Search

Abaxial epidermal cells of developing faba bean (Vicia faba) cotyledons are modified to a transfer cell morphology and function. In contrast, the adaxial epidermal cells do not form transfer cells but can be induced to do so when excised cotyledons are cultured on an agar medium. The first fenestrated layer of wall ingrowths is apparent within 24 h of cotyledon exposure to culture medium. The time course of wall ingrowth formation was examined further. By 2 h following cotyledon excision, a 350 nm thick wall was deposited evenly over the outer periclinal walls of adaxial epidermal cells and densities of cytoplasmic vesicles increased. After 3 h in culture, 10% of epidermal cells contained small projections of wall material on their outer periclinal walls. Thereafter, this percentage rose sharply and reached a maximum of 90% by 15 h. Continuous culture of cotyledons on a medium containing 6-methyl purine (an inhibitor of RNA synthesis) completely blocked wall ingrowth formation. In contrast, if exposure to 6-methyl purine was delayed for 1 h at the start of the culture period, the adaxial epidermal cells were found to contain small wall ingrowths. Treating cotyledons for 1 h with 6-methyl purine at 15 h following cotyledon excision halted further wall ingrowth development. We conclude that transfer cell induction is rapid and that signalling and early events leading to wall ingrowth formation depend upon gene expression. In addition, these gene products have a high turnover rate.

Transfer cell formation in cotyledons of developing faba bean (Vicia faba L.) seeds coincides with an abrupt change in seed apoplasm composition from one dominated by hexoses to one in which sucrose is the principal sugar. On the basis of these observations, we tested the hypothesis that sugars induce and/or sustain transfer cell development. To avoid confounding effects of in planta developmental programs, we exploited the finding that adaxial epidermal cells of cotyledons, which do not become transfer cells in planta, can be induced to form functional transfer cells when cotyledons are cultured on an agar medium. Growth rates of cotyledons cultured on hexose or sucrose media were used to inform choice of sugar concentrations. The same proportion of adaxial epidermal cells of excised cotyledons were induced to form wall ingrowths independent of sugar species and concentration supplied. In all cases, induction of wall ingrowths coincided with a marked increase in the intracellular sucrose-to-hexose ratio. In contrast, further progression of wall ingrowth deposition was correlated positively with intracellular sucrose concentrations that varied depending upon external sugar species and supply. Sucrose symporter induction and subsequent maintenance behaved identically to wall ingrowth formation in response to an external supply of hexoses or sucrose. However, in contrast to wall ingrowth formation, induction of sucrose symporter activity was delayed. We discuss the possibility of intracellular sugars functioning both as signals and substrates that induce and control subsequent development of transfer cells.

Medicago sativa var. Gabes is a perennial glycophyte that develops new shoots even in high salinity (150 mM NaCl). In the upper exporting leaves, K(+) is high and Na(+) is low by comparison with the lower leaves, where Na(+) accumulation induces chlorosis after 4 weeks of NaCl treatment. By secondary ion mass spectroscopy, a low Na(+)/K(+) ratio was detected in the phloem complex of blade veins in these lower leaves. By transmission electron microscopy, the ultrastructural features were observed in the phloem complex. In the upper leaves of both control and NaCl-treated plants, companion cells in minor veins were found to be transfer cells. These cells may well be involved in the intravenous recycling of ions and in Na(+) flowing out of exporting leaves. Under the effect of NaCl, companion cells in the main veins develop transfer cell features, which may favor the rate of assimilate transport from exporting leaves toward meristems, allowing the positive balance necessary for the survival in salt conditions. These features no longer assist the lower leaves when transfer cells are necrotized in both minor and main veins of NaCl-treated plants. As transfer cells are the only degenerating phloem constituent, our observations emphasize their role in controlling nutrient (in particular, Na(+)) fluxes associated with the stress response.

