Observations of the pattern of secondary wall development in the hypodermis of onion (Allium cepa) roots

Summary This brief communication reports the appearance, under certain circumstances, of root hypodermal cells with transfer cell labyrinths. These cells lie at regular intervals around the hypodermis at the bases of onion bulb roots. They are narrower (smaller tangential dimension) than unmodified cells but have the same radial dimension. These narrow cells contain small vacuoles, their main volume being composed of a cytoplasm rich in organelles, especially mitochondria. When treated with a low concentration of lanthanum nitrate solution, the tracer accumulates in the outer tangential wall and in small vacuoles and vesicles.     

Biophysical characterization of a large conductance anion channel in hypodermal membranes of the gastrointestinal nematode,Ascaris suum

Patch-clamp recordings from muscle- and cuticle-facing hypodermal membranes of the gastrointestinal nematode Ascaris suum reveal a high-conductance, voltage- sensitive Ca(2+) -dependent Cl(-) channel. The hypodermal channel has a conductance of 195 pS in symmetrical 160 mM NaCl. The open probability of the channel is highly voltage-sensitive, and channel activity is not observed when Ca(2+) is reduced to <100 microM. The channel is permeable to organic anions that are major end-products of carbohydrate metabolism in A. suum, including acetate, butyrate and 2-methylvalerate. The conductances and relative permeabilities of these organic anions are inversely related to size, with 2-methylvalerate being only approximately 3% as permeable as Cl(-). The diameter of the channel pore was 12.3+/-0.2 A, calculated from the relative permeability coefficients of Cl(-) and the organic anions. Results of this study are consistent with the hypothesis that the large conductance anion channel in A. suum hypodermal membranes provides a low energy pathway for organic anion excretion from the hypodermal compartment, followed by diffusion across the aqueous channels of the cuticle matrix.     

Establishment of a tissue-specific RNAi system in C. elegans

In C. elegans, mosaic analysis is a powerful genetic tool for determining in which tissue or specific cells a gene of interest is required. For traditional mosaic analysis, a loss-of-function mutant and a genomic fragment that can rescue the mutant phenotype are required. Here we establish an easy and rapid mosaic system using RNAi (RNA mediated interference), using a rde-1 mutant that is resistant to RNAi. Tissue-specific expression of the wild type rde-1 cDNA in rde-1 mutants limits RNAi sensitivity to a specific tissue. We established hypodermal-and muscle-specific RNAi systems by expressing rde-1 cDNA under the control of the lin-26 and hlh-1 promoters, respectively. We confirmed tissue-specific RNAi using two assays: (1) tissue-specific knockdown of GFP expression, and (2) phenocopy of mutations in essential genes that were previously known to function in a tissue-specific manner. We also applied this system to an essential gene, ajm-1, expressed in hypodermis and gut, and show that lethality in ajm-1 mutants is due to loss of expression in hypodermal cells. Although we demonstrate tissue-specific RNAi in hypodermis and muscle, this method could be easily applied to other tissues.     

Posidonia oceanica seedling root structure and development

The seedling root system of the seagrass Posidonia oceanica consists of a primary root and up to four adventitious roots. Under culture, germination and early growth began with the emergence of the primary root in the first week. Then the two adventitious root primordia originally present in the seed emerged at 3 and 5 weeks respectively, followed successively by further adventitious roots. Primary roots reached 17 mm at 4 weeks, but then their growth decreased markedly. In contrast the adventitious roots showed a pattern of continued elongation. Anatomical observations of both primary and adventitious roots revealed a multilayered hypodermis of thick-walled cells enclosing a wide cortex (99% of the root transverse area) and narrow stele. A well-distinguished endodermis was only observed in the primary roots, while differentiated xylem elements were found solely in the adventitious roots, but it is unclear to what degree differences between the two root types are due to different root maturity or to their role in water and nutrient uptake. Overall, the P. oceanica seedling root system is composed of multiple, rapidly formed roots which are strong yet flexible due to a large proportion of cortical tissue and further strengthened by a multilayered hypodermis, characteristics which could potentially facilitate initial anchorage and establishment.     

