Progress on trichome development regulated by phytohormone signaling

Trichomes are specialized structures that develop from epidermal cells in the aerial parts of plants, and are an excellent model system to study all aspects of cell differentiation including cell fate determination, cell cycle regulation, cell polarity and cell expansion. The development of the trichome is a process of integration of both external signals and endogenous developmental programs. During recent years, molecular analysis of trichome development at different stages has been well studied, and through the mutant phenotypes and the function of corresponding genes, the underlying mechanism has been revealed in a first glimpse. This paper offers a mini-view on this integration process with emphasis on the effects of plant hormone signaling on trichome development in plants through GLABROUS INFLORESCENCE STEMS (GIS) family and subfamily genes.     

拟南芥表皮毛发育的分子机制

拟南芥(Arabidopsis thaliana)表皮毛是存在于地上部分表皮组织的一种特化的、典型的单细胞结构。近几年, 对其发育的分子调控机制的研究取得了很大进展, 已克隆出大量的控制表皮毛不同发育阶段的基因, 通过对这些基因的功能解析揭示出表皮毛发育及生长调节的内在分子机制。该文对拟南芥表皮毛发育的最新研究进展进行综述, 并展望了关于表皮毛的研究方向及潜在的应用价值。     

植物表皮毛研究进展

表皮毛是大多数植物地上部分表皮组织所延伸出来的一种特化的毛状结构附属物。表皮毛在植物表皮层和环境间构筑了一道天然的物理屏障, 不但对植物的生长发育具有重要意义, 而且还具有非常高的应用价值和经济价值。近几年, 研究者从不同植物中不断克隆出新的表皮毛发育相关基因, 在揭示植物调控表皮毛生长发育的分子机制方面取得很大进展。该文综述了植物表皮毛的最新研究进展, 并展望了植物表皮毛的研究方向及应用开发价值。     

拟南芥GLABROUS1两个新等位突变体的筛选和鉴定

植物细胞命运决定机制的解析一直以来都是植物发育生物学研究的核心。模式植物拟南芥的表皮毛形成过程是研究植物细胞命运决定的优良模式系统。为了筛选和鉴定控制拟南芥表皮毛形成的新因子,我们进行了大规模的正向遗传筛选,获得了两株莲座叶表皮毛不能形成或数量显著减少的突变体f08-01和vat002-07。通过对突变基因的克隆和遗传互补实验,确定这两株突变体是GLABROUS1(GL1)的新等位突变体。GL1是调控植物表皮毛发生的关键遗传因子,其表达模式在表皮毛形成过程中受到严格调控,但是具体的机制还不清楚。通过遗传互补实验,证实GL1的3'非编码区在表皮毛形成过程中至关重要,并且3'非编码区的序列影响GL1 mRNA的稳定性和转录激活。     

拟南芥表皮毛发育的分子机制

拟南芥（Arabidopsis thaliana）表皮毛是存在于地上部分表皮组织的一种特化的、典型的单细胞结构。近几年,对其发育的分子调控机制的研究取得了很大进展,已克隆出大量的控制表皮毛不同发育阶段的基因,通过对这些基因的功能解析揭示出表皮毛发育及生长调节的内在分子机制。该文对拟南芥表皮毛发育的最新研究进展进行综述,并展望了关于表皮毛的研究方向及潜在的应用价值。     

ZFP5 encodes a functionally equivalent GIS protein to control trichome initiation

The Arabidopsis thaliana trichome development is a model system for understanding various aspects of plant cell development and differentiation. The C2H2 zinc finger proteins GIS, GIS2, and ZFP8 play important roles in controlling trichome initiation. In our recent study, we reported that a new C2H2 zinc finger protein, ZINC FINGER PROTEIN 5 (ZFP5), controls trichome cell development through GA signaling. ZFP5 acts upstream of GIS gene family and key trichome initiation regulators, and ZFP8 is the direct target gene of ZFP5. Here we show that ZFP5 encodes a protein functionally equivalent to GIS and GIS2 in controlling trichome initiation. Furthermore, similar to GIS2, ZFP5 is not involved in trichome branching.     

