts of the gut was
also observed. In the ascending colon, the greatest increase was observed on day 5 at a
0.5% concentration of chia mucilage, while in the transverse and descending colon, the
increase was observed mainly on day 3 after the administration of chia mucilage. However,
an increase was also observed in the transverse and descending colon on day 5 and day 8 at
a 0.8% and 0.5% chia mucilage [
21
,
22
]. Recent studies suggest that flaxseed mucilage also
exhibits antibacterial activity against several Gram-positive and Gram-negative bacteria
using the agar well diffusion method and disk diffusion method. Mucilage showed strong
antibacterial properties against all strains tested except Listeria monocytogenes [
23
]. There
was also a potential to improve the course of chronic obstructive pulmonary diseases when
the Pharmacopeial Unani formulation: linctus of flax mucilage [
24
] was used as the test
drug. In Iranian traditional medicine, mucilage from quince seeds is used to treat skin
wounds and burns. In a study on mucilage in rabbits, it was concluded that mucilage
from quince seeds increases the level of growth factors in the wound fluids are involved
in tissue repair, and therefore has good potential to promote wound healing at a 10–20%
concentration [
25
]. The healing effects against the T-2 toxin-induced dermal toxicity in
rabbits has also been demonstrated for mucilage obtained from quince seeds. This mucilage
probably preserves the wound surface proteins whose synthesis is inhibited by the T-2
toxin. In addition, it is thought to act as a barrier against microorganisms and may also
activate the growth factors and thereby facilitate skin healing [
26
]. In medicine, there
is potential to use mucilage as a polymer capable of retaining water, for example, for
wound dressings. An antibacterial wound dressing was prepared by the lyophilization of
basil mucilage and with the addition of the antibacterial agent zinc oxide nanoparticles
(ZnO-NPs). Hydrogen bonding and electrostatic interaction were confirmed between the
slime and ZnO-NPs molecules. The resulting product was non-adhesive and non-toxic,
with reasonable mechanical and thermal properties, which were further enhanced by the
addition of ZnO to promote antibacterial capabilities. It was confirmed that the porosity,
swelling and water retention of the product were suitable for use as a wound dressing.
Due to its good porosity, basil mucilage gel is able to absorb a high volume of exudate
from the wound surface. Water retention capacity is one of the most important properties
of wound dressing because it allows the holding of water molecules within its structure.
Nutraceuticals 2022,2257
The addition of ZnO-NPs slightly decreases porosity and swelling, but slightly increases
water retention [
27
]. Mucilage has the potential to be used as a superdisintegrant in the
production of pharmaceutical tablets by direct compression with other excipients and in
wet granulation technology where the mucilage from basil seeds (Ocimum basilicum L.)
was successfully used to produce the drug metoprolol tartarate [
28
]. Similarly, mucilage
from plantain (Plantago psyllium L.) at a 3% (w/w) concentration can also be used as a drug
binder. Studies indicate that paracetamol with this formulation is released more slowly
than the traditional drug [
29
]. The Ocimum basilicum L. seed mucilage can also be used as a
nasal gel containing paracetamol [
30
]. The mucilage from the seeds of Lallemantia royleana
(BE NT H.) itself exhibits analgesic effects, and was used to create a mixture of commercial
2% lidocaine gel and a mucilage-containing gel (0.01 g
·
ml
−1
), which increased the efficacy
of this local anesthetic [31].
5. Potential Uses of Mucilage in Agriculture and Industry
Mucilaginous substances have potential in agriculture, food, cosmetics and pharma-
ceutical industries (Table 1) [
32
]. In the food industry, chia mucilage can be used as a
low-fat source of fiber. The addition of 7.5% chia seed mucilage to a yogurt recipe reduced
the degree of syneresis during storage compared to full-fat yogurt and improved the nutri-
tional value of the yogurt by increasing the fiber content. In addition, the resulting yogurt
had a higher consistency, firmness, viscosity and better resistance to stress. The sensory
acceptability of the resulting yogurts in terms of acidity, creaminess and viscosity was
similar to full-fat yogurts [
33
]. Similarly, the addition of flaxseed mucilage increased the
viscosity and decreased yogurt syneresis. In addition, it decreased the cohesiveness and
increased the stickiness of the blended yogurt, while its addition in combination with car-
boxymethylcellulose resulted in decreased stickiness, increased cohesiveness and elasticity.
The mucilage of flax with the addition of carboxymethylcellulose resulted in an increase
in Lactobacillus bulgaricus in the blended yogurt, although the addition of mucilage alone
had little effect on the growth of this lactic bacterium. On the other hand, the addition of
mucilage itself had a considerable effect on the growth of Streptococcus thermophilus [
34
].
