Outermost Cell Layer Research Articles

Immunohistochemical localization of cytokeratins and involucrin in calcifying epithelioma: comparative studies with normal skin

The expression of cytokeratins and involucrin varies greatly in different epithelia, and this raises the possibility that detailed analysis of these epidermal proteins might provide a means of identifying various skin tumours. The present study was conducted to determine the immunohistochemical distribution of cytokeratins and involucrin in calcifying epithelioma of Malherbe, in order to elucidate the nature and differentiation of this tumour. To correlate the immunohistochemical profile with the most frequent histological patterns, we categorized the basophilic, transitional, shadow, and squamoid cells, and the shreds of keratin. Comparative studies with normal skin showed that the shadow and transitional cells corresponded to hair cortex cells, the squamoid cells to the outer root sheath, the basophilic cells adjacent to the stroma to the outermost cell layer of the outer root sheath between the lower permanent portion and upper transient portion of the follicles, and the basophilic cells adjacent to the transitional cells to the hair matrix. The expression of cytokeratins in most shreds of keratin was similar to that in squamoid cells. Calcifying epithelioma was, therefore, shown to be composed of tumour cells differentiating into both the hair cortex and outer root sheath. These tumour cells were differentiated from basophilic cells, which showed the same staining patterns as the outermost cell layer of the outer root sheath between the lower permanent portion and upper transient portion of the hair follicles, supporting the hypothesis that the keratinocytes in the outermost cell layer can differentiate into the transitional portion of the follicle and anagen hair.

Adenosine receptor expression and function in bladder uroepithelium

The uroepithelium of the bladder forms an impermeable barrier that is maintained in part by regulated membrane turnover in the outermost umbrella cell layer. Other than bladder filling, few physiological regulators of this process are known. Western blot analysis established that all four adenosine receptors (A1, A2a, A2b, and A3) are expressed in the uroepithelium. A1 receptors were prominently localized to the apical membrane of the umbrella cell layer, whereas A2a, A2b, and A3 receptors were localized intracellularly or on the basolateral membrane of umbrella cells and the plasma membrane of the underlying cell layers. Adenosine was released from the uroepithelium, which was potentiated 10-fold by stretching the tissue. Administration of adenosine to the serosal or mucosal surface of the uroepithelium led to increases in membrane capacitance (where 1 microF approximately 1 cm(2) tissue area) of approximately 30% or approximately 24%, respectively, after 5 h. Although A1, A2a, and A3 selective agonists all stimulated membrane capacitance after being administrated serosally, only the A1 agonist caused large increases in capacitance after being administered mucosally. Adenosine receptor antagonists as well as adenosine deaminase had no effect on stretch-induced capacitance increases, but adenosine potentiated the effects of stretch. Treatment with U-73122, 2-aminoethoxydiphenylborate, or xestospongin C or incubation in calcium-free Krebs solution inhibited adenosine-induced increases in capacitance. These data indicate that the uroepithelium is a site of adenosine biosynthesis, that adenosine receptors are expressed in the uroepithelium, and that one function of these receptors may be to modulate exocytosis in umbrella cells.

Protein Interactions of Uroplakin IIIa in Urinary Bladder Umbrella Cells

The mucosal surface of the urinary bladder forms a dynamic barrier that prevents pathogens and toxic components of urine from entering the underlying tissue layers. An important protein component of the apical membrane of the outermost umbrella cell layer is uroplakin IIIa (UP3a). At present, little is known about the proteins that interact with the 52 amino acid cytoplasmic tail of UP3a. Yeast two-hybrid analysis showed that the cytoplasmic tail of UP3a interacts with several proteins, including Rho-GTPase regulatory proteins and proteins involved in signal transduction pathways. The aim of this project is to determine which amino acids in the UP3a tail sequence are required for interactions with these proteins. We hypothesize that mutating various regions of the UP3a tail will eliminate these protein interactions, allowing us to define specific amino acids that are important for protein-protein interactions. Site-directed mutagenesis was used to create mutants of the UP3a tail, which were expressed in yeast and then mated with yeast expressing UP3a tail interacting proteins in a two-hybrid experiment. Mated yeast were grown on plates lacking nutritional factors, which only allow growth when a positive protein interaction occurs. Preliminary results indicate that the N-terminal region of the UP3a tail is required for interactions with the UP3a interacting proteins. The study of the protein interactions of the UP3a tail will give us a better understanding of how UP3a contributes to umbrella cell function, the function of the UP3a tail, and overall bladder physiology.