Root hair formation and the development of transfer cells in the rhizodermis was investigated in various existing auxin-related mutants of Arabidopsis thaliana and in the tomato mutant diageotropica. Wild-type Arabidopsis plants showed increased formation of root hairs when the seedlings were cultivated in Fe- or P-free medium. These extranumerary hairs were located in normal positions and in positions normally occupied by nonhair cells, e.g., over periclinal walls of underlying cortical cells. Defects in auxin transport or reduced auxin sensitivity inhibited the formation of root hairs in response to Fe deficiency completely but did only partly affect initiation and elongation of hairs in P-deficient roots. Application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid or the auxin analog 2,4-dichlorophenoxyacetic acid did not rescue the phenotype of the auxin-resistant axr2 mutant under control and Fe-deficient conditions, indicating that functional AXR2 product is required for translating the Fe deficiency signal into the formation of extra hairs. The development of extra hairs in axr2 roots under P-replete conditions was not affected by auxin antagonists, suggesting that this process is independent of auxin signaling. In roots of tomato, growth under Fe-deficient conditions induced the formation of transfer cells in the root epidermis. Transfer cell frequency was enhanced by application of 2,4-dichlorophenoxyacetic acid but was not inhibited by the auxin transport inhibitor N-1-naphthylphthalamic acid. In the diageotropica mutant, which displays reduced sensitivity to auxin, transfer cells appeared to develop in both Fe-sufficient and Fe-deficient roots. Similar to the wild type, no reduction in transfer cell frequency was observed after application of the above auxin transport inhibitor. These data suggest that auxin has no primary function in inducing transfer cell development; the formation of transfer cells, however, appears to be affected by the hormonal balance of the plants.

Transfer-cell-like wall ingrowth deposition is induced in adaxial epidermal cells ofVicia faba L. cotyledons grown in vitro in the presence of high hexose concentrations. We have further characterised this putative transfer cell induction system by examining initiation of secondary-wall ingrowth deposition and expression of sucrose transport-related genes. Wall ingrowth deposition, as wall thickening and scattered papillate ingrowths, was visualised on the outer periclinal walls of adaxial epidermal cells of cotyledons after 1 day in culture. Over the next 2 days, wall deposition increased significantly to form a distinct band of discrete and coalescing ingrowths. Thereafter, further wall deposition was arrested. Densities of Golgi, endoplasmic reticulum, and mitochondria increased concurrently with wall ingrowth deposition. Transcripts of a H+/sucrose symporter (SUT) and a sucrose-binding protein (SBP) were detected by in situ hybridisation in differentiating transfer cells. Antibodies raised against an H+-ATPase immunolocalised evenly around the perimeter of the adaxial epidermal cells in day 1 cotyledons. Thereafter, labelling became increasingly localised to the developing wall ingrowth regions. In contrast, SBP antibodies immunolocalised exclusively to wall ingrowth regions. However, SBP exhibited a transient pattern of expression, being detected only in 2-day-cultured cotyledons. A proton gradient, sufficient to facilitate sulphorhodamine G accumulation, was established by adaxial epidermal cells after 1 day in culture. [14C]sucrose uptake by cotyledons became sensitive to an inhibitor of carrier-mediated transport of sucrose,para-chloromercuribenzene sulfonic acid after 2 days. The initial 37% inhibition of sucrose transport by this compound declined to 5% for cotyledons cultured for 3 days. Collectively, these results suggest that differentiation of the key functional characteristics of transfer cells can be induced in vitro, providing an exciting tool for further exploration of transfer cell development.