The mucilage from chia seeds can serve as a substitution for some oil in mayonnaise, thus
increasing its stability, textural parameters and reducing the amount of fats [
35
]. Similarly,
the addition of chia mucilage to pie dough reduces the fat content and increases fiber
and protein contents [
36
], and some studies have shown that chia mucilage can replace
emulsifiers and stabilizers in the preparation of ice cream [
37
]. Mucilage can also be used
to encapsulate important substances, such as probiotics, which can improve the functional
properties of food. It has been shown that quince seed mucilage is able to increase the
survival rate of Lactobacillus rhamnosus up to 72
◦
C by encapsulation, and is also suitable as
a transport matrix in the gastrointestinal environment when the bacteria are released at an
appropriate time after reaching the intestinal tract [
38
]. The mucilage and soluble proteins
from chia and flax seeds can be used as encapsulating material for two probiotic bacteria:
Bifidobacterium infantis and Lactobacillus plantarum [
39
]. Using the electrospinning method,
it was possible to incorporate the flavonoid hesperetin into basil mucilage nanofibers in
conjunction with polyvinyl alcohol. After a successful encapsulation, there was an increase
in resistance to high temperatures (from 182
◦
C to 314
◦
C) and a decrease in their release
rate in acidic environments (pH 1.2) [
40
]. Vitamin A was also encapsulated by a similar
principle using watercress seed mucilage and polyvinyl alcohol. Again, its stability in
acidic environments and against high temperatures was enhanced [
41
]. Last but not least,
mucilage can be used to produce biodegradable and antimicrobial edible films that increase
the shelf life of food. Films made out of the psyllium seed mucilage, oregano extract and
glycerol as a plasticizer had effective antimicrobial activities against Staphylococcus aureus
and Escherichia coli and extended the postharvest shelf life of strawberries to 16 days [42].
Nutraceuticals 2022,2258
Table 1. Application of mucilage in industry and agriculture.
Application Area Plant Source Applied Form Achieved Properties Reference
Food industry
Salvia hispanica L.,
Linum usitatissimum L. Additive in yogurts
Improved nutritional
properties, syneresis and
viscosity
Refs. [33,34]
Salvia hispanica L. Additive in mayonnaise
Increased stability, reducing fat
Ref. [35]
Salvia hispanica L. Additive in cakes Improved nutritional qualities Ref. [36]
Salvia hispanica L. Additive in ice cream Replacement for stabilizers
and emulsifiers Ref. [37]
Salvia hispanica L.
Linum usitatissimum L.
Cydonia oblonga MIL LE R
Encapsulation of probiotics
Better resistance in the
digestive tract Refs. [38,39]
Ocimum basilicum L.
Lepidium sativum L.
Encapsulation of vitamins
and flavonoids
Better resistance in the
digestive tract Refs. [40,41]
Plantago psyllium L. Production of edible films Increased food shelf life Ref. [42]
Pharmaceutical
industry
Lallemantia royleana
(BE NT H.) Formation of gels Healing effects against dermal
toxicity and burns Ref. [31]
Ocimum basilicum L. Wound dressing formation Antimicrobial effects Ref. [27]
Ocimum basilicum L.
Plantago psyllium L.
Formation of medicinal
tablets
Slower release, replacement of
chemical preparations Refs. [28,29]
Ocimum basilicum L. Formation of nasal gel Analgesic effects Ref. [30]
Cosmetics Salvia hispanica L. Gel formation UV-protective effects Ref. [43]
Agriculture Salvia hispanica L. Hydrogels in arid areas Retention of water Refs. [44,45]
Engineering industry Linum usitatissimum L. Biocomposite binder
Inexpensive and biocompound
Ref. [46]
In cosmetics, chia seed mucilage has promising potential due to its high photostability
under UV light and muco-adhesion, which promotes the adhesion of the formulation to
the mucosa [
43
]. In agriculture, mucilage can be used as a hydrogel that retains water in
the rhizosphere, which, in addition, reduces surface tension and increases soil viscosity
and the hysteresis index [
44
]. Therefore, it is potentially possible to use mucilage for plant
growth in arid deserts [
45
]. In the industry, mucilage is used as a binder for biocomposite
materials in which plant fibers serve as a reinforcing component [46].
6. Physical and Chemical Properties of Mucilage
As a natural product, the composition of mucilage can vary in space and time de-
pending on a variety of external and internal conditions [
47
]. In addition, there are also
significant variations in the chemical composition and functional properties of mucilage
among different plant species and varieties (Table 2) [
48
]. In general, the seed mucilage of
different plants is mainly composed of polysaccharides. Mucilaginous polysaccharides are
a source of energy for microorganisms, absorb water, exchange cations and allow the plant
to adhere to solid surfaces in the rhizosphere [
49
]. The composition of polysaccharides is
mainly influenced by the enzymes secreted by the plant during water imbibition along
with mucilage [
5
]. The mucilage coat of myxodiaspores seeds represents a modified cell
wall. Chemically, it is mainly composed of the polysaccharide groups typical for the cell
wall, mainly hemicelluloses (cellulose type of mucilage—e.g., Neopallasia pectinata (PALL.)
POLJAKOV), but very often pectins are the main component (pectin type of mucilage—e.g.,
Linum usitatissimum L.) [
50
]. The flax mucilage of the Eden cultivar mainly consists of
rhamnogalacturonan-I (52–62%), which is influenced by the enzymes rhamnogalacturonase
and
β
-d-galactosidase, and arabinoxylan (27–36%), which is related to the activity of the
enzymes
α
-l-arabinofuranosidase,
β
-d-xylosidase and
β
-xylanase. The highest value of xy-
lanase activity was observed after 4 h of seed hydration, resulting in the low viscosity of the
Nutraceuticals 2022,2259
polysaccharides, which mainly contained pectic sugars. Maximum glycosidase activities
were observed 24 to 48 hours after the application of water hydration, and mucilaginous
substances, which were tightly bound to the cell walls, were released. The presence of
β
-d
xylosidase and
α
-l-arabinofuranosidase activities was also confirmed [
5
]. By their high
molecular weight, the polysaccharides of linseed mucilage represent about 3 to 9% of the
total weight of the seed and are divided into two components: neutral and acidic. The neu-
tral component is composed of D-xylose, L-arabinose and D-galactose in a ratio of 6.2:3.5:1,
while the acidic component contains L-rhamnose, L-fucose, L-galactose and D-galacturonic
acid in a ratio of 2.6:1:1.4:1.7 [
48
,
51
]. On average, flax varieties with yellow seeds were
found to have a higher content of neutral polysaccharides (arabinoxylans) due to the pres-
ence of the s1 gene, while brown seeds had a higher content of acidic polysaccharides
(pectins) [
52
]. In addition to polysaccharides, they also contain glycoproteins and various
bioactive components, such as tannins, alkaloids and steroids to a lesser extent [
32
,
49
,
53
].