The actin cytoskeleton is required to elaborate and maintain spatial patterning during trichome cell morphogenesis in Arabidopsis thaliana

Arabidopsis thaliana trichomes provide an attractive model system to dissect molecular processes involved in the generation of shape and form in single cell morphogenesis in plants. We have used transgenic Arabidopsis plants carrying a GFP-talin chimeric gene to analyze the role of the actin cytoskeleton in trichome cell morphogenesis. We found that during trichome cell development the actin microfilaments assumed an increasing degree of complexity from fine filaments to thick, longitudinally stretched cables. Disruption of the F-actin cytoskeleton by actin antagonists produced distorted but branched trichomes which phenocopied trichomes of mutants belonging to the 'distorted' class. Subsequent analysis of the actin cytoskeleton in trichomes of the distorted mutants, alien, crooked, distorted1, gnarled, klunker and wurm uncovered actin organization defects in each case. Treatments of wild-type seedlings with microtubule-interacting drugs elicited a radically different trichome phenotype characterized by isotropic growth and a severe inhibition of branch formation; these trichomes did not show defects in actin cytoskeleton organization. A normal actin cytoskeleton was also observed in trichomes of the zwichel mutant which have reduced branching. ZWICHEL, which was previously shown to encode a kinesin-like protein is thought to be involved in microtubule-linked processes. Based on our results we propose that microtubules establish the spatial patterning of trichome branches whilst actin microfilaments elaborate and maintain the overall trichome pattern during development.     

Cell polarity in Arabidopsis trichomes

Arabidopsis leaf trichomes are unicellular hairs that display a highly characteristic cell form that has a fixed orientation with respect to the basal-distal leaf axis. The genetic, molecular and cell biological analysis of trichome morphogenesis reveal that various cellular processes need to be coordinated including regulation of the cell cycle, the cell size and the actin and tubulin cytoskeleton. Here we will focus on what is known about the establishment and maintenance of positional information during trichome formation.     

Trichome morphogenesis in Arabidopsis

Trichomes (plant hairs) in Arabidopsis thaliana are large non-secreting epidermal cells with a characteristic three-dimensional architecture. Because trichomes are easily accessible to a combination of genetic, cell biological and molecular methods they have become an ideal model system to study various aspects of plant cell morphogenesis. In this review we will summarize recent progress in the understanding of trichome morphogenesis.     

Gene expression profiling of the different stages of Arabidopsis thaliana trichome development on the single cell level.

Leaf hairs (trichomes) of Arabidopsis thaliana are a model system for studying cell development, differentiation and cell cycle regulation. To exploit this model system with ultimate spatial resolution we applied single cell sampling, thus avoiding the averaging effect induced by complex tissue mixtures. In particular, we analysed gene expression profiles of two selected stages of the developing trichome: trichome initial cells and mature trichomes, as well as pavement cells. Ten single cells per sample were collected by glass microcapillaries and used for the generation of radioactive probes for subsequent hybridization to nylon filters representing approximately 8000 genes of A. thaliana. Functional categorization of genes transcribed in trichome initials, mature trichomes and pavement cells demonstrated involvement of these surface cells in the stress response. In silico promoter analysis of genes preferentially expressed in trichome initials revealed enrichment in MYB-binding sites and presence of elements involved in hormonal, metal, sulphur response and cell cycle regulation. Three candidate genes preferentially expressed in trichome initials were selected for further analysis: At3g16980 (putative RNA polymerase II), At5g15230 (GASA4) and At4g27260 (GH3.5, WES1). Promoter:GUS studies confirmed expression of the putative RNA polymerase II and the gibberellin responsive GASA4 in trichome initials and partially in mature trichomes. Functional implication of the three selected candidates in trichome development and hence in cell cycle regulation in A. thaliana is discussed. We suggest that these genes are involved in differentiation and initiation of endocycling during trichome development.     

Organized F-actin is essential for normal trichome morphogenesis in Arabidopsis

Actin microfilaments form a three-dimensional cytoskeletal network throughout the cell and constitute an essential throughway for organelle and vesicle transport. Development of Arabidopsis trichomes, unicellular structures derived from the epidermis, is being used as a genetic system in which to study actin-dependent growth in plant cells. The present study indicates that filamentous actin (F-actin) plays an important role during Arabidopsis trichome morphogenesis. For example, immunolocalization of actin filaments during trichome morphogenesis identified rearrangements of the cytoskeletal structure during the development of the mature cell. Moreover, pharmacological experiments indicate that there are distinct requirements for actin- and microtubule-dependent function during trichome morphogenesis. The F-actin-disrupting drug cytochalasin D does not affect the establishment of polarity during trichome development; however, maintenance and coordination of the normal pattern of cell growth are very sensitive to this drug. In contrast, oryzalin, an agent that depolymerizes microtubules, severely inhibits cell polarization. Furthermore, cytochalasin D treatment phenocopies a known class of mutations that cause distorted trichome morphology. Results of an analysis of cell shape and microfilament structure in wild-type, mutant, and drug-treated trichomes are consistent with a role for actin in the maintenance and coordination of an established growth pattern.     