The unusual thick-walled cells in contact with host and parasite vessels, first noted by Calvin 1967 in sinkers (structures composed of tracheary elements and parenchyma that originate from parasite bark strands that grow centripetally to the host vascular cambium and become embedded by successive development of xylem) of the mistletoePhoradendron macrophyllum (Englem.) Cockerell, have been investigated by modern methods of microscopy. The wall is thickest in cells abutting large-diameter host vessels, less so against smaller host vessels and those abutting sinker vessels. Transmission electron microscopy reveals the wall to be complex, consisting of a basement primary wall, upon which two developments of secondary-wall material occur. These are represented by lignified thickenings, in the form of flanges, and a labyrinth of wall ingrowths characteristic of a transfer cell. The wall ingrowths occur mostly in the primary-wall regions between the flanges, but when in contact with a large host vessel the ingrowths also differentiate on top of the flanges. Cells with such a transfer cell labyrinth have not been previously reported in the endophytic system of a mistletoe. The cells are confined to the xylary portion of the primary haustorium and sinkers. InP. macrophyllum, however, the cells differ from ordinary transfer cells in that they have differentiated as part of a flange parenchyma cell. This arrangement represents a novel anatomical situation. The name flange-walled transfer cell is used for these cells. The xylem of primary haustorium and sinkers also contain numerous ordinary flange cells. In both flange-walled transfer cells and ordinary flange cells the flanges are lignified and form a reticulate pattern of thickenings, separated by rounded areas of primary pit fields. The extent of development of the flange wall can vary in different parts of a sinker. At the host interface, the existence of a flange-walled transfer cell in direct contact with a vessel reflects a site associated with high loading into the parasite. Similarly, a labyrinth against a sinker vessel indicates a site of unloading from surrounding sinker tissue into the vessel for subsequent longdistance transport within the parasite.

Abnormally thick cell walls of a clonal maize cell line with the labyrinth wall morphology found in transfer cells were analyzed and compared to the relatively thin and even archetypical walls of a sister cell line. Despite a drastic difference in wall morphology between the transfer and archetypical cell walls, the chemical composition of the walls was essentially the same. There were no major differences in the glycosyl residue composition, in the amount of total lipid, and in the amount of total protein. The amounts of wall material released by chemical extraction of cellulosic, hemicellulosic, and pectic fractions were the same for the two types of walls. There were some differences in the protein profile and in the inorganic ion content between the transfer and archetypical walls. These results indicate that profound changes in wall morphology can be brought about in the absence of gross changes in wall composition and suggest that major changes in time- or place-dependent deposition and/or subtle changes in arrangement of rare wall constituents may be responsible.

The epidermal transfer cells in developingVicia faba L. cotyledons are highly polarized. Extensive wall ingrowths occur on their outer periclinal walls and extend part way down both anticlinal walls. This ingrowth development serves to increase the surface area of the plasma membrane and thus maximize porter-dependent uptake of sugars from the seed apoplasm. In contrast, the inner periclinal walls of these transfer cells do not form wall ingrowths. We have commenced a study of the mechanisms responsible for establishing this polarity by first analysing the microtubule (MT) cytoskeleton in developing transfer cells. Thin sections of fixed cotyledons embedded in methacrylate resin were processed for immunofluorescence microscopy using monoclonal anti-β-tubulin and counterstained with Calcofluor White to visualize wall ingrowths. In epidermal cells of young cotyledons where wall ingrowths were yet to develop, MT labelling was detected around all cortical regions of the cell. However, in cells where wall ingrowths were clearly established, MT labelling was detected almost exclusively in cortical regions adjacent to the wall ingrowths. Little, if any, MT labelling was detected on the anticlinal or inner periclinal walls of these cells. This distribution of MTs was most prominent in cells with well developed wall ingrowths. In these cells, a subpopulation of MTs were also detected emanating from the subcortex and extending towards the wall ingrowth region. The possible role of MT distribution in establishing transfer cell polarity and wall ingrowth formation is discussed.