The main constituent of the mucilage of Lepidium perfoliatum L. species is the highly methyl
esterified homogalacturonan (HG). In addition, a significant amount of callose and hemi-
cellulose and a small amount of weakly methyl esterified HG were present in the seed coat
mucilage of L. perfoliatum L. [
2
]. Lallemantia royleana (BE NT H.) seed mucilage, similar to
other mucilage, is mainly composed of carbohydrates (76.74%), of which the most abun-
dant monosaccharides are galactose (36.28%) and arabinose (35.96%). The less abundant
monosaccharides are rhamnose (15.18%), xylose (7.38%) and glucose (5.20%). In addition
to carbohydrates, the mucilage of L. royleana (BE NT H.) seeds is also composed of protein
(3,86%), ash (9,92%) and moisture (9,48%). Overall, it contains 82.56
±
1.6
µ
g GAE/mg
of phenolic compounds [
54
]. A similar polysaccharide content of Lallemantia royleana
(BE NT H.) mucilage (Figure 2) was also determined by [
55
]. The researchers observed that
Lallemantia royleana (BE NT H.) mucilage consisted of arabinose (37.88%), galactose (33.54%),
rhamnose (18.44%), xylose (6.02%) and glucose (4.11%) [
55
]. The mucilage from basil is
mainly composed of high-molecular-weight polysaccharides (2320 kDa), which consist
of glucose, galactose, mannose, arabinose, xylose and rhamnose. The polysaccharides of
basil mucilage are slightly acidic due to the presence of uronic acid (6.51%) [
56
]. Chia seed
mucilage contains 93.8% carbohydrates, which form the following monosaccharide units:
xylose, glucose, arabinose, galactose, glucuronic acid and galacturonic acid [
57
]. These
subsequently form D-xylosyl and D-glucosyl residues in a 2:1 ratio. Additionally, it contains
22 to 25% 4-0-methyl-D-glucuronopyranosyl residues. The acetates of xylitol, glucitol and
4-O-methylglucitol are present in a ratio of 8:4:3. Another component of the polymer is
4-O-methyl-D-glucuronic acid [
58
]. The mucilage from the seeds of Hyptis suaveolens L.
contains acidic and neutral heteropolysaccharides in a ratio of approximately 1:1. The
neutral polysaccharides are composed of galactose, glucose and mannose, which form
the polysaccharides galactoglucan (30%) and galactoglucomannan (70%), while the acidic
polysaccharides contain residues of fucose, xylose and 4-O-methylglucuronic acid [
21
,
59
].
The total carbohydrate content of watercress mucilage is 87.4%, of which the most abundant
carbohydrates are mannose (38.9%), arabinose (19.4%), galacturonic acid (8.0%), fructose
(6.8%), glucuronic acid (6.7%), galactose (4.7%), rhamnose (1.9%) and glucose (1.0%) [60].
Table 2. Carbohydrate composition of some seed mucilages.
Plant Source of Seed Mucilage Carbohydrates Reference
Linum usitatissimum L. Rhamnogalacturonan and arabinoxylan Ref. [5]
Linum usitatissimum L. D-xylose, L-arabinose, D-galactose, L-ramnose, L-fucose,
L-galactose, D-galacturonic acid Ref. [51]
Lepidium perfoliatum L. Methylesterified homogalacturonan, callose, hemicellulose Ref. [2]
Lallemantia royleana BEN TH. Galactose, arabinose, rhamnose, xylose, glucose Refs. [54,55]
Ocimum basilicum L. Glucose, galactose, mannose, arabinose, xylose, rhamnose Ref. [56]
Nutraceuticals 2022,2260
Table 2. Cont.
Plant Source of Seed Mucilage Carbohydrates Reference
Salvia hispanica L. Xylose, glucose, arabinose, galactose, glucuronic acid,
galacturonic acid Ref. [57]
Salvia hispanica L. Residues of D-xylosyl, D-glucosyl,
4-0-methyl-D-glucuronopyranosyl Ref. [58]
Hyptis suaveolens L. Galactose, glucose, mannose, galactoglucan,
galactoglucomannan, fucose, xylose, 4-O-methylglucuronic acid
Refs. [21,59]
Lepidium sativum L. Mannose, arabinose, galacturonic acid, fructose, glucuronic
acid, galactose, rhamnose, glucose Ref. [60]
Nutraceuticals 2022, 2, FOR PEER REVIEW 7
polysaccharides (arabinoxylans) due to the presence of the s1 gene, while brown seeds
had a higher content of acidic polysaccharides (pectins) [52]. In addition to polysaccha-
rides, they also contain glycoproteins and various bioactive components, such as tannins,
alkaloids and steroids to a lesser extent [32,49,53]. The main constituent of the mucilage
of Lepidium perfoliatum L. species is the highly methyl esterified homogalacturonan (HG).
