Өсімдіктердегі жұқа түкшелер

Fine hair like growth on plants

Trichomes ('/t//r//aɪ//k//oʊ//m//z//, /'/t//r//ɪ//k//oʊ//m//z/; ) 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. The trichomes of cyanobacteria 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.

Non glandular trichomes serve as structural protection against a variety of abiotic stressors, including water losses, extreme temperatures and UV radiation, and biotic threats, such as pathogen or herbivore attack. For example, the model plant C. salviifolius 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 known role of flavonoids is to block out the shortest wavelengths to protect the plant; this differs from their role in glandular trichomes. 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, many enzymes are regulated. For example, just prior to the root hair development, there is a point of elevated phosphorylase activity. Many of what scientists know about trichome development comes from the model organism Arabidopsis thaliana, because their trichomes are simple, unicellular, and non glandular. The development pathway is regulated by three transcription factors: R2R3 MYB, basic helix loop helix, and WD40 repeat. The three groups of TFs form a trimer complex (MBW) and activate the expression of products downstream, which activates trichome formation. However, just MYBs alone act as an inhibitor by forming a negative complex.

Plant phytohormones have an effect on the growth and response of plants to environmental stimuli. Some of these phytohormones are involved in trichome formation, which include gibberellic acid (GA), cytokinins (CK), and jasmonic acids (JA). GA stimulates growth of trichomes by stimulating GLABROUS1 (GL1), however, both SPINDLY and DELLA proteins repress the effects of GA, so less of these proteins create more trichomes. 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.

Arabidopsis thaliana trichomes are classified as being aerial, epidermal, unicellular, tubular structures.

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.