In developing seeds ofVicia faba, transfer cells line the inner surface of the seed coat and the juxtaposed epidermal surface of the cotyledons. Circumstantial evidence, derived from anatomical and physiological studies, indicates that these cells are the likely sites of sucrose efflux to, and influx from, the seed apoplasm, respectively. In this study, expression of an H+/sucrose symporter-gene was found to be localised to the epidermal-transfer cell complexes of the cotyledons. The sucrose binding protein (SBP) gene was expressed in these cells as well as in the thin-walled parenchyma transfer cells of the seed coat. SBP was immunolocalised exclusively to the plasma membranes located in the wall ingrowth regions of the transfer cells. In addition, a plasma membrane H+-ATPase was most abundant in the wall ingrowth regions with decreasing levels of expression at increasing distance from the transfer cell layers. The observed co-localisation of high densities of a plasma membrane H+-ATPase and sucrose transport proteins to the wall ingrowths of the seed coat and cotyledon transfer cells provides strong evidence that these regions are the principal sites of facilitated membrane transport of sucrose to and from the seed apoplasm.

Immediately prior to seed fill, a dermal transfer cell complex, comprised of epidermal and subepidermal cells, differentiates on the abaxial surface of the cotyledons in seed ofVicia faba. Over the period of differentiation of this complex in vivo, the principal sugars of the seed apoplasmic sap change from hexoses, glucose and fructose, to sucrose. Cotyledons were removed from seeds before differentiation of the transfer cell complex and cultured for 6 days on an agar-based medium in the dark with their abaxial surface in contact with a medium containing either 100 mM hexoses (glucose and fructose in equimolar concentrations) or 100 mM sucrose. On both media, cotyledon growth rate was maintained throughout the culture period at, or above, that of in vivo grown cotyledons of equivalent developmental age. When cotyledons were cultured on a medium containing glucose and fructose, epidermal cells of both the ab- and adaxial surfaces developed wall ingrowths on their outer periclinal walls and their cytoplasm became dense, vesicular, and rich in mitochondria. Extensive ingrowth deposition also occurred on walls of the subepidermal cells and several rows of underlying storage cells where they abutted intercellular spaces. This latter ingrowth development was apparent on both cotyledon surfaces, but extended into more of the underlying cell layers on the abaxial surface at the funicular end of the cotyledon. In in vivo grown cotyledons, such ingrowth development is restricted to the subepidermal cells of the abaxial surface. Ingrowth morphology was commensurate with that of transfer cells of in vivo grown cotyledons. In contrast to the observed induction on a medium containing glucose and fructose, cotyledons cultured with sucrose as the sole sugar source exhibited no ingrowth deposition or small wall ingrowths in some abaxial epidermal cells. While the potential sugar signalling mechanism is unknown, this culture system offers an exciting opportunity to explore the molecular biology of transfer cell development.

Sunflowers are known to respond to Fe deficiency (-Fe) with a typical root tip swelling and the formation of root hairs and transfer cells in the rhizodermis. The possible regulation of this process was examined by a comparative study of root morphology and cytology of intact seedlings (Helianthus annuus L. cv. Giganteus) under -Fe and hormonal treatment in nutrient solution. Longitudinal sections of -Fe roots showed root tip swelling is due to cessation of cell elongation and isodiarnetric volume increase of the cortical cells. Enhanced cell division in the pericycle leads to the formation of lateral root primordia in the swollen zone. Xylem vessel differentiation is markedly accelerated and accompanied by early differentiation of the casparian band in the endodermis. Exogenous application of IAA (10−8-10−7 M) via the nutrient solution to Fe sufficient plants causes symptoms which closely mimick the characteristics of Fe deficiency including root hair development. Moreover, rhizodermal cells produce peripheral protuberances reminiscent of -Fe transfer cells. Ethylene-releasing ethephon (10−4M) also causes subapical swelling and root hair formation. However, wall protuberance development is less pronounced. ABA (10−5 M) leads to similar root thickening and root hair formation but without any comparable transfer cell differentiation. From the striking similarities between -Fe and IAA treatment it is concluded that this hormone (possibly in cooperation with ethylene) is involved in the Fe stress response of sunflower roots. The importance of a continuous polar IAA transport for this process is discussed.

