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Trichomes (/ˈtrkmz/ or /ˈtrɪkmz/), from the Greek τρίχωμα (trichōma) meaning "hair", are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a plant is an indumentum, and the surface bearing them is said to be pubescent.

Algal trichomes

Certain, usually filamentous, algae have the terminal cell produced into an elongate hair-like structure called a trichome.[example needed] The same term is applied to such structures in some cyanobacteria, such as Spirulina and Oscillatoria. The trichomes of cyanobacteria may be unsheathed, as in Oscillatoria, or sheathed, as in Calothrix.[1] These structures play an important role in preventing soil erosion, particularly in cold desert climates.[citation needed] The filamentous sheaths form a persistent sticky network that helps maintain soil structure.

Plant trichomes

Sticky trichomes of a carnivorous plant, Drosera capensis with a trapped insect, contain proteolytic enzymes.
Trichomes on Cannabis, rich in cannabinoids.
Trichomes on the surface of a Solanum scabrum leaf
Certain, usually filamentous, algae have the terminal cell produced into an elongate hair-like structure called a trichome.[example needed] The same term is applied to such structures in some cyanobacteria, such as Spirulina and Oscillatoria. The trichomes of cyanobacteria may be unsheathed, as in Oscillatoria, or sheathed, as in Calothrix.[1] These structures play an important role in preventing soil erosion, particularly in cold desert climates.[citation needed] The filamentous sheaths form a persistent sticky network that helps maintain soil structure.

Plant trichomes

Sticky trichomes of a carnivorous plant, Drosera capensis with a trapped insect, contain proteolytic enzymes.
Trichomes on Cannabis, rich in cannabinoids.
Trichomes on the surface of a [2]
  • Presence of cytoplasm
  • Glandular (secretory) vs. Eglandular
  • Tortuous, Simple (unbranched and unicellular), Peltate (scale-like), Stellate (star-shaped)[3]
  • Adaxial vs. abaxial, referring to whether trichomes are present, respectively, on the upper surface (adaxial) or lower surface (abaxial) of a leaf or other lateral organ.
  • In a model organism, C. salvifolius, there are more adaxial trichomes present on this plant because this surface suffers from more UV, solar irradiance light stress than the abaxial surface.[4]

    Trichomes can protect the plant from a large range of detriments, such as UV light, insects, transpiration, and freeze intolerance.[5]

    Aerial surface hairs

    Trichomes on plants are epidermal outgrowths of various kinds. The terms emergences or prickles refer to outgrowths that involve more than the epidermis. This distinction is not always easily applied (see Wait-a-minute tree). Also, there are nontrichomatous epidermal cells that protrude from the surface.[example needed]

    A common type of trichome is a hair. Plant hairs may be unicellular or multicellular, branched or unbranched. Multicellular hairs may have one or several layers of cells. Branched hairs can be dendritic (tree-like) as in kangaroo paw (Anigozanthos), tufted, or stellate (star-shaped), as in Arabidopsis thaliana.

    Another common type of trichome is the scale or peltate hair, that has a plate or shield-shaped cluster of cells attached directly to the surface or borne on a stalk of some kind. Common examples are the leaf scales of bromeliads such as the pineapple, Rhododendron and sea buckthorn (Hippophae rhamnoides).

    Any of the various types of hairs may be glandular, producing some kind of secretion, such as the essential oils produced by mints and many other members of the family Lamiaceae.

    Many terms are used to describe the surface appearance of plant organs, such as stems and leaves, referring to the presence, form and appearance of trichomes. Examples include:

    • glabrous, glabrate – lacking hairs or trichomes; surface smooth
    • hirsute – coarsely hairy
    • hispid – having bristly hairs
    • articulate – simple pluricellular-uniseriate hairs
    • downy – having an almost wool-like covering of long hairs
    • pilose – pubescent with long, straight, soft, spreading or erect hairs
    • puberulent – minutely pubescent; having fine, short, usually erect, hairs
    • pubescent – bearing hairs or trichomes of any type
    • strigillose – minutely strigose
    • strigose – having straight hairs all pointing in more or less the same direction as along a margin or midrib
    • tomentellous – minutely tomentose
    • tomentose – covered with dense, matted, woolly hairs
    • villosulous – minutely villous
    • villous – having long, soft hairs, often curved, but not matted

    The size, form, density and location of hairs on plants are extremely variable in their presence across species and even within a species on different plant organs. Several basic functions or advantages of having surface hairs can be listed. It is likely that in many cases, hairs interfere with the feeding of at least some small herbivores and, depending upon [4]

    Trichomes can protect the plant from a large range of detriments, such as UV light, insects, transpiration, and freeze intolerance.[5]

    Aerial surface hairs

    Trichomes on plants are epidermal outgrowths of various kinds. The terms emergences or prickles refer to outgrowths that involve more than

    Trichomes can protect the plant from a large range of detriments, such as UV light, insects, transpiration, and freeze intolerance.[5]

    Trichomes on plants are epidermal outgrowths of various kinds. The terms emergences or prickles refer to outgrowths that involve more than the epidermis. This distinction is not always easily applied (see Wait-a-minute tree). Also, there are nontrichomatous epidermal cells that protrude from the surface.[example needed]

    A common type of trichome is a hair. Plant hairs may be unicellular or multicellular, branched or unbranched. Multicellular hairs may have one or several layers of cells. Branched h

    A common type of trichome is a hair. Plant hairs may be unicellular or multicellular, branched or unbranched. Multicellular hairs may have one or several layers of cells. Branched hairs can be dendritic (tree-like) as in kangaroo paw (Anigozanthos), tufted, or stellate (star-shaped), as in Arabidopsis thaliana.