In addition, a significant amount of callose and hemicellulose and a small amount of
weakly methyl esterified HG were present in the seed coat mucilage of L. perfoliatum L.
[2]. Lallemantia royleana (BENTH.) seed mucilage, similar to other mucilage, is mainly com-
posed of carbohydrates (76.74%), of which the most abundant monosaccharides are galac-
tose (36.28%) and arabinose (35.96%). The less abundant monosaccharides are rhamnose
(15.18%), xylose (7.38%) and glucose (5.20%). In addition to carbohydrates, the mucilage
of L. royleana (BENTH.) seeds is also composed of protein (3,86%), ash (9,92%) and moisture
(9,48%). Overall, it contains 82.56 ± 1.6 μg GAE/mg of phenolic compounds [54]. A similar
polysaccharide content of Lallemantia royleana (BENTH.) mucilage (Figure 2) was also de-
termined by [55]. The researchers observed that Lallemantia royleana (BENTH.) mucilage
consisted of arabinose (37.88%), galactose (33.54%), rhamnose (18.44%), xylose (6.02%)
and glucose (4.11%) [55]. The mucilage from basil is mainly composed of high-molecular-
weight polysaccharides (2320 kDa), which consist of glucose, galactose, mannose, arabi-
nose, xylose and rhamnose. The polysaccharides of basil mucilage are slightly acidic due
to the presence of uronic acid (6.51%) [56]. Chia seed mucilage contains 93.8% carbohy-
drates, which form the following monosaccharide units: xylose, glucose, arabinose, galac-
tose, glucuronic acid and galacturonic acid [57]. These subsequently form D-xylosyl and
D-glucosyl residues in a 2:1 ratio. Additionally, it contains 22 to 25% 4-0-methyl-D-glucu-
ronopyranosyl residues. The acetates of xylitol, glucitol and 4-O-methylglucitol are pre-
sent in a ratio of 8:4:3. Another component of the polymer is 4-O-methyl-D-glucuronic
acid [58]. The mucilage from the seeds of Hyptis suaveolens L. contains acidic and neutral
heteropolysaccharides in a ratio of approximately 1:1. The neutral polysaccharides are
composed of galactose, glucose and mannose, which form the polysaccharides galactoglu-
can (30%) and galactoglucomannan (70%), while the acidic polysaccharides contain resi-
dues of fucose, xylose and 4-O-methylglucuronic acid [21,59]. The total carbohydrate con-
tent of watercress mucilage is 87.4%, of which the most abundant carbohydrates are man-
nose (38.9%), arabinose (19.4%), galacturonic acid (8.0%), fructose (6.8%), glucuronic acid
(6.7%), galactose (4.7%), rhamnose (1.9%) and glucose (1.0%) [60].
Figure 2. Difference in carbohydrate composition of Lallemantia royleana (BENTH.) seed mucilage be-
tween two studies [54,55].
36% 36%
15%
74%
52%
34% 38%
18%
60%
41%
GALACTOSE ARABINOSE RHAMNOSE XYLOSE GLUCOSE
Lallemantia royleana (Benth.) (Behbahani and Fooladi, 2018)
Lallemantia royleana (Benth.) (Razavi et al., 2015)
Figure 2.
Difference in carbohydrate composition of Lallemantia royleana (BEN TH.) seed mucilage
between two studies [54,55].
When comparing the mucilage from several plants, it was observed that the lipid
content of mucilage generally tended to be low. For example, the lipid content in the
mucilage of yellow mustard was only 0.2%, 0.5 to 0.7% in flax, 4.76% in tamarind and
1.85% in watercress seeds. However, mucilage lipids provide important functions for the
plant, improving their water uptake and desorbing the adsorbed phosphorus on the soil
particles in the rhizosphere. The amount of protein varies considerably from plant to
plant, with Indian plantain seed mucilage containing 0,94% protein, Artemisia sphaerocephala
(KRASCH.) mucilage up to 24,1%, tamarind seed mucilage 14,78% and linseed mucilage
having a protein content of between 4,4 and 15,1%. Mucilage proteins break down mucilage
polysaccharides into the forms available to microorganisms, they respond to biotic and
abiotic stresses, and mobilize nutrients in the rhizosphere [
49
,
53
,
61
]. An average mineral
content of plant mucilage is 5.6% and they are also important in the exchange of cations
between the plant and the rhizosphere and improve the coupling of the liquid phase
of the soil with the water content [
49
]. Altogether, six chemical elements—copper, zinc,
cobalt, lead, chromium, chromium and cadmium—have been detected in the mucilage of
flaxseed [
48
]. The most abundant chemical element in cress mucilage is calcium (0.17%),
but it also contains sodium, potassium and magnesium [60].