Anatomical Differentiation and Photosynthetic Adaptation in Brown Algae

The photosynthetic parameters of dark- adapted minimum fluorescence (Fo) and maximum quantum yield of charge separation in PSII (Fv/Fm) were measured in transverse sections of eight species of marine Phaeophyceae (species of Laminariales, Fucales, Desmarestiales, Chordariales) using pulse amplified modulation (PAM) fluorometry. Within each transverse section fluorescence was measured in three regions corresponding to outer cortical and meristoderm cells, inner cortical cells and innermost medullary cells. Minimum fluorescence declined from 19-74% (mean of 39%) from outermost to innermost cells. Maximum quantum yield varied from 0.51-0.59 in outermost cell layers and this was reduced to 0.23-0.40 in innermost cell layers, with an average reduction of 50%. Despite the reduction Fo in medullary cells (inner), medullas of all species showed maximum quantum yields consistent with a photosynthetic role in carbon fixation. These results show that medullary cells of complex brown algae have more than a role in structure, storage or transport, and may also provide an important role in carbon fixation.

Interferon-gamma alters the phagocytic activity of the mouse trophoblast

Interferon-gamma (IFN-gamma) mediates diverse functions in bone marrow-derived phagocytes, including phagocytosis and microbe destruction. This cytokine has also been detected at implantation sites under both physiological and pathological conditions in many different species. At these particular sites, the outermost embryonic cell layer in close contact with the maternal tissues, the trophoblast exhibits intense phagocytic activity. To determine whether IFN-gamma affects phagocytosis of mouse-trophoblast cells, ectoplacental cone-derived trophoblast was cultured and evaluated for erythrophagocytosis. Phagocytic activity was monitored ultrastructurally and expressed as percentage of phagocytic trophoblast in total trophoblast cells. Conditioned medium from concanavalin-A-stimulated spleen cells significantly enhanced trophoblast phagocytosis. This effect was blocked by pre-incubation with an anti-IFN-gamma neutralizing antibody. Introduction of mouse recombinant IFN-gamma (mrIFN-gamma) to cultures did not increase cell death, but augmented the percentage of phagocytic cells in a dose-dependent manner. Ectoplacental cones from mice deficient for IFN-gamma receptor alpha-chain showed a significant decrease of the phagocytosis, even under mrIFN-gamma stimulation, suggesting that IFN-gamma-induced phagocytosis are receptor-mediated. Reverse transcriptase-PCR analyses confirmed the presence of mRNA for IFN-gamma receptor alpha and beta-chains in trophoblast cells and detected a significant increase in the mRNA levels of IFN-gamma receptor beta-chain, mainly, when cultured cells were exposed to IFN-gamma. Immunohistochemistry and Western blot analyses also revealed protein expression of the IFN-gamma receptor alpha-chain. These results suggest that IFN-gamma may participate in the phagocytic activation of the mouse trophoblast, albeit the exact mechanism was not hereby elucidated. Protective and/or nutritional fetal benefit may result from this physiological response. In addition, our data also shed some light on the understanding of trophoblast tolerance to inflammatory/immune cytokines during normal gestation.

Distribution of chlorophyll-bearing organelles in the shoot apex of a range of dicotyledonous plants

Confocal laser scanning microscopy was used to study the distribution of the smallest detectable autofluorescing, chlorophyll-bearing structures in fresh, 40 microm thick longitudinal sections of the shoot apex of four dicotyledonous plants (Arabidopsis thaliana, Nicotiana glauca, Lupinus alba, and Spinacia oleracea). In all species, the smallest chlorophyll-bearing particles were found in the outermost cell layers (L1 and L2) of the shoot apex. Their distribution between these layers differed in each species. The smallest such particles were about 0.5-1.0 microm in maximum dimension, approximating the size of a single granum in the developing leaf. Their size and abundance increased with increasing cell age and distance from the peak of the apex. Immediately beneath the L1 and L2 layers was a zone largely devoid of these particles. Below this nonfluorescing zone, in the region where the derivatives of the meristematic zone differentiate into cells of the central pith region, the size and abundance of the chlorophyll-bearing particles increased progressively with increasing distance from the nonfluorescing zone. The presence of these small autofluorescing particles in the L1 and L2 cell layers of the shoot apex places the development of photosystem II fluorescence at an earlier stage of leaf development than previously observed. The use of confocal laser scanning microscopy to study unfixed sections provides another useful metabolic marker for mapping patterns of differentiation and development in the cells of the shoot apex.