Transfer cells in the nucellar projection of wheat grains at 25 ±3 days after anthesis have been examined using light and electron microscopy. Within the nucellar tissue, a sequential increase in non-polarized wall ingrowth differentiation and cytoplasmic density was evident. Cells located near the pigment strand were the least differentiated. The degree of differentiation increased progressively in cells further removed from the pigment strand and the cells bordering the endosperm cavity had degenerated. Four stages of transfer cell development were identified at the light microscope level. Wall ingrowth differentiation followed a sequence from a papillate form through increased branching (antler-shaped ingrowths) which ultimately anastomosed to form a complex labyrinth. The final stage of wall ingrowth differentiation was compression which resulted in massive ingrowths. In parallel with wall ingrowth deposition cytoplasmic density increased. During wall deposition, paramural and multivesicular bodies were prominent and were in close association with the wall ingrowths. The degeneration phase involved infilling of cytoplasmic islets within the wall ingrowths. This was accompanied by complete loss of the protoplast. The significance of this transfer cell development for sucrose efflux to the endosperm cavity was assessed by computing potential sucrose fluxes across the plasma membrane surface areas of the nucellar projection cells. Transfer cell development amplified the total plasma membrane surface area by 22 fold. The potential sucrose flux, when compared with maximal rates of facilitated membrane transport of sugars, indicated spare capacity for sucrose efflux to the endosperm cavity. Indeed, when the total flux was partitioned between the nucellar projection cells at the three stages of transfer cell development, the fully differentiated stage III cells located proximally to the endosperm cavity alone exhibited spare transport capacity. Stage II cells could accommodate the total rate of sucrose transfer, but stage I cells could not. It is concluded that the nucellar projection tissue of wheat provides a unique opportunity to study transfer cell development and the functional role of these cells in supporting sucrose transport.

A survey is presented of the architecture of secondary wall ingrowths in transfer cells from various taxa based on scanning electron microscopy. Wall ingrowths are a distinguishing feature of transfer cells and serve to amplify the plasma membrane surface area available for solute transport. Morphologically, two categories of ingrowths are recognized: reticulate and flange. Reticulate-type wall ingrowths are characterized by the deposition of small papillae that emerge from the underlying wall at discrete but apparently random loci, then branch and interconnect to form a complex labyrinth of variable morphology. In comparison, flange-type ingrowths are deposited as curvilinear ribs of wall material that remain in contact with the underlying wall along their length and become variously elaborate in different transfer cell types. This paper discusses the morphology of different types of wall ingrowths in relation to existing models for deposition of other secondary cell walls.

Crystalline bodies, observed within the intercellular spaces of the gametophyte-sporophyte junction of the hornwortPhaeoceros laevis, were composed of alternating light and dark bands about 6 nm in width. They stained positively with all protein stains employed but not with periodic acid-Schiffs reagent suggesting a protein composition. The crystals were degraded during development indicating possible utilization of the protein components. The occurrence of intercellular protein crystals is unusual and further work is necessary to determine the exact nature of these crystals and their function, if any.

The epithelial cells of the septal nectaries ofGasteria andAloe have a “second wall layer” consisting of numerous irregular wall protuberances along the outer wall. They, thus, are typical transfer cells. The wall protuberances develop a short time before the nectar secretion begins. At this time and during the secretory phase the amount of microtubules is at a minimum. After the secretion phase the protuberances disappear gradually while often callose is formed between them. Simultaneously a “third wall layer” is deposited which covers the remnants of the “second layer”. During the first developmental stages the cells contain amyloplasts with changing amounts of starch. In the redifferentiation phase they become chloroplasts with a crystalline prolamellar body and small stacks of thylakoids.

