Trichomes (// or //), 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.
Certain, usually filamentous, algae have the terminal cell produced into an elongate hair-like structure called a trichome. The same term is applied to such structures in some cyanobacteria, such as Spirulina and Oscillatoria. Cyanobacteria trichomes may be unsheathed, as in Oscillatoria, or sheathed, as in Calothrix. These structures play an important role in preventing soil erosion, particularly in cold desert climates. The filamentous sheaths form a persistent sticky network that helps maintain soil structure.
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.
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).
In describing the surface appearance of plant organs, such as stems and leaves, many terms are used in reference to the presence, form, and appearance of trichomes. The most basic terms used are glabrous—lacking hairs— and pubescent—having hairs. Details are provided by:
- 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 curly, 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
Hairs on plants are extremely variable in their presence across species and even within a species, such as their location on plant organs, size, density, and therefore functionality. However, 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.
Trichome and Root Hair Development
Both trichomes and root hairs, the rhizoids of many vascular plants, are lateral outgrowths of a single cell of the epidermal layer. Root hairs form from trichoblasts, the hair-forming cells on the epidermis of a plant root. Root hairs vary between 5 and 17 micrometres in diameter, and 80 to 1,500 micrometres in length (Dittmar, cited in Esau, 1965). Root hairs can survive for two to three weeks and then die off. At the same time new root hairs are continually being formed at the top of the root. This way, the root hair coverage stays the same. It is therefore understandable that repotting must be done with care, because the root hairs are being pulled off for the most part. This is why planting out may cause plants to wilt.
The genetic control of patterning of trichomes and roots hairs shares similar control mechanisms. Both processes involve a core of related transcription factors that control the initiation and development of the epidermal outgrowth. Activation of genes that encode specific protein transcription factors (named GLABRA1 (GL1), GLABRA3 (GL3) and TRANSPARENT TESTA GLABRA1 (TTG1)) are the major regulators of cell fate to produce trichomes or root hairs. When these genes are activated in a leaf epidermal cell, the formation of a trichrome is initiated within that cell. GL1, GL3. and TTG1 also activate negative regulators, which serve to inhibit trichrome formation in neighboring cells. This system controls the spacing of trichomes on the leaf surface. Once trichome are developed they may divide or branch. In contrast, root hairs only rarely branch. During the formation of trichomes and root hairs, may enzymes are regulated. For example, just prior to the root hair development, there is a point of elevated phosphorylase activity.
Significance for taxonomy
The type, presence and absence and location of trichomes are important diagnostic characters in plant identification and plant taxonomy. In forensic examination, plants such as Cannabis sativa can be identified by microscopic examination of the trichomes. Although trichomes are rarely found preserved in fossils, trichome bases are regularly found and, in some cases, their cellular structure is important for identification.
Significance for plant molecular biology
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. Trichomes also serve as models for cell differentiation as well as pattern formation in plants.
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.
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.
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). 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.
Stinging trichomes vary in their morphology and distribution between species, however similar effects on large herbivores implies they serve similar functions. In areas susceptible to herbivory, higher densities of stinging trichomes were observed. In Urtica, the stinging trichomes induce a painful sensation lasting for hours upon human contact. This sensation has been attributed as a defense mechanism against large animals and small invertebrates, and plays a role in defense supplementation via secretion of metabolites. Studies suggest that this sensation involves a rapid release of toxin (such as histamine) upon contact and penetration via the globular tips of said trichomes.
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