Chemical versus mechanical inhibition of fouling in the red alga Dilsea carnosa

In this study, we used field experiments in natural populations to test whether the low degree of fouling found naturally on the red alga Dilsea carnosa (Schmidel) O. Kuntze (1893) is due to chemical inhibition by antifouling metabolites. Extracts with concentrations volumetrically equiva- lent to whole algal tissue were incorporated into stable gels, which served as settlement substrata for potential fouling organisms. The gels were placed in the field during several time periods, covering all seasons. We also investigated the fouling intensity on living D. carnosa plants to be able to compare fouling on algae with fouling on gels. The extracts inhibited recruitment of a few fouling organisms, but they were not effective against the dominant bryozoan fouling species Electra pilosa and Membranipora membranacea. Furthermore, a relatively high number of these species occasion- ally recruited onto D. carnosa plants. Hence, chemical inhibition of fouling could not explain the low degree of fouling of bryozoan species found naturally on D. carnosa. Instead, based on field observa- tions showing that D. carnosa is able to shed its epidermis, we hypothesised that the low degree of fouling is primarily due to cuticle peeling, whereby the alga sloughs off the outermost cell layer in order to remove associated fouling organisms. This hypothesis was tested in a field survey in which D. carnosa plants were marked and surveyed for about 5 mo. The results show that individual algae that experienced a radical drop in their fouling cover had distinct traces of cuticle peeling. Hence, this study suggests that the dominant fouling organisms are not chemically inhibited by D. carnosa. Instead, the low degree of fouling found naturally on D. carnosa is probably a consequence of a mechanical defence, whereby the alga sloughs off the outermost cell layer in order to remove associ- ated epibiota.

Distinct roles of Lotus japonicus SYMRK and SYM15 in root colonization and arbuscule formation.

• The colonization of Lotus japonicus roots by the arbuscular mycorrhizal fungus Glomus intraradices was analysed in plant mutants affected in the symbiosis genes, SYM15 or SYMRK. SYMRK encodes an LRR receptor-like kinase that is, like the SYM15 gene, essential for both mycorrhizal and rhizobial symbioses. • Different colonization patterns were observed in growing vs meristematically arrested roots. • Three steps in the interaction were differentially impaired in the mutants: surface opening, where the anticlinal cell walls of two adjacent epidermal cells separate from each other in the vicinity of fungal hyphae; intracellular passage of hyphae through an exodermal cell and an adjacent cell of the outermost cortical layer; and arbuscule formation in cells of the two innermost cortical layers. • The combined results indicate that LjSYMRK is required for the intracellular passage through exodermis and outermost cortical cell layer whereas LjSYM15 is required for surface opening and arbuscule formation.

Peroxidases and the metabolism of capsaicin in Capsicum annuum L.

Capsaicinoids are acid amides of C9 - C11 branched-chain fatty acids and vanillylamine. These compounds are responsible for the pungency of the Capsicum species and of cultivars regarded as hot peppers. Moreover, it has been suggested that these compounds play an ecological role in seed dispersal. Because they are used in the pharmacological, food and pesticide industries, much attention has been paid on knowing how their accumulation is controlled, both in the fruit and in cell cultures. Such control involves the processes of biosynthesis, conjugation and catabolism. Recent progress has been made on the biosynthetic pathway, and several of the genes coding for biosynthetic enzymes have been cloned and expression studies performed. With regard to catabolism, cumulative evidence supports that capsaicinoids are oxidized in the pepper by peroxidases. Peroxidases are efficient in catalyzing in vitro oxidation of both capsaicin and dihydrocapsaicin. These enzymes are mainly located in placental and the outermost epidermal cell layers of pepper fruits, as occurs with capsaicinoids, and some peroxidases are present in the organelle of capsaicinoid accumulation, that is, the vacuole. Hence, peroxidases are in the right place for this function. The products of capsaicin oxidation by peroxidases have been characterized in vitro, and some of them have been found to appear in vivo in the Capsicum fruit. Details on the kinetics and catalytic cycle for capsaicin oxidation by peroxidases are also discussed.

