毛状体

毛状体（trichome）泛指是植物、藻类、地衣及部分原生生物上的纤细生长物或毛状附属体。毛状体的结构和功能各不相同，有柔毛、刚毛、腺毛、鳞片、乳突毛等等多种。植物上任何形式的毛形成的覆盖，称为（indumentum），而带有毛被的表面，称为“具被毛”（pubescent）。
藻类.
一些（主要是丝状）藻类的边缘细胞会形成类似于毛发的长条结构，称为藻丝。这个词也用作蓝菌门中的类似结构，可见于螺旋藻属和颤藻属。蓝菌的藻丝有些有鞘（如眉藻属），有些无鞘（如颤藻属）。藻丝在防止寒冷荒漠气候的土壤受侵蚀方面极其重要。藻丝的鞘交叉形成持久的粘性网络，有助于维持土壤结构。
植物.
通常植物是为了适应干旱、严寒，对抗强蒸腾、强紫外线、昆虫及植食性动物等的危害而长出毛状体；其可截留水滴、隔绝低温、或使掠食者无法吞咽。植物毛状体特征各异，在植物的种类和单个植物的器官之间都有不同。毛状体按照这些特征进行分类。一些决定性的特征包括：
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.
Polyphenols.
Non-glandular trichomes in the genus "Cistus" were found to contain presences of ellagitannins, 糖苷s, and 山柰酚 derivatives. The ellagitannins have the main purpose of helping adapt in times of nutrient-limiting stress.
Trichome and root hair development.
Both trichomes and 根毛s, the 假根s of many 维管植物s, 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 根. Root hairs vary between 5 and 17 micrometers in diameter, and 80 to 1,500 micrometers 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, many enzymes are regulated. For example, just prior to the root hair development, there is a point of elevated 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.
Phytohormones.
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.
Some other phytohormones that promote growth of trichomes include brassinosteroids, ethylene, and salicylic acid. This was understood by conducting experiments with mutants that has little to no amounts of each of these substances. In every case, there was less trichome formation on both plant surfaces, as well as incorrect formation of the trichomes present.
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 "大麻草" can be identified by microscopic examination of the trichomes. Although trichomes are rarely found preserved in 化石s, trichome bases are regularly found and, in some cases, their cellular structure is important for identification.
"Arabidopsis thaliana" trichome classification.
"Arabidopsis thaliana" trichomes are classified as being aerial, epidermal, unicellular, tubular structures.
Significance for plant molecular biology.
In the model plant "拟南芥", trichome formation is initiated by the GLABROUS1 protein. Knockouts of the corresponding gene lead to glabrous plants. This 表型 has already been used in 基因编辑技术 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.
Uses.
Bean leaves have been used historically to trap s in houses in 东欧. 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 ("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.
Defense.
Plants may use trichomes in order to deter herbivore attacks via physical and/or chemical means, e.g. in specialized, s of "荨麻属" (Nettle) species that deliver inflammatory chemicals such as 组织胺. 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 "", for example, are able to physically free themselves from trichomes, are able to bite off trichomes, and are able to form 丝绸 blankets in order to navigate the leaves better.
Stinging trichomes.
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.