Clonal analysis of epidermal patterning during maize leaf development

In plants, specialized epidermal cells are arranged in semiordered patterns. In grasses such as maize, stomata and other specialized cell types differentiate in linear patterns within the leaf epidermis. A variety of mechanisms have been proposed to direct patterns of epidermal cell differentiation. One class of models proposes that patterns of cellular differentiation depend on the lineage relationships among epidermal cells. Another class of models proposes that epidermal patterning depends on positional information rather than lineage relationships. In the dicot epidermis, cell lineage is an important factor in the patterning of stomata, but not trichomes. In this study, the role of cell lineage in the linear patterning of stomata and bulliform cells in the maize leaf epidermis is investigated. Clones of epidermal cells in juvenile leaves were marked by excision of dSpm from gl15-m and in adult leaves by excision of Ds2 from bz2-m. These clones were analyzed in relation to patterns of stomata and bulliform cells, testing specific predictions of clonal origin hypotheses for the patterning of these cell types. We found that the great majority of clones analyzed failed to satisfy these predictions. Our results clearly show that lineage does not account for the linear patterning of stomata and bulliform cells, implying that positional information must direct the differentiation patterns of these cell types in maize.     

Microtubule patterning in apical epidermal cells ofVinca minor preceding leaf emergence

Summary Serial thin sectioning for electron microscopy was carried out on the cortical cytoplasm of surface cells of the apical dome ofVinca minor. The cellulose reinforcement pattern in the outer epidermal walls forming this surface is known to correlate well with the decussate phyllotaxis pattern. The purpose of this study was to determine the location of microtubules immediately under these epidermal walls as a first step toward finding out how the cellulose pattern arises. First, correspondence between the patterns of microtubules and cellulose was checked. Second, the role of potential orienting cues for the alignment of microtubule arrays in specific cells was evaluated.Microtubule arrays which were well or moderately ordered (70% of the total interphase cells) generally had alignment parallel to the adjacent leaf base, as has been seen for cellulose. The aligned features or cues potentially correlating with a given array were: (1) orientation and length of the previous anticlinal cross-wall, (2) alignment of microtubules in adjacent cells, and (3) direction of inferred stretch, parallel to the nearby leaf bases. All three features were found to agree with the microtubule alignment in 17 of 34 cells with ordered arrays. At least two features agreed in 33 of the 34 cases. All 34 cells with ordered arrays had at least one feature parallel to the array. Random association between microtubule orientation and these features would lead to such correlations less than 0.01% of the time. Thirty percent of the interphase cells showed no obvious order. Most of these cells were located in the central linear corridor region of the apex. The unordered cells were more likely than the ordered cells to have more than one orientation specified by the potential cues; i.e., no single orientation parallel to all of the cues existed. This indicates that uniformity of the orientation cues may be as important as their direction.     

Distinct and overlapping roles of single-repeat MYB genes in root epidermal patterning

Cell specification in the root epidermis of Arabidopsis generates a position-dependent pattern of root-hair cells and non-hair cells. Here we conduct a comprehensive analysis of the five members of a single-repeat R3 MYB gene family, including CAPRICE (CPC), TRIPTYCHON (TRY), ENHANCER of TRY and CPC 1, 2, and 3 (ETC1, ETC2, and ETC3), and study their role and functional relationship in root epidermal cell specification. Based on genetic and expression analyses, CPC, TRY and ETC1, but not ETC2 or ETC3, promote the hair cell fate by inhibiting non-hair specification. Further, we find that single-repeat MYB activity is required for epidermal patterning throughout root development, beginning during embryogenesis. We also identify a novel regulatory interaction whereby GLABRA2 (GL2) promotes TRY (but not CPC or ETC1) expression in the root epidermis, which generates a second lateral inhibition feedback loop. Gene fusion experiments combining CPC regulatory elements with protein-coding regions of each single-repeat MYB gene suggest that all five proteins are functionally similar, although TRY and ETC2 exhibit distinctions from CPC/ETC1/ETC3. These results provide new insight into the function of these single-repeat MYBs and suggest that divergence of their regulatory sequences is largely responsible for their distinct roles in epidermal cell patterning.     