A new FTIR-ATR cell for drug diffusion studies

The drug diffusion of most compounds, particularly hydrophilic molecules through the skin is limited by the permeation of the outermost cell layers of the epidermis, the stratum corneum(SC). For this reason it is of interest to characterize drug diffusion processes through this skin layer. A new FTIR-ATR cell was developed for non-invasive real time measurements of drug diffusion. The diffusion of water through an artificial polyethyleneglycol-polydimethylsiloxane membrane was studied. Additionally the diffusion of urea in human SC was analyzed. Based on a mathematical model the diffusion coefficients were derived. We could reveal that this cell associates the advantages of the Franz diffusion cell and the FTIR-ATR spectroscopy as a new powerful method for determining drug diffusion through biological membranes.

The epicardium and epicardially derived cells (EPDCs) as cardiac stem cells.

After its initial formation the epicardium forms the outermost cell layer of the heart. As a result of an epithelial-to-mesenchymal transformation (EMT) individual cells delaminate from this primitive epicardial epithelium and migrate into the subepicardial space (Pérez-Pomares et al., Dev Dyn 1997; 210:96-105; Histochem J 1998a;30:627-634). Several studies have demonstrated that these epicardially derived cells (EPDCs) subsequently invade myocardial and valvuloseptal tissues (Mikawa and Fischman, Proc Natl Acad Sci USA 1992;89:9504-9508; Mikawa and Gourdie, Dev Biol 1996;174:221-232; Dettman et al., Dev Biol 1998;193:169-181; Gittenberger de Groot et al., Circ Res 1998;82:1043-1052; Manner, Anat Rec 1999;255:212-226; Pérez-Pomares et al., Dev. Biol. 2002b;247:307-326). A subset of EPDCs continue to differentiate in a variety of different cell types (including coronary endothelium, coronary smooth muscle cells (CoSMCs), interstitial fibroblasts, and atrioventricular cushion mesenchymal cells), whereas other EPDCs remain in a more or less undifferentiated state. Based on its specific characteristics, we consider the EPDC as the ultimate 'cardiac stem cell'. In this review we briefly summarize what is known about events that relate to EPDC development and differentiation while at the same time identifying some of the directions where EPDC-related research might lead us in the near future.

Regulation of phyllotaxis by polar auxin transport.

The regular arrangement of leaves around a plant's stem, called phyllotaxis, has for centuries attracted the attention of philosophers, mathematicians and natural scientists; however, to date, studies of phyllotaxis have been largely theoretical. Leaves and flowers are formed from the shoot apical meristem, triggered by the plant hormone auxin. Auxin is transported through plant tissues by specific cellular influx and efflux carrier proteins. Here we show that proteins involved in auxin transport regulate phyllotaxis. Our data indicate that auxin is transported upwards into the meristem through the epidermis and the outermost meristem cell layer. Existing leaf primordia act as sinks, redistributing auxin and creating its heterogeneous distribution in the meristem. Auxin accumulation occurs only at certain minimal distances from existing primordia, defining the position of future primordia. This model for phyllotaxis accounts for its reiterative nature, as well as its regularity and stability.

Stabilisation of cables of fibronectin with micromolar concentrations of copper: in vitro cell substrate properties

We have previously described the production of large cables of fibronectin, a large extracellular matrix cell adhesion glycoprotein, which has a potential application in tissue engineering. Here we have stabilised these cables for longer survival and looked at their ultrastructural cell–substrate behaviour in vitro. Dissolution experiments showed that low concentrations of copper not only caused significant material stabilisation but left pores which could promote cell ingrowth, as we have previously reported with Fn-mats. Indeed, the greatest amount of cell ingrowth was observed for copper treated cables. Immunostaining showed S-100 + multi-layers of cells around the edge of cables while ultrastructural analysis confirmed the presence of a mixture of fibroblasts and bipolar cells associated with fragments of basal lamina, which is a Schwann cell phenotype. Interestingly, the outermost layers of cells consisted of S-100 − cells, presumed fibroblasts, apparently ‘capping’ the Schwann cells. Toxicity tests revealed that Schwann cells were only able to grow at the lowest concentration of copper used (1 μ m) while fibroblasts grew at all concentrations tested. These results could be used to design biomaterials with optimum properties for promoting cellular ingrowth and survival in tissue engineered grafts which may be used to improve peripheral nerve repair.