    Another common type of trichome is the scale or peltate hair, that has a plate or shield-shaped cluster of cells attached directly to the surface or borne on a stalk of some kind. Common examples are the leaf scales of bromeliads such as the pineapple, Rhododendron and sea buckthorn (Hippophae rhamnoides).

    Any of the various types of hairs may be glandular, producing some kind of secretion, such as the essential oils produced by mints and many other members of the family Lamiaceae.

    Many terms are used to describe the surface appearance of plant organs, such as stems and leaves, referring to the presence, form and appearance of trichomes. Examples include:

    The size, form, density and location of hairs on plants are extremely variable in their presence across species and even within a species on different plant organs. Several basic functions or advantages of having surface hairs can be listed. It is likely that in many cases, hairs interfere with the feeding of at least some small herbivores and, depending upon stiffness and irritability to the palate, large herbivores as well. Hairs on plants growing in areas subject to frost keep the frost away from the living surface cells. In windy locations, hairs break up the flow of air across the plant surface, reducing transpiration. Dense coatings of hairs reflect sunlight, protecting the more delicate tissues underneath in hot, dry, open habitats. In addition, in locations where much of the available moisture comes from fog drip, hairs appear to enhance this process by increasing the surface area on which water droplets can accumulate.[citation needed]

    Glandular trichomes

    Glandular trichomes have been vastly studied, even though they are only found on about 30% of plants. Their function is to secrete metabolites for the plant. Some of these metabolites include:

    • terpenoids, which have many functions in the plant related to growth and development[6]
    • phenylpropanoids, which have a role in many plant pathways, such as secondary metabolites, stress response, and act as the mediators of plant interactions in the environment[7]

      Glandular trichomes have been vastly studied, even though they are only found on about 30% of plants. Their function is to secrete metabolites for the plant. Some of these metabolites include:

      • terpenoids, which have many functions in the plant related to growth and development[6]
      • phenylpropanoids, which have a role in

        Non-glandular trichomes are important for plant protection against UV light.[4]

        The model plant, Cistus salvifolius, is found in areas of high-light stress and poor soil conditions, along the Mediterranean coasts. It contains non-glandular, stellate and dendritic trichomes that have the ability to synthesize and store polyphenols that both affect absorbance of radiation and plant desiccation. These trichomes also contain acetylated flavonoids, which can absorb UV-B, and non-acetylated flavonoids, which absorb the longer wavelength of UV-A. In non-glandular trichomes, the only role of flavonoids is to block out the shortest wavelengths to protect the plant, which differs from in glandular trichomes.[4]

        Polyphenols

        Non-glandular trichomes in the genus Cistus were found to contain presences of ellagitannins, [4]

        Non-glandular trichomes in the genus Cistus were found to contain presences of ellagitannins, glycosides, and kaempferol derivatives. The ellagitannins have the main purpose of helping adapt in times of nutrient-limiting stress.[4]

        Trichome and root hair development

        In the model pla

        In the model plant Arabidopsis thaliana, trichome formation is initiated by the GLABROUS1 protein. Knockouts of the corresponding gene lead to glabrous plants. This phenotype has already been used in genome editing experiments and might be of interest as visual marker for plant research to improve gene editing methods such as CRISPR/Cas9.[17][18] Trichomes also serve as models for cell differentiation as well as pattern formation in plants.[19]

        Bean leaves have been used historically to trap bedbugs in houses in Eastern Europe. The trichomes on the bean leaves capture the insects by impaling their feet (tarsi). The leaves would then be destroyed.[20]

        Trichomes are an essential part of nest building for the European wool carder bee (Anthidium manicatum). This bee species incorporates trichomes into their nests by scraping them off of plants and using them as a lining for their nest cavities.[21]

        Defense

        Plants may use trichomes in order to deter herbivore attacks via physical and/or chemical means, e.g. in specialized, stinging hairs of Urtica (Nettle) species that deliver inflammatory chemicals such as histamine. Studies on trichomes have been focused towards crop protection, which is the result of deterring herbivores (Brookes et al. 2016).Trichomes are an essential part of nest building for the European wool carder bee (Anthidium manicatum). This bee species incorporates trichomes into their nests by scraping them off of plants and using them as a lining for their nest cavities.[21]

        Plants may use trichomes in order to deter herbivore attacks via physical and/or chemical means, e.g. in specialized, stinging hairs of Urtica (Nettle) species that deliver inflammatory chemicals such as histamine. Studies on trichomes have been focused towards crop protection, which is the result of deterring herbivores (Brookes et al. 2016).[22] However, some organisms have developed mechanisms to resist the effects of trichomes. The larvae of Heliconius charithonia, for example, are able to physically free themselves from trichomes, are able to bite off trichomes, and are able to form silk blankets in order to navigate the leaves better.[23]

        Stinging trichomes