Ultrastructural and biochemical characterization of the epidermal hairs of the seeds of Cuphea procumbens

Rehydration of desiccated Cuphea seeds results in a rapid morphological change in the seed. Within 20 min thread like epidermal hairs are present on the seed surface. The hairs, which are highly ordered helical structures, are present in the epidermal cells of the desiccated seed. Following emergence the hairs increase in length by means of an eversion process, the mechanism for which is proposed in the text. The hairs were purified to homogeneity and found to be composed of 55% carbohydrate and 45% protein. Following -elimination of the carbohydrate using NaOH/NaBH₄ one major protein of MW 31,000 was seen upon polacrylamide gel electrophoresis in the presence of sodium dodecylsulphate. The protein, here termed helexin, probably plays a major structural role in determining the helical shape of the hairs.     

Directionality of wingless protein transport influences epidermal patterning in the Drosophila embryo.

Active endocytotic processes are required for the normal distribution of Wingless (Wg) protein across the epidermal cells of each embryonic segment. To assess the functional consequences of this broad Wg distribution, we have devised a means of perturbing endocytosis in spatially restricted domains within the embryo. We have constructed a transgene expressing a dominant negative form of shibire (shi), the fly dynamin homologue. When this transgene is expressed using the GAL4-UAS system, we find that Wg protein distribution within the domain of transgene expression is limited and that Wg-dependent epidermal patterning events surrounding the domain of expression are disrupted in a directional fashion. Our results indicate that Wg transport in an anterior direction generates the normal expanse of naked cuticle within the segment and that movement of Wg in a posterior direction specifies diverse denticle cell fates in the anterior portion of the adjacent segment. Furthermore, we have discovered that interfering with posterior movement of Wg rescues the excessive naked cuticle specification observed in naked (nkd) mutant embryos. We propose that the nkd segment polarity phenotype results from unregulated posterior transport of Wg protein and therefore that wild-type Nkd function may contribute to the control of Wg movement within the epidermal cells of the segment.     

Cell death and selective adhesion reorganize the dorsoventral boundary for zigzag patterning of Drosophila wing margin hairs

Animal tissues and organs are comprised of several types of cells, which are often arranged in a well-ordered pattern. The posterior part of the Drosophila wing margin is covered with a double row of long hairs, which are equally and alternately derived from the dorsal and ventral sides of the wing, exhibiting a zigzag pattern in the lateral view. How this geometrically regular pattern is formed has not been fully understood. In this study, we show that this zigzag pattern is created by rearrangement of wing margin cells along the dorsoventral boundary flanked by the double row of hair cells during metamorphosis. This cell rearrangement is induced by selective apoptosis of wing margin cells that are spatially separated from hair cells. As a result of apoptosis, the remaining wing margin cells are rearranged in a well-ordered manner, which shapes corrugated lateral sides of both dorsal and ventral edges to interlock them for zigzag patterning. We further show that the corrugated topology of the wing edges is achieved by cell-type specific expression and localization of four kinds of NEPH1/nephrin family proteins through heterophilic adhesion between wing margin cells and hair cells. Homophilic E-cadherin adhesion is also required for attachment of the corrugated dorsoventral edges. Taken together, our results demonstrate that sequential coordination of apoptosis and epithelial architecture with selective adhesion creates the zigzag hair alignment. This may be a common mechanism for geometrically ordered repetitive packing of several types of cells in similarly patterned developmental fields such as the mammalian organ of Corti.     

The evolution of root hairs and rhizoids

Background

Almost all land plants develop tip-growing filamentous cells at the interface between the plant and substrate (the soil). Root hairs form on the surface of roots of sporophytes (the multicellular diploid phase of the life cycle) in vascular plants. Rhizoids develop on the free-living gametophytes of vascular and non-vascular plants and on both gametophytes and sporophytes of the extinct rhyniophytes. Extant lycophytes (clubmosses and quillworts) and monilophytes (ferns and horsetails) develop both free-living gametophytes and free-living sporophytes. These gametophytes and sporophytes grow in close contact with the soil and develop rhizoids and root hairs, respectively.

Scope

Here we review the development and function of rhizoids and root hairs in extant groups of land plants. Root hairs are important for the uptake of nutrients with limited mobility in the soil such as phosphate. Rhizoids have a variety of functions including water transport and adhesion to surfaces in some mosses and liverworts.