Ultrastructural and cytochemical study of plocamium cartilagineum (plocamiales, rhodophyta) from Ross sea (antarctica)

Morphological, ultrastructural, and cytochemical characteristics of the red alga Plocamium cartilagineum from Antarctica have been studied and compared with those of the same species from the Mediterranean Sea. Distinct regions with possibly different functions were recognisable in the bushy thallus of Plocamium cartilagineum. Chloroplasts were concentrated in the outermost cell layer, while in the other cortex cells abundant floridean starch accumulated. No starch was found in the large medullary cells, characterised by the insertion in their walls of several large and lenticular pit connections devoid of plug caps and proteic in nature. Sulphated polysaccharides were distributed in the extracellular compartments of all the regions of the thallus, except in the surface layer. These acidic phycocolloids were particularly concentrated in the cell wall proper, which also exhibited a well organised fibrillar component probably consisting of cellulose. The distinctive feature of the Antarctic thalli, compared with the Mediterranean ones, was the lack, in the former, of recognisable phycobilisomes on the thylakoid surfaces. This peculiarity had already been noticed in the other Antarctic red alga Iridaea cordata, collected, like Plocamium cartilagineum, in summer and from ice‐free waters. The loss of phycobilisomes may be a defence mechanism activated by benthic shade‐adapted red algae from Antarctica to protect the photosynthetic apparatus against damage from high light intensities.

A calcium/calmodulin kinase pathway connects brain-derived neurotrophic factor to the cyclic amp-responsive transcription factor in the rat hippocampus

Brain-derived neurotrophic factor (BDNF) plays fundamental roles in synaptic plasticity in rat hippocampus. Recently, using rat hippocampal slices, we found that BDNF induces activation of calcium/calmodulin-dependent protein kinase 2 (CaMKII), a critical mediator of synaptic plasticity. CaMKII in turn activates the p38 subfamily of mitogen-activated protein kinases (MAPK) and its downstream effector, MAPK-activated protein kinase 2 (MAPKAPK-2). Herein, we determined whether some kinases of this pathway connect BDNF to the cyclic AMP response element -binding protein (CREB), a transcription factor also involved in plasticity and survival. Crude cytosolic and nuclear fractions were prepared from hippocampal slices of adult rat, and then kinase involvement in CREB phosphorylation was studied with a combination of pharmacologic inhibition and antibody depletion. In addition, the regional localization of this signaling pathway was immunohistochemically investigated. We show that: (i) the BDNF-stimulated CaMKII cascade phosphorylates the key positive regulatory site of CREB via its end MAPKAPK-2 component; (ii) this process appears to be highly localized in the outermost cell layer of the dentate gyrus. The present findings suggest that CaMKII is involved in neurotrophic-dependent activation of CREB in the dentate gyrus. Such a signaling process could be important for controlling synaptic plasticity in this major area for the afferent inputs to the hippocampal formation.

Isolation and properties of floral defensins from ornamental tobacco and petunia.

The flowers of the solanaceous plants ornamental tobacco (Nicotiana alata) and petunia (Petunia hybrida) produce high levels of defensins during the early stages of development. In contrast to the well-described seed defensins, these floral defensins are produced as precursors with C-terminal prodomains of 27 to 33 amino acids in addition to a typical secretion signal peptide and central defensin domain of 47 or 49 amino acids. Defensins isolated from N. alata and petunia flowers lack the C-terminal domain, suggesting that it is removed during or after transit through the secretory pathway. Immunogold electron microscopy has been used to demonstrate that the N. alata defensin is deposited in the vacuole. In addition to the eight canonical cysteine residues that define the plant defensin family, the two petunia defensins have an extra pair of cysteines that form a fifth disulfide bond and hence define a new subclass of this family of proteins. Expression of the N. alata defensin NaD1 is predominantly flower specific and is most active during the early stages of flower development. NaD1 transcripts accumulate in the outermost cell layers of petals, sepals, anthers, and styles, consistent with a role in protection of the reproductive organs against potential pathogens. The floral defensins inhibit the growth of Botrytis cinerea and Fusarium oxysporum in vitro, providing further support for a role in protection of floral tissues against pathogen invasion.

Regulation of shoot epidermal cell differentiation by a pair of homeodomain proteins in Arabidopsis.

