Root

mucilage Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of m ...

is made of plant-specific

polysaccharides Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with w ...

or long chains of sugar molecules. This polysaccharide secretion of

root exudate Plant root exudates are fluids emitted through the roots of plants. These secretions influence the rhizosphere around the roots to inhibit harmful microbes and promote the grow of self and kin plants. Plant root systems can grow to be complex due t ...

forms a gelatinous substance that sticks to the caps of roots. Root mucilage is known to play a role in forming relationships with soil-dwelling life forms. Just how this root mucilage is secreted is debated, but there is growing evidence that mucilage derives from ruptured cells. As roots penetrate through the soil, many of the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

s surrounding the caps of roots are continually shed and replaced. These ruptured or lysed cells release their component parts, which include the polysaccharides that form root mucilage. These polysaccharides come from the

Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles ins ...

and plant

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

, which are rich in plant-specific polysaccharides. Unlike

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

cells, plant cells have a cell wall that acts as a barrier surrounding the cell providing strength, which supports plants just like a skeleton. This cell wall is used to produce everyday products such as

timber Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, wi ...

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distributed ...

, and natural

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...

s, including

cotton Cotton is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus ''Gossypium'' in the mallow family Malvaceae. The fiber is almost pure cellulose, and can contain minor perce ...

. Root mucilage is a part of a wider secrete from plant roots known as root exudate. Plant roots secrete a variety of organic molecules into the surrounding soil, such as

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

s, DNA,

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...

s and

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

s, which are the building blocks of life. This collective secretion is known as root exudate. This root exudate prevents root infection from

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

and

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

, helps the roots to penetrate through the soil, and can create a micro-climate that is beneficial to the plant.

Root mucilage composition

To determine the sugars within root mucilage,

monosaccharide Monosaccharides (from Greek ''monos'': single, '' sacchar'': sugar), also called simple sugars, are the simplest forms of sugar and the most basic units (monomers) from which all carbohydrates are built. They are usually colorless, water-solub ...

analysis and monosaccharide linkage analysis are undertaken. Monosaccharide linkage analysis involves methylating the root mucilage, which contains polysaccharides. The root mucilage is

hydrolysed Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

using acid to break down the polysaccharides into their monosaccharide components. The subsistent monosaccharides are then reduced to open their rings. The open ring monosaccharides are then acetylated, and separated typically by using gas chromatography, although

liquid chromatography In chemical analysis, chromatography is a laboratory technique for the Separation process, separation of a mixture into its components. The mixture is dissolved in a fluid solvent (gas or liquid) called the ''mobile phase'', which carries it ...

is also used. The masses of the monosaccharides are then detected using

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

. The

gas chromatography Gas chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. Typical uses of GC include testing the purity of a particular substance, ...

retention times and the mass spectrometry chromatogram are used to identify how the monosaccharides are linked to form the polysaccharides that make root mucilage. For monosaccharide analysis, which reveals the sugars that make root mucilage, scientists hydrolyse the root mucilage using acid, and put the samples directly through gas chromatography linked to mass spectrometry. Several scientists have determined the composition of plant root mucilage using

analysis and linkage analysis, showing that

Maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...

(Zea mays) root mucilage contains high levels of

galactose Galactose (, '' galacto-'' + '' -ose'', "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molec ...

xylose Xylose ( grc, ξύλον, , "wood") is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes an aldehyde functional gro ...

arabinose Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group. For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structurally ...

rhamnose Rhamnose (Rha, Rham) is a naturally occurring deoxy sugar. It can be classified as either a methyl-pentose or a 6-deoxy-hexose. Rhamnose predominantly occurs in nature in its L-form as L-rhamnose (6-deoxy-L-mannose). This is unusual, since most o ...

, and

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, using ...

, and lower levels of

uronic acid 300px, The Fischer projections of glucose and glucuronic acid">glucose.html" ;"title="Fischer projections of glucose">Fischer projections of glucose and glucuronic acid. Glucose's terminal carbon's primary alcohol group has been oxidized to a ...

mannose Mannose is a sugar monomer of the aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several congenital disorders of glycosylation ...

fucose Fucose is a hexose deoxy sugar with the chemical formula C6H12O5. It is found on ''N''-linked glycans on the mammalian, insect and plant cell surface. Fucose is the fundamental sub-unit of the seaweed polysaccharide fucoidan. The α(1→3) link ...

, and

glucuronic acid Glucuronic acid (from Greek γλεῦκος "''wine, must''" and οὖρον "''urine''") is a uronic acid that was first isolated from urine (hence the name). It is found in many gums such as gum arabic (c. 18%), xanthan, and kombucha tea and ...

Wheat Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...

(Triticum aestivum) root mucilage also contains high levels of xylose, arabinose, galactose, glucose, and lower levels of rhamnose, glucuronic acid and mannose.

Cowpea The cowpea (''Vigna unguiculata'') is an annual herbaceous legume from the genus ''Vigna''. Its tolerance for sandy soil and low rainfall have made it an important crop in the semiarid regions across Africa and Asia. It requires very few input ...

(Vigna unguiculata) also contains high levels of arabinose, galactose, glucose, fucose, and xylose, and lower levels of rhamnose, mannose, and glucuronic acid. Many other plants have had their root mucilage composition determined using monosaccharide analysis and monosaccharide linkage analysis. With the following monosaccharides determined as well as their linkages, scientists have determined the presence of

pectin Pectin ( grc, πηκτικός ': "congealed" and "curdled") is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal, chemical component of ...

arabinogalactan protein Arabinogalactan-proteins (AGPs) are highly glycosylated proteins (glycoproteins) found in the cell walls of plants. Each one consists of a protein with sugar molecules attached (which can account for more than 90% of the total mass). They are memb ...

xyloglucan Xyloglucan is a hemicellulose that occurs in the primary cell wall of all vascular plants; however, all enzymes responsible for xyloglucan metabolism are found in Charophyceae algae.LEV Del Bem and M Vincentz (2010) Evolution of xyloglucan-relate ...

, arabinan, and

xylan Xylan (; ) ( CAS number: 9014-63-5) is a type of hemicellulose, a polysaccharide consisting mainly of xylose residues. It is found in plants, in the secondary cell walls of dicots and all cell walls of grasses. Xylan is the third most abundan ...

, which are plant-specific polysaccharides within the root mucilage of plants.

Importance and role of root mucilage

Plants use up to 40% of their energy secreting root mucilage, which they generate from

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

that takes place in the leaves. Root mucilage plays a role in developing a symbiotic relationship with the soil-dwelling fungi. This important relationship is known to affect 94% of land plants, and benefits plants by increasing water and nutrient uptake from the soil, particularly phosphorus. In return, the fungi receive food in the form of

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or ma ...

s from the plant in the form of broken-down root mucilage. Without this relationship, many plants would struggle to gain sufficient water or nutrients. Root mucilage also helps soil to stick to roots. The purpose of this is to maintain the plant's contact with the soil so that the plant can regulate the levels of water it can absorb, decrease friction so that roots can penetrate through the soil, and maintain a

micro-climate A microclimate (or micro-climate) is a local set of atmospheric conditions that differ from those in the surrounding areas, often with a slight difference but sometimes with a substantial one. The term may refer to areas as small as a few squ ...

. Root mucilage contributes to the particular hydrophysical properties of the

rhizosphere The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Soil pores in the rhizosphere can contain many bacteria and other microor ...

, which can affect the plant's response to water deficit. For example, root mucilage can reduce

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

and store water in the rhizosphere.

References

{{reflist Polysaccharides

Root mucilage composition

Importance and role of root mucilage

See also

References