The structure of the phloem was studied in stem and leaf ofArtemisia afra Jacq., with particular attention being given to the sieve element walls. Both primary and secondary sieve elements of stem and midvein have nacreous walls, which persist in mature cells. Histochemical tests indicated that the sieve element wall layers contained some pectin. Sieve element wall layers lack lignin. Sieve elements of the minor veins (secondary and tertiary veins) lack nacreous thickening, although their walls may be relatively thick. These walls and those of contiguous transfer cells are rich in pectic substances. Transfer cell wall ingrowths are more highly developed in tertiary than in secondary veins.

The morphology of wall ingrowths in xylem and phloem transfer cells inHelianthemum is different. It is possible to use nematode infection to induce the formation of giant cells which abut both xylem and phloem elements to test whether ingrowth morphology is controlled by the solutes presumed to be transported across the plasmalemma of the cells. This experiment has been done and it is found that although wall ingrowths develop against both xylem and phloem, the giant cells exhibit only the ingrowth structure characteristic of xylem transfer cells.

Observations at ultrastructural level have been added to earlier descriptions of the vascular tissue inParthenium argentatum (guayule). Large transfer cells with pronounced cell wall projections were found in the phloem of the minor veins in the leaves. These cells have every appearance of being modified for the purpose of collecting photosynthates. In the large veins of the leaf the elements of the phloem, engaged in bulk transport of solutes, showed reduced transfer cell characteristics. Within these larger veins of the leaf nacreous thickenings of the sieve tubes was very common. Similar thickenings were found in the petiole and the stem and these persisted to maturity. Histochemical tests indicated a high level of pectic substances in these walls. In the stem, particularly in regions where rubber deposition was significant, the companion cells in the phloem had elaborate cell wall ingrowths. The adjacent parenchyma cells were adequately provided with pits. Occasionally transfer cells were found in the xylem.

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.

The ontogeny of transfer cells associated with tracheary elements in the rhizome of the rosette plant,Hieracium floribundum is described. The events preceding the initiation of wall ingrowths in parenchymatous cells associated with tracheary elements determining them as transfer cells vary with the type of tracheary element and the position within the rhizome. Parenchyma cells adjacent to protoxylem and metaxylem elements close to the rhizome apex usually divide before wall ingrowth formation whereas this tendency is decreased in the basipetal direction in the rhizome. When parenchyma cells do not divide before wall ingrowth formation, the wall ingrowths are poorly developed and the cells remain vacuolated. If cell division does occur, the daughter cell next to the tracheary element always becomes a transfer cell and the other develops as a xylem parenchyma cell. Within a vascular bundle there is a gradient in intensity of wall ingrowth formation, with the transfer cells associated with protoxylem elements having the most elaborate wall ingrowths, and the transfer cells associated with the late metaxylem elements having few or no wall ingrowths. Parenchyma cells associated with secondary xylem elements lack wall ingrowths. At the end of primary growth and the establishment of secondary growth within a bundle, successive cytological changes occur in the xylem transfer cells, leading ultimately to the loss of wall ingrowths, the thickening of cell walls and the complete vacuolation of the once-richly cytoplasmic cells. Although the thickened walls of other primary xylem parenchyma cells become lignified at this time, the transfer cell walls remain unlignified until the rhizome is very old.