Nutraceuticals 2022,2261
Although the chemical composition of mucilage is well known, its structural or-
ganization is unclear. The fibrillar character of the individual mucilage components is
demonstrated by both the pectic and cellulosic types of mucilage. However, due to the
presence of cellulose microfibrils, cellulose mucilage is much more organized [
50
]. Using
critical point drying (CPD) and scanning electron microscopy (SEM), the structural details
of mucilage were resolved down to the nanoscale. The mucilaginous fibrillar components
generally form a network of cellulose fibers that serve as a scaffold for other polysaccharide
fibers, which often branch out and are found between or on the surface of the cellulose
fibers. The cellulose fibrils are long, thick, unbranched and, by being attached to the surface
of the seeds, prevent the loss of the mucilage cover by mechanical impact. Interestingly,
the structural organization of mucilage varies among plant species, which is important for
water binding and storage [
4
]. Pectic mucilage, on the other hand, has a fibrous, convoluted
and more homogeneous structure than the cellulosic type [50].
7. Functional Properties of Plant Seed Mucilage
The mucilaginous substances of the plants are odorless, colorless and tasteless. In
addition, they are non-toxic and biodegradable [
32
]. Mucilage can also exhibit good
photostability; for example, mucilage obtained from the seeds of Salvia hispanica L. showed
a degradation percentage of 6.6% after 120 min under UV light [
43
]. Three parameters in
the extraction of mucilage have a great influence on the functional properties of mucilage—
temperature, pH and water/seed ratio. It has been observed that the maximum values
of extraction, viscosity, emulsion stability, foam stability, solubility and water absorption
capacity (9.3 g/g) of the Eruca sativa MILL. seed mucilage could be achieved at an extraction
temperature of 65.5 ◦C, pH 4 and a water-to-seed ratio of 60:1 [62].
A very important indicator of the quality of mucilage is its molecular weight because
the polymer chains interact when the mucilage dissolves, and mucilage with a high molec-
ular weight can improve its viscosity. This property can be used to improve the texture
of foods and it also affects the mouthfeel of the consumer [
63
]. The molecular weight of
mucilage also affects the emulsifying and foaming properties [
64
]. The mucilage of different
plants has different molecular weights, for example, the mucilage from the seeds of Hyptis
suaveolens L. contains an anionic fraction responsible for swelling and viscous behavior with
an average molar mass of 0.35
×
10
6
g
·
mol
−1
, while the neutral polysaccharide fraction
(in a 1:1 ratio) exhibits an average molar mass of 0.047
×
10
6
g.mol
−1
[
59
]. The neutral
component of flaxseed mucilage has a lower molecular weight (1.47
×
10
6
g
·
mol
−1
) than
the acidic part (1851
×
10
6
g
·
mol
−1
) [
65
]. The molecular weight of the Lallemantia royleana
BENTH. in WALL. seed mucilage is 1.19
×
10
6
g
·
mol
−1
,Salvia hispanica L. 2.3
×
10
6
g
·
mol
−1
,
and the molecular weight of the Ocimum basilicum L. seed is 2.32
×
10
6
g
·
mol
−1
[
54
,
66
].
Another study of Lallemantia royleana BEN TH . in WALL. seed mucilage showed that the
molecular weight was 1.294 ×106g·mol−1[55].
The solubility of mucilage improves with increasing temperatures, where the lowest
solubility values for flax mucilage were observed at 20
◦
C (24.52% to 30.95%) and the
highest at 80
◦
C (64.5% to 69.15%) [
48
]. It was observed that the mucilage from both white
and black chia seeds showed similar solubility values between 30 and 60
◦
C. Black chia seed
mucilage showed the greatest solubility at 70
◦
C (80.65%), while the solubility of white chia
seed mucilage remained constant [
67
]. The solubility of Eruca sativa MIL L. at 65.5
◦
C was
28.5% [
61
] and the solubility of Lepidium perfoliatum L. seed mucilage was approximately
20% at 60 ◦C [68].
Furthermore, mucilage exhibits thermostable properties with high degradation tem-
peratures, for example, tamarind seed mucilage starts to lose weight at 175
◦
C and chia seed
mucilage at 244
◦
C [
49
,
53
]. Black chia seed mucilage has a higher thermal decomposition
temperature (286.8 ◦C) than white chia seed mucilage (269.4 ◦C) [67].
Another property of mucilage is its ability to retain water, which is dependent on pore
size, capillary action and the amount of protein components present in the mucilage. Flax
mucilage has a higher water retention capacity compared to microbial xanthan mucilage
Nutraceuticals 2022,2262
and lower water retention capacity compared to plant guar mucilage [
69
]. The mucilage
from the seeds of Lepidium perfoliatum L. showed a similar trend; the water absorption
capacity (around 20 g.g
−1
) was lower than guar but almost identical to xanthan. It is
suggested that the lower water absorption rate by L. perfoliatum L. seed mucilage compared
to guar is due to the strong degree of interaction between the polysaccharide chains and
hence the lower interaction with water [
68
]. In tamarind seed mucilage, the water holding
and oil retention capacities have been shown to increase with temperature [
61
]. The
water absorption capacity of basil seed mucilage is higher (35.16–38.96 g
·
g
−1
) than its oil
absorption capacity (5.40–17.38%) [
70
]. The water absorption capacity of chia seed mucilage
is 54.24
±
0.47 g
·
g
−1
and the water holding capacity is greater (35.49
±
0.24 g
·
g
−1
) than its
oil holding capacity (7.72
±
0.36 g
·
g
−1
) [
67
]. The water absorption capacity of Eruca sativa
Mill. was 9.3 g·g−1[62].