C. elegans peb-1 mutants exhibit pleiotropic defects in molting, feeding, and morphology

Caenorhabditis elegans PEB-1 is a novel DNA-binding protein expressed in most pharyngeal cell types and outside the pharynx in the hypodermis, hindgut, and vulva. Previous RNAi analyses indicated that PEB-1 is required for normal morphology of these tissues and growth; however, the peb-1 null phenotype was unknown. Here we describe the deletion mutant peb-1(cu9) that not only exhibits the morphological defects observed in peb-1(RNAi) animals, but also results in penetrant larval lethality characterized by defects in pharyngeal function and molting. Consistent with a function in molting, we found that PEB-1 was detectable in all hypodermal and hindgut cells underlying the cuticle. Comparison to molting-defective lrp-1(ku156) mutants revealed that the peb-1(cu9) mutants were particularly defective in shedding the pharyngeal cuticle, and this defect likely contributed to feeding defects and lethality. Most markers of pharyngeal cell differentiation examined were expressed normally in peb-1(cu9) mutants; however, g1 gland cell expression of a kel-1Colon, two colonsgfp reporter was reduced. As g1 gland cells have prominent functions during molting, we suggest defective gland cell differentiation contributes to peb-1(cu9) molting defects. In comparison, other peb-1 mutant phenotypes, including hindgut abnormalities, appeared independent of the molting defect. Similar phenotypes resulted from late loss of pha-4 function, suggesting that PEB-1 and PHA-4 have common functions in some tissues where they are co-expressed.     

Root anatomy and spatial pattern of radial oxygen loss of eight true mangrove species

The root anatomical features of eight mangrove species in Hong Kong were similar, with large aerenchymal lacunae in the cortex for efficient internal oxygen transfer and an outer barrier consisting of an epidermis and hypodermis to prevent oxygen loss. The spatial pattern of radial oxygen loss (ROL) was also comparable, with more oxygen lost from the tip than that from the basal and mature zones. However, the aerenchyma in the cortex, the barrier and the extent of ROL varied along the root and these variations were species-specific. The whole root of Avicennia marina (Forsk.) Vierh., Acanthus ilicifolius L., Aegiceras corniculatum (Linn.) Blanco, Kandelia obovata Sheue, Liu & Yong (previously known as Kandelia candel (L.) Druce) and Heriteria littoralis Dryand. ex W. Ait. had schizogenous aerenchyma, while the aerenchyma of Lumnitzera racemosa Willd. and Bruguiera gymnorrhiza (L.) Poir changed from schizogenous in the root tip to lysigenous in the other parts of the root. Excoecaria agallocha L. displayed the opposite pattern, from lysigenous in the root tip to schizogenous further up. Among the eight species, the roots of A. marina and A. ilicifolius had the largest areas of aerenchyma air spaces, but they also had the weakest barrier. On the other hand, H. littoralis had the least longitudinal oxygen transfer because of its smaller area of aerenchyma air spaces in its root. The tolerance of mangrove species to waterlogged soil followed the order of A. marina (most foreshore species) > A. ilicifolius > K. obovata > A. corniculatum > B. gymnorrhiza > E. agallocha > L. racemosa > H. littoralis (most landward species), which is related to their anatomical features of root cortex, epidermis and hypodermis.     

Do leaves in Cyperoideae (Cyperaceae) have a multiple epidermis or a hypodermis?

In Cyperaceae, leaf anatomical characters, in particular the presence of a hypodermis or of a multiple epidermis, have contributed in taxonomic and phylogenetic studies. In this family, the leaf epidermis is often described as uniseriate, and the cells of the subepidermal layers having no chloroplasts are treated as hypodermis. Both tissues have a different ontogenetic origin and hence are not homologous. The aim of the present work was to verify the origin of the subepidermal layers in eight species belonging to Cyperoideae. All species studied presented multiple epidermal layers that were confirmed by leaf ontogeny. In Fimbristylis complanata, F. dichotoma, Pycreus flavescens and P. polystachyos the mature leaves present multiple epidermal layers with cells of the distinct layers similar in shape and size; in the other species studied these cells are different. Especially in the latter case, a multiple epidermis is easily interpreted erroneously as a hypodermis, possibly leading to erroneous evolutionary conclusions. Making correctly distinction between a hypodermis and a multiple epidermis, and hence in case of doubt investigating the origin of the questioned tissue, is compulsory in order to use both characters in a phylogenetic context. Though in the past often called ‘hypodermis’, our leaf ontogenetical observations show that in all species studied, the subepidermical layers constitute a multiple epidermis, originating from the protodermis.     