Conclusions

A similar gene regulatory network controls the development of rhizoids in moss gametophytes and root hairs on the roots of vascular plant sporophytes. It is likely that this gene regulatory network first operated in the gametophyte of the earliest land plants. We propose that later it functioned in sporophytes as the diploid phase evolved a free-living habit and developed an interface with the soil. This transference of gene function from gametophyte to sporophyte could provide a mechanism that, at least in part, explains the increase in morphological diversity of sporophytes that occurred during the radiation of land plants in the Devonian Period.     

The cytoskeleton of Cobaea seed hairs:

The cell wall of Cobaea scandens seed hairs developed in a characteristic sequence, with the deposition of a cellulose thread onto a pectic swelling layer was the final event. The cellulose thread was intracellularly accompanied by a band of 10–18 microtubules. During the formation of the swelling layer the microtubules were homogeneously distributed; they ran circumferentially normal to the cell axis. When cellulose-thread formation started, the microtubules became arranged in a helical band. The density of the microtubules varied during the different phases of development. The highest density was observed before cellulosethread formation and ranged from 6–15 m·m^-2. The length of the microtubules, 20–30 m, was determined by direct measurements, as well as estimated from the total microtubular length in a given area and the counted free ends. With the indirect immunofluorescence technique the microtubules of the band stained inhomogeneously. Those which were located at the edges of the band fluoresced more intensely than those of the central part. Attempts to visualize actin filaments in the hair cells with rhodaminyl-conjugated phalloidin resulted in a homogeneous staining of the area of the microtubular band, indicating that actin filaments may be present in this region. Though, in thin sections and dry-cleaved cells, filamentous structures were observed between the microtubules, caution is expressed that the observed fluorescence was, indeed, due to actin filaments. The role of the filamentous structures is discussed with respect to formation and maintenance of the microtubular band. Microtubules apparently did not cross coated pits which were visualized in the plasma membrane through the dry-cleaving technique.Abbreviations IFT indirect immunofluorescence technique - RP rhodaminyl-conjugated phalloidin - SEM scanning electron microscopy     

A crucial role for Fgfr2-IIIb signalling in epidermal development and hair follicle patterning

To understand the role Fgf signalling in skin and hair follicle development, we analysed the phenotype of mice deficient for Fgfr2-IIIb and its main ligand Fgf10. These studies showed that the severe epidermal hypoplasia found in mice null for Fgfr2-IIIb is caused by a lack of the basal cell proliferation that normally results in a stratified epidermis. Although at term the epidermis of Fgfr2-IIIb null mice is only two to three cells thick, it expresses the classical markers of epidermal differentiation and establishes a functional barrier. Mice deficient for Fgf10 display a similar but less severe epidermal hypoplasia. By contrast, Fgfr2-IIIb-/-, but not Fgf10-/-, mice produced significantly fewer hair follicles, and their follicles were developmentally retarded. Following transplantation onto nude mice, grafts of Fgfr2-IIIb-/- skin showed impaired hair formation, with a decrease in hair density and the production of abnormal pelage hairs. Expression of Lef1, Shh and Bmp4 in the developing hair follicles of Fgfr2-IIIb-/- mice was similar to wild type. These results suggest that Fgf signalling positively regulates the number of keratinocytes needed to form a normal stratified epidermis and to initiate hair placode formation. In addition, Fgf signals are required for the growth and patterning of pelage hairs.     

WEREWOLF, a MYB-related protein in Arabidopsis, is a position-dependent regulator of epidermal cell patterning

The formation of the root epidermis of Arabidopsis provides a simple and elegant model for the analysis of cell patterning. A novel gene, WEREWOLF (WER), is described here that is required for position-dependent patterning of the epidermal cell types. The WER gene encodes a MYB-type protein and is preferentially expressed within cells destined to adopt the non-hair fate. Furthermore, WER is shown to regulate the position-dependent expression of the GLABRA2 homeobox gene, to interact with a bHLH protein, and to act in opposition to the CAPRICE MYB. These results suggest a simple model to explain the specification of the two root epidermal cell types, and they provide insight into the molecular mechanisms used to control cell patterning.