Mucilage proteins are characterized by their good foaming properties; foam sta-
bility increases with increasing the mucilage concentration. Chia seed mucilage has
96.5
±
1.6% foam stability at a 0.1% concentration and 97.8
±
1.2% at a 0.3% concen-
tration [
67
]. Foam stabilization is also affected by the water/seed ratio (negatively) and
temperature (positively) during mucilage extraction. Quince seed mucilage had a 94.89%
emulsion stability and a 21.36% foam stability [
71
] and Eruca sativa MILL. mucilage had an
emulsion stability of 87% and foam stability of 87.5% [
62
]. The foam stability of Lepidium
perfoliatum L. seed gum also increased with increasing concentrations, but was lower com-
pared to xanthan and guar gums at similar concentrations. This trend was probably due to
the differences in viscosity of the continuation phase [68].
Mucilage can also form a cold-solidifying thermo-reversible gel. The strength of this
gel is influenced by the dissolution temperature, pH and addition of minerals. With higher
dissolution temperatures, the strength of the gel increases, and the addition of NaCl and
complex phosphate salt decreases the strength. If we want to increase the strength, we
can add CaCl
2
at a low concentration (<0.3 wt.%), and its strength decreases at higher
concentrations [
72
]. The strength of Hyptis suaveolens (L.) POIT. seed mucilage gel also
increased by the addition of sucrose (1, 3, 5, 10 and 20% w/v) to a 0.5% mucilage dispersion.
This caused the gel to exhibit its shear-thinning behavior to a lesser extent, which had a
stabilizing effect [73].
As the concentration of mucilage increases, its viscosity increases as well. The vis-
cosity and elasticity are also influenced by chemical composition, with both variables
increasing at a higher concentration of xylose and lower concentration of uronic acid.
The viscosity of linseed mucilage ranges from 0.02 to 0.28 Pa
·
s, while the viscosity of
basil seed mucilage ranges from 0.19 to 0.714 Pa
·
s. Depending on the variety and concen-
tration, mucilage can behave as a viscous liquid, viscoelastic liquid or almost an elastic
body [
70
,
74
]. The water/seed ratio during extraction had the highest effect on the viscosity
of the quince seed mucilage, and increasing the extraction time at temperatures of up to
45
◦
C decreased the viscosity. Under optimum extraction conditions, the viscosity of the
mucilage was 1.47396 Pa
·
s [
71
]. The viscosity of Eruca sativa MIL L. in optimal conditions
was 0.357 Pa
·
s [
62
]. The viscosity of the Lepidium perfoliatum L. seed gum decreased with
the increasing shear rate. The highest viscosity (approximately 3 Pa
·
s) was noted at a shear
rate of approximately 15 (1
·
s
−1
). The comparison of the viscosity of Lepidium perfoliatum L.
seed gum with other commercial gums with the same shear rate showed that the viscosity
of this gum was higher than in locust beans, lower than in guar and almost identical to the
viscosity of xanthan. As with other types of mucilage, increasing the concentration of the
solution leads to an increase in the viscosity of L. perfoliatum seed mucilage, and increasing
the temperature up to 65
◦
C leads to a decrease in viscosity. Interestingly, the addition of
NaCl, KCl, CaCl
2
and MgCl
2
salts also influenced the viscosity of the mucilage, showing a
rapid decrease in viscosity after the addition of 0.2% of any of the salts [
68
]. Although the
mucilage from Lallemantia royleana BENTH. exhibited a similar molecular weight to most
seed mucilage, the intrinsic viscosity (23.06 dL·g−1) was higher [55].
Nutraceuticals 2022,2263
8. Gene Regulation of Seed Mucilage Synthesis
The epidermal cells of plants that secrete mucilage are influenced by several genes
during the development phase, leading to changes in their extracellular matrices. Most
research has focused on the epidermal cell genes of Arabidopsis thaliana L. Research on the
COBRA-LIKE 2 (COBL2) gene, a member of the COBRA-LIKE gene family, found that it
has a specialized function in maintaining a proper cellulose deposition in the seed mu-
cilage [
75
]. Additionally, the MUM 2 gene, a member of glycosyl hydrolase family 35, was
identified. Its localization is in the cell wall of A. thaliana, with the MUM 2 protein entering
the apoplast via the endoplasmic reticulum and the Golgi apparatus network. Overall, the
MUM2 gene exhibits β-galactosidase activity and has a negligible effect on the amount of
mucilage produced or the seed morphology; on the other hand, it is essential for the proper
structure of the produced mucilage [
76
]. The
β
-galactosidase activity of the MUM2 gene
may also be complemented by the TESTA-ABUNDANT2 (TBA2), PEROXIDASE36 (PER36)
and MUCILAGE-MODIFIED4 (MUM4) genes, and thus may be involved in modifying the
polysaccharide composition of seed mucilage [
77
]. It was possible to isolate a sequence
of 308 base pairs of the MUM4 gene that controls the expression of the reporter gene in
both A. thaliana L. and Camelina sativa (L.) Crantz seed coat cells and is regulated by the
same cascade of transcription factors as endogenous MUM4 [
78
]. KNAT3 and KNAT7,
members of the KNOX class II gene family, act as positive regulators of the biosynthetic
gene RG-I MUCILAGE-MODIFIED 4 (MUM4, AT1G53500) and thus affect the produc-
tion of mucilage in A. thaliana L. at early developmental stages [
79
]. The mucilage from
A. thaliana L. is mainly composed of rhamnogalacturonan I, the size of which is influenced
by the MUCILAGE-RELATED70 (MUCI70) gene with glycosyltransferase activity. Addi-
tionally, the CuAO
α
1 gene encoding a putative copper amine oxidase of clade 1a affects
the production of pectin and influences the amount of rhamnogalacturonan I in the outer
mucilage layer [
80
]. The MUM1 gene in A. thaliana L. encodes the transcription factor LEU-
NIG_HOMOLOG (LUH), which is localized in the nucleus. According to the research, the
LUH/MUM1 transcriptional activator could be a positive regulator of the gene-encoding
enzymes required for the extrusion of mucilage—MUM2, SUBSILIN PROTEASE1.7 and
β
-
XYLOSIDASE1 [
81
]. The A. thaliana L. gene GALACTURONOSYLTRANSFERASE-LIKE5
(AtGATL5), which is localized in both the endoplasmic reticulum and Golgi system, could
also be involved in the regulation of the final size of mucilage rhamnogalacturonan I [
82
].