The Fat-like cadherin CDH-4 controls axon fasciculation, cell migration and hypodermis and pharynx development in Caenorhabditis elegans

Cadherins are one of the major families of adhesion molecules with diverse functions during embryonic development. Fat-like cadherins form an evolutionarily conserved subgroup characterized by an unusually large number of cadherin repeats in the extracellular domain. Here we describe the role of the Fat-like cadherin CDH-4 in Caenorhabditis elegans development. Cdh-4 mutants are characterized by hypodermal defects leading to incompletely penetrant embryonic or larval lethality with variable morphogenetic defects. Independently of the morphogenetic defects cdh-4 mutant animals also exhibit fasciculation defects in the ventral and dorsal cord, the major longitudinal axon tracts, as well as migration defects of the Q neuroblasts. In addition CDH-4 is essential for establishing and maintaining the attachment between the buccal cavity and the pharynx. Cdh-4 is expressed widely in most affected cells and tissues during embryogenesis suggesting that CDH-4 functions to ensure that proper cell contacts are made and maintained during development.     

Epidermal and hypodermal characteristics in North American Cactoideae (Cactaceae)

Dermal and hypodermal anatomical features of 70 species representing 21 genera of North American Cactoideae were studied. Results show that all species examined have parallelocytic stomata and anticlinal wall surface varies from straight to undulate. Cuticle thickness is mostly narrow (1–10 μm) contrary to the general opinion that cuticle is thick in most cacti; however, few species such as Ariocarpus fissuratus and several species of Pachycereus show a distinctive thick cuticle. More than 80% of the species studied have a single-layered epidermis. Papillae occur in eight species belonging to four genera. Notable papillae are a feature shared by all members of Peniocereus subg. Peniocereus. Other species show a bullate surface produced by irregular patches of secondary epidermal cell divisions. Commonly, the hypodermis is composed of more than two cell layers with distinctive collenchymatous walls as reported in many South American species. Silica bodies, prismatic crystals, druses, sphaerocrystals, and tannins are the most common cellular inclusions that distinguish several genera and appear to have taxonomic value. However, a more thorough search in species of Cephalocereus, Coryphantha, Echinocereus, Mammillaria, Neobuxbaumia, Pilosocereus, and Turbinicarpus is needed to support the previous assertion. Electronic Publication     

Multiple regulatory elements with spatially and temporally distinct activities control the expression of the epithelial differentiation gene lin-26 in C. elegans

Epithelial differentiation is a very early event during development of most species. The nematode Caenorhabditis elegans, with its well-defined and invariant lineage, offers the possibility to link cell lineage, cell fate specification and gene regulation during epithelial differentiation. Here, we focus on the regulation of the gene lin-26, which is required for proper differentiation of epithelial cells in the ectoderm and mesoderm (somatic gonad). lin-26 expression starts in early embryos and remains on throughout development, in many cell types originating from different sublineages. Using GFP reporters and mutant rescue assays, we performed a molecular dissection of the lin-26 promoter and could identify almost all elements required to establish its complex spatial and temporal expression. Most of these elements act redundantly, or synergistically once combined, to drive expression in cells related by function. We also show that lin-26 promoter elements mediate activation in the epidermis (hypodermis) by the GATA factor ELT-1, or repression in the foregut (pharynx) by the FoxA protein PHA-4. Taken together, our data indicate that lin-26 regulation is achieved to a large extent through tissue-specific cis-regulatory elements.