The ultrastructure of quadrifid and bifid hairs which line the inside of the trap in the bladderwortUtricularia monanthos are described. Both types of hair consist of a pedestal cell resting on a special epidermal cell which bears two and four terminal cells in bifids and quadrifids, respectively. The pedestal cell is a transfer cell with an extensive labyrinth of wall ingrowths and cytoplasm containing numerous well differentiated mitochondria. The wall ingrowths contain little structural material and appear to undergo changes in width depending on the activity of the trap. The proximal region of the lateral wall is devoid of ingrowths and is completely impregnated with opaque material slightly different in appearance from that of the adjoining impregnated walls. Numerous compound plasmodesmata connect the protoplast of the pedestal cell to that of the basal epidermal cell. The terminal cells of quadrifids and bifids show a striking differentiation of protoplast and wall associated with their specialization into stalk and arms. The protoplast of the stalk is narrow and contains mostly tubular ER. Abundant simple plasmodesmata traverse the wall between the foot of the stalk and the pedestal cell. The protoplast of the arm displays a large central vacuole, with the nucleus and numerous mitochondria concentrated towards the base of the arm. The walls of the stalk are thick and heavily impregnated with cuticular material in their outer regions, but in the arms the cuticle is very thin and consists of small separate cutin cystoliths. The walls of the arms exhibit short unbranched ingrowths. The structure and function of the hairs is discussed. Materials are probably absorbed by the arms and transported to the pedestal cell. It is suggested that once substances have passed through the cuticle of the arm they are transported by the terminal cells, either symplasticallyvia the protoplast or apoplastically through the walls into the labyrinth of the pedestal cell. Because of the continuous impregnation in the proximal half of the lateral wall of the pedestal cell, all substances must pass through the protoplast of the pedestal cell if they are to be transported into the walls of the trap.

Anabaena azollae is associated with two types of multicellular epidermal trichomes inAzolla leaf cavities, the simple and branched hairs. The observation of transfer cell ultrastructure in some hair cells led to speculation that the cavity hairs might participate in metabolite exchange between the symbionts. The developmental ontogeny of cavity trichomes is described here, using transmission electron microscopy, with a goal of improving our understanding of possible functions of these structures in the symbiosis. The observations have established that all cells of simple and branched hairs develop the structural characteristics of transfer cells, but not simultaneously. Rather, there is an acropetal succession of transfer cell ultrastructure beginning in terminal cells, moving to body cells where present, and ending in stalk cells. The transfer cell stage is followed immediately by senescence in all hair cells. The timing of transfer cell differentiation, considered together with information from other studies, suggests that branched hairs may be involved in exchange of fixed nitrogen between the symbionts, while simple hairs may participate in exchange of fixed carbon fromAzolla toAnabaena.

The development of the suspensor (consisting of a basal cell and a few chalazal cells) inAlisma plantagoaquatica andA. lanceolatum was investigated using cytochemical methods, light and electron microscopy. The basal cell becomes differentiated during the first three days of embryo development. As a result of endopolyploidization the volume of the nucleus rapidly increases, as does the quantity of chromatin it contains and the size of the nucleolus. As basal cell grows, its cytoplasm increases in volume and the number of organelles increase, and wall ingrowths begin to form on the walls at the micropylar pole of the cell. The full development and functioning of the suspensor occurs during the next three days. The enormous basal cell then attains its maximum degree of differentiation: its nucleus reaches a ploidy of 256n or 512n, the micropylar transfer wall is fully developed, as is the cytoplasm, rich in proteins, ribonucleic acids (RNA) and organelles, particularly dictyosomes and long cisternae of the rough endoplasmic reticulum. The chalazal suspensor cells joining the embryo proper to the basal cell also become differentiated. In the seven-day embryo the suspensor begins to degenerate which coincides with the cellularization of the endosperm at the micropylar pole of the embryo sac. The senescence of the suspensor involves the degradation of the nucleus, increasing cytoplasmic vacuolization, and a distinct decrease in protein and RNA content, first in the basal cell, then in the chalazal suspensor cells. Analysis of the development and ultrastructure of the basal suspensor cell suggests that it plays the role of an active metabolic transfer cell, translocating nutrients from the maternal tissues via the chalazal suspensor cells to the growing embryo proper.