Zinc finger protein5 is required for the control of trichome initiation by acting upstream of zinc finger protein8 in Arabidopsis

Arabidopsis (Arabidopsis thaliana) trichome development is a model system for studying cell development, cell differentiation, and the cell cycle. Our previous studies have shown that the GLABROUS INFLORESCENCE STEMS (GIS) family genes, GIS, GIS2, and ZINC FINGER PROTEIN8 (ZFP8), control shoot maturation and epidermal cell fate by integrating gibberellins (GAs) and cytokinin signaling in Arabidopsis. Here, we show that a new C2H2 zinc finger protein, ZFP5, plays an important role in controlling trichome cell development through GA signaling. Overexpression of ZFP5 results in the formation of ectopic trichomes on carpels and other inflorescence organs. zfp5 loss-of-function mutants exhibit a reduced number of trichomes on sepals, cauline leaves, paraclades, and main inflorescence stems in comparison with wild-type plants. More importantly, it is found that ZFP5 mediates the regulation of trichome initiation by GAs. These results are consistent with ZFP5 expression patterns and the regional influence of GA on trichome initiation. The molecular analyses suggest that ZFP5 functions upstream of GIS, GIS2, ZFP8, and the key trichome initiation regulators GLABROUS1 (GL1) and GL3. Using a steroid-inducible activation of ZFP5 and chromatin immunoprecipitation experiments, we further demonstrate that ZFP8 is the direct target of ZFP5 in controlling epidermal cell differentiation.     

Secreting glandular trichomes: more than just hairs

Secreting glandular plant trichome types which accumulate large quantities of metabolic products in the space between their gland cell walls and cuticle permit the plant to amass secretions in a compartment that is virtually outside the plant body. These structures not only accumulate and store what are often phytotoxic oils but they position these compounds as an apparent first line of defense at the surface of the plant. Recent advances in methods for isolation and study of trichome glands have allowed more precise analysis of gland cell metabolism and enzymology. Isolation of mutants with altered trichome phenotypes provides new systems for probing the genetic basis of trichome development. These advances and their continuation can pave the way for future attempts at modification of trichome secretion. The biochemical capability of glandular secreting trichomes and the potential for its future manipulation to exploit this external storage compartment is the focus of this review.     

植物不同类型表皮毛调控模型研究进展

表皮毛是植物地上部分表皮细胞向外突出延伸的特化毛状结构,不仅可以保护植物免受病虫的危害,还具有一定的经济和药用价值,对其调控的分子机制的阐明有利于植物的分子设计育种和遗传改良。近年来,模式植物拟南芥表皮毛形成的调控模式基本被阐明,其他植物表皮毛的调控机制也取得很大进展。鉴于此,文中综述了拟南芥和棉花(单细胞表皮毛)及番茄和青蒿(多细胞表皮毛)在基因和激素水平上对表皮毛的发育调控,同时简要介绍了其他典型单、双子叶植物表皮毛相关的研究进展,最后,展望了植物表皮毛的研究方向和应用前景。     

GLABROUS INFLORESCENCE STEMS (GIS) is required for trichome branching through gibberellic acid signaling in Arabidopsis

Cell differentiation generally corresponds to the cell cycle, typically forming a non-dividing cell with a unique differentiated morphology, and Arabidopsis trichome is an excellent model system to study all aspects of cell differentiation. Although gibberellic acid is reported to be involved in trichome branching in Arabidopsis, the mechanism for such signaling is unclear. Here, we demonstrated that GLABROUS INFLORESCENCE STEMS (GIS) is required for the control of trichome branching through gibberellic acid signaling. The phenotypes of a loss-of-function gis mutant and an overexpressor showed that GIS acted as a repressor to control trichome branching. Our results also show that GIS is not required for cell endoreduplication, and our molecular and genetic study results have shown that GIS functions downstream of the key regulator of trichome branching, STICHEL (STI), to control trichome branching through the endoreduplication-independent pathway. Furthermore, our results also suggest that GIS controls trichome branching in Arabidopsis through two different pathways and acts either upstream or downstream of the negative regulator of gibbellic acid signaling SPINDLY (SPY).