The A. thaliana L. UUAT1 gene encodes a protein localized in the Golgi apparatus that trans-
ports the UDP-glucuronic acid and UDP-galacturonic acid
in vitro
. UDP-glucuronic acid
is a precursor of many seed mucilage polysaccharides and, after synthesis in the cytosol,
it is transported to the Golgi apparatus lumen where it is converted to UDP-galacturonic
acid, UDP-arabinose and UDP-xylose. This suggests that the UUAT1 gene has a key role in
the composition of seed mucilage [
83
]. CELLULOSE SYNTHASE 5 (CESA5)/MUCILAGE-
MODIFIED 3 (MUM3), MUM5/MUCI21, SALT-OVERLY SENSITIVE 5 (SOS5) and FEI2
gene influences the adherence of A. thaliana mucilage. While MUM5 and CESA5 act
as synergists by providing the adhesion of pectin to the seed through cellulose and xy-
lan biosynthesis, SOS5 and FEI2 encode an arabinogalactan protein [
84
]. The PECTIN
METHYLESTERASE INHIBITOR6 gene promotes mucilage release in A. thaliana L. by
inhibiting the activities of endogenous pectin methylesterase that demethylate homo-
galacturonan [
85
]. The genes A. thaliana L. TRANSPARENT TESTA 8, SUBTILISIN-LIKE
SERINE PROTEASE, GALACTUROSYL TRANSFERASE-LIKE 5, MUCILAGE-MODIFIED
4, AGAMOUS-LIKE MADS-BOX PROTEIN AGL62, GLYCOSYL HYDROLASE FAMILY 17
and UDP-GLUCOSE FLAVONOL 3-O-GLUCOSYLTRANSFERASE play a role in mucilage
synthesis and release, seed coat development and anthocyanin biosynthesis, and are among
the promising candidate genes of flaxseed [
86
]. The gene-encoding pectin methylesterases
(PMEs), which control the level of pectin methylesterification, influence the structure and
organization of A. thaliana mucilage. Of the PMEs observed, the PME58 gene showed the
highest expression [
87
]. The direct activation of this gene is provided by two transcription
factors in A. thaliana L., BLH2 and BLH4, which are significantly expressed in mucilage-
Nutraceuticals 2022,2264
secreting cells and thus positively regulate PMEs. In addition to PME58, they also affect
the expression of the genes PECTIN METHYLESTERASE INHIBITOR6, SEEDSTICK, and
MYB52 [
88
]. Conversely, the MUD1 gene, which encodes a nuclear RING domain protein
and is highly expressed in the developing seed coat of A. thaliana L., negatively regulates
the PME levels. MUD1 expression causes a reduction in the expression of PME-related
genes, including MYB52, LUH, SBT1.7, PMEI6 and PMEI14 [89].
The production of mucilage at different developmental stages from the Aechmea sphae-
rocephala (GAUDICH.) Baker seeds is influenced by 21 key regulatory genes (AsNAM-1 to
AsNAM-17, AsAP2-1, AsAP2-2, AsKNAT7 and AsTTG1) whose expressions were different
at 10, 20, 30, 40, 50, 60 and 70 days after flowering. In the period of 10 to 30 days after
flowering, both the AsNAM and AsAP2 genes stimulated the production of mucilage by
their expression. In the period of 40 to 70 days after flowering, the expressions of AsNAM
and AsAP2 were reduced, and conversely, the increase in AsKNAT7 expression inhibited
the formation of mucilage [
90
]. The transcription factors MYB-bHLH-WD40 (MBW) and
APETALA2 (AP2) had a key effect on the production of mucilage in the A. sphaerocephala
(GAUDICH.) Baker seeds. The increased accumulation of UDP-glucose was mediated by an
increased expression of phosphoglucomutase (pgm) and uridine glucose diphosphorylase
(UGPase) and decreased expression of UDP-glucose 4-epimerase (GALE), UDP-glucose
6-dehydrogenase (UGDH) and UDP-glucose 4,6-dehydratase (RHM). The accumulation of
UDP-xylose (UDP-Xyl) was influenced by an increased expression of UDP-apiose/xylose
synthase (AXS) and decreased expression of UDP-arabinose 4-epimerase (UXE) [
91
]. The
transparent testa glabra 1 (TTG1) gene encodes the transcription factor of Lepidium perfo-
liatum that plays a role in epidermal cell differentiation and the release of mucilage. This
gene is 1032 bp long, it encodes 343 predicted amino acids and contains WD40 motifs [
92
].