The mature dome-shaped glands which cover the outer surfaces of the trap, leaves, anchor and runner stolons inU. monanthos are described using conventional and some high voltage transmission electron microscopy. The glands occur as scattered ordinary external glands and as a compact clump of vestibule glands at the entrance to the doorway. Each gland rests on a basal epidermal cell and consists of a single pedestal and terminal cell. Vestibule and leaf glands differ slightly from the other glands mainly in the structure of the outer wall of the terminal cell. Nuclear crystals are prominent and the cytoplasm of the pedestal and terminal cells contains tubular structures usually aggregated near the nucleus. The pedestal cell is a transfer cell with short wall protuberances on the outer wall, conspicuous mitochondria and a heavily impregnated lateral wall. The terminal cell often has an outer wall that is greatly thickened and a protoplast that may degenerate early. In the most developed cells the protoplast remains active for a long period and the outer wall is differentiated into several layers. The outermost layer is cuticularized consisting of an open meshwork of deposits. In leaf glands a local polysaccharide mass is usually developed within the cuticularized region. The inner non-impregnated region of the outer wall may show four layers. In vestibule glands fewer layers are present and the wall shows prominent lamellations. Some ordinary external glands differentiate a sponge-like substructure within the inner wall. The ultrastructure and function of the glands are discussed. We support the concept that mature external glands are responsible for secreting water, with those on traps being particularly active during the resetting of the organ. Our work provides a structural basis for recent suggestions by other workers that the mechanism of secretion probably involves establishing a standing osmotic gradient within the gland.

Cells of the root epidermis ofPisonia grandis R. Br. at the interface with the mycorrhizal fungus are modified as transfer cells. The length of wall profile in transverse section is increased 1.7-fold by the wall ingrowths, on average, over the outer tangential wall and the outer third of the radial walls; this corresponds to a 1.3—fold increase in wall profile length over the whole cell. These increases in length of wall profile approximate—slightly underestimating-the amplification of surface area of the epidermal cells that results from the ingrowths. The surface area between the symbionts in thePisonia mycorrhiza is less amplified than in classical ectomycorrhizas with a Hartig net: this may be functionally adequate because of the extremely high nutrient status of theP. grandis habitat.

The development of external glands on traps and stolons ofU. monanthos has been studied using transmission electron microscopy. During early differentiation of the epidermis some cells remain narrow and develop a protuberance which subsequently divides into a terminal and a pedestal cell, with the remainder of the original cell forming the basal epidermal cell of the gland. The lateral wall of the pedestal cell soon becomes densely impregnated throughout its thickness, and this is followed by the formation of discontinuous cuticular deposits within the primary wall of the terminal cell. The outer wall of the terminal cell then usually undergoes extensive secondary wall thickening beginning with the formation of ingrowths which for a period characterize the cell as a transfer cell. Later, at the stage when traps begin capturing prey, these ingrowths are overlain by further layers of secondary wall material. Concomitantly, in the pedestal cell, wall ingrowths become fully differentiated on the outer transverse wall and persist throughout the remaining life of the gland. The function of external glands during early ontogeny is discussed. At the stage when the terminal cell is differentiated as a transfer cell it is suggested that the gland is mainly responsible for absorbing solutes from the external medium. Once traps commence capturing prey the gland may become modified for a role in water secretion, facilitated by the differentiation of the pedestal cell as a transfer cell, and by the formation of a thick outer wall in the terminal cell.

A study of the fine structure of minor veins of mature leaves of 975 species and 242 families of Angiosperms shows that transfer cells are widespread amongst herbaceous Dicotyledons, are much rarer in woody Dicotyledons, and are virtually absent from the Monocotyledons. The evolutionary significance of the distribution of the cells amongst and within orders, families and minor groupings is discussed. Four types of transfer cell are recognized in minor veins, all possessing irregular ingrowths of wall material protruding into their protoplasts, and all being regarded as modified parenchyma of the minor vein. Two types occur in phloem. One (the A-cell), with ingrowths distributed right round its periphery, is associated specifically with the sieve elements. The other (the B-cell) occurs more generally throughout the phloem and has zones of wall ingrowths oriented towards sieve elements and their associated companion cells or A-cells. Two other types (C- and D-cells) occur in xylem parenchyma and bundle sheath respectively, and have ingrowths only on walls in contact with or in close proximity to