In higher plants, the outermost cell layer (L1) of the shoot apex gives rise to the epidermis of shoot organs. Our previous study demonstrated that an 8-bp motif named the L1 box functions as a cis-regulatory element for L1-specific gene expression in the shoot system of Arabidopsis: We show here that PROTODERMAL FACTOR2 (PDF2), a member of the HD-GL2 class of homeobox genes, is expressed exclusively in the L1 of shoot meristems and that recombinant PDF2 protein specifically binds to the L1 box in vitro. Although knockout mutants of PDF2 and ATML1, another L1-specific HD-GL2 class gene sharing the highest homology with PDF2, display normal shoot development, the double mutant results in severe defects in shoot epidermal cell differentiation. This suggests that PDF2 and ATML1 are functionally interchangeable and play a critical role in maintaining the identity of L1 cells, possibly by interacting with their L1 box and those of downstream target-gene promoters.

New species, notes and key to the aeroaquatic genera Beverwykella and Ramicephala gen. nov.

Two new aeroaquatic fungi, Ramicephala sphaerospora gen. sp. nov. and Beverwykella clathrata sp. nov., are described and illustrated from tropical and subtropical latitudes. B. clathrata, a species similar to B. cerebriformis, deviates from the latter in having a clathrate outermost cell layer instead of tightly appressed cells, a feature unique in Beverwykella. The second species, Ramicephala sphaerospora gen. sp. nov., has a unique combination of features concerning conidial ontogeny and morphology, and is therefore placed in a new genus. The genus most similar to Ramicephala is Beverwykella, from which it differs in having: (1) globose, non-flattened conidia; (2) a large central globose cell giving rise to centrifugally branching, tightly packed chains of cells; and (3) a compact peripheral cell layer, the cells of which are tightly interlocked in a zipper-like form. Similarities of R. sphaerospora to species of Pseudaegerita, Candelabrum and Cristulariella are discussed. Illustrations of and a key to all described species of Ramicephala and Beverwykella are provided.

Direct induction of somatic embryogenesis from immature zygotic embryo of cashewnut ( Anacardium occidentale L.)

Direct somatic embryogenesis was induced in precociously germinated immature zygotic embryos (IZEs) of cashewnut ( Anacardium occidentale L.). The IZEs were cultured on semisolid Murashige and Skoog (MS) basal medium with 0.5% activated charcoal and incubated in dark. After 4–5 weeks, somatic embryos were induced directly from the radical tip of IZEs on medium with 2,4-dichlorophenoxyacetic acid (2,4-D) (5 μM)+ BAP (5 μM)+ gibberellic acid (GA 3) (3 μM) with 10% frequency of embryogenic response, which was 9% when 2,4-D (5 μM) was used alone or with Kinetin (Kin) (5 μM). The somatic embryos were transferred to medium with abscissic acid (ABA) (20 μM) and maltose (3%) for further maturation and, studies are being carried out for conversion of somatic embryos into plantlets. A histological study of somatic embryo development was carried out where it was seen that the somatic embryos arose from the outermost cell layer of radical tip.

A model for the development of the rhizobial and arbuscular mycorrhizal symbioses in legumes and its use to understand the roles of ethylene in the establishment of these two symbioses

We propose a model depicting the development of nodulation and arbuscular mycorrhizae. Both processes are dissected into many steps, using Pisum sativum L. nodulation mutants as a guideline. For nodulation, we distinguish two main developmental programs, one epidermal and one cortical. Whereas Nod factors alone affect the cortical program, bacteria are required to trigger the epidermal events. We propose that the two programs of the rhizobial symbiosis evolved separately and that, over time, they came to function together. The distinction between these two programs does not exist for arbuscular mycorrhizae development despite events occurring in both root tissues. Mutations that affect both symbioses are restricted to the epidermal program. We propose here sites of action and potential roles for ethylene during the formation of the two symbioses with a specific hypothesis for nodule organogenesis. Assuming the epidermis does not make ethylene, the microsymbionts probably first encounter a regulatory level of ethylene at the epidermis – outermost cortical cell layer interface. Depending on the hormone concentrations there, infection will either progress or be blocked. In the former case, ethylene affects the cortex cytoskeleton, allowing reorganization that facilitates infection; in the latter case, ethylene acts on several enzymes that interfere with infection thread growth, causing it to abort. Throughout this review, the difficulty of generalizing the roles of ethylene is emphasized and numerous examples are given to demonstrate the diversity that exists in plants.Key words: AM, epidermis, evolution, pea, rhizobia, sym mutant.