An overview of the genes/transcription factors, their function in the mucilage process and
spatial localization is shown in Tables 3and 4.
Table 3. Function of genes/transcription factors in the mucilage process.
Function in the Process Genes/Transcription Factors Reference
Mucilage synthesis and release
Transparent testa 8; subtilisin-like serine protease; galacturosyl
transferase-like 5; mucilage-modified 4; agamous-like
MADS-box protein AGL62; glycosyl hydrolase family 17;
pectin methylesterase inhibitor 6
Refs. [85,86]
Mucilage amount Mucilage-modified 2 (MUM2) Ref. [76]
Mucilage proper structure Mucilage-modified 2 (MUM2) Ref. [76]
Mucilage polysaccharide composition Mucilage-modified 2 (MUM2) + testa-abundant 2 (TBA2);
peroxidase 36 (PER36); mucilage-modified 4 (MUM4) Ref. [77]
Mucilage production Knotted arabidopsis thaliana 3 (KNAT3) and knotted
arabidopsis thaliana 7 (KNAT7) Ref. [79]
Mucilage cellulose deposition Cobra-like 2 (COBL2) Ref. [75]
Mucilage composition UDP-uronic acid transporter1 (UUAT 1) Ref. [83]
Mucilage extrusion Leunig homolog (LUH)/mucilage-modified 1 (MUM 1);
enzymes MUM 2; subsilin protease 1.7; beta-xylosidase 1 Ref. [81]
Mucilage adherence Cellulose synthase 5 (CESA5)/mucilage-modified 3 (MUM3) Ref. [84]
Mucilage structure and organization Pectin methylesterase 8 (PME 8) + BLH 2 and BLH 4 Refs. [87,88]
Mucilage rhamnogalacturonan I size
Mucilage-related 70 (MUCI 70); galacturonosyltransferase-like
5 (GATL 5) Ref. [80]
Mucilage rhamnogalacturonan I amount Copper amine oxidase 1 (CuAOX 1) Ref. [80]
Notes: genes are shown in italics.
Nutraceuticals 2022,2265
Table 4. Spatial localizations of some genes included in the mucilage process.
Spatial Localization Genes/Transcription Factors Reference
Epidermal cells Cobra-like 2 (COBL2) Ref. [75]
Cell wall Mucilage-modified 2 (MUM2) Ref. [76]
Seed coat cells Mucilage-modified 4 (MUM4) Ref. [77]
Mucilage-secreting cells BLH 2 and BLH 4 Ref. [88]
Nucleus Leunig homolog LUH Ref. [81]
Endoplasmic reticulum; Golgi apparatus Galacturonosyltransferase-like 5 (GATL5) Ref. [82]
Golgi apparatus UDP-uronic acid transporter1 (UUAT 1) Ref. [83]
Developing seed coat Mucilage defect 1 (MUD1) Ref. [89]
Notes: genes are shown in italics.
9. Summary
Specific cells of some plants can produce hydrophilic mucilage in the Golgi apparatus
and subsequently secrete it into the apoplastic space. This mucilage has several vital
functions for the plant: it protects the seeds from desiccation, fixes the seeds in the soil,
protects the seeds from predation, influences seed germination and serves as a source of
energy for the seeds. In addition, it is priceless in agriculture and the food industry because
it serves as an additive in various foods, and it is also used in the production of edible films
and the encapsulation of probiotics. It is also used in human and veterinary medicines
as it has antihypercholesterolemic, antibacterial, laxative, healing, anti-inflammatory and
anticarcinogenic effects, and it influences glucose metabolism and acts as a prebiotic. It can
be used in the manufacture of tablet medicines and for wound dressings.
Mucilage is mainly composed of polysaccharides, which vary between the species
and varieties, but it also contains other components, such as proteins, lipids, ash, moisture,
phenolics and minerals to a lesser extent. The mucilaginous substances of plants are
odorless, colorless and tasteless; they have a high degradation temperature; good foaming
properties and a high water retention capacity. In the future, mucilaginous substances have
great potential to be used as potential nutraceuticals in disease prevention and treatment.
10. Future Perspectives
For the development of functional foods, food supplements or nutraceuticals, it is
necessary to research more extensively the genotypic variability of the biochemical com-
position of mucilage and its biological and other properties (according to the purpose of
use). The identification of the specific plant genotype reflecting the appropriate/required
parameters of seed mucilage is crucial for advancing the usability of this potential nutraceu-
tical. Therefore, detailed knowledge of the molecular mechanisms behind the regulation
of mucilage biosynthesis mainly at the epigenetic level (microRNAs) should become the
focus of future research.
Author Contributions:
Conceptualization, M.K. and K.R.; methodology, M.K., K.R., L’.H. and T.K.;
validation, K.R., M.K., L’.H. and T.K.; writing—original draft preparation, M.K. and K.R.; writing—
review and editing, K.R., M.K., L’.H. and T.K.; visualization, M.K.; supervision, K.R.; project adminis-
tration, K.R. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Nutraceuticals 2022,2266
Acknowledgments:
This publication was created thanks to the support under the Operational
Programme Integrated Infrastructure for the project: Long-term strategic research of prevention,
intervention and mechanisms of obesity and its comorbidities, IMTS: 313011V344, co-financed by the
European Regional Development Fund.
Conflicts of Interest: The authors declare no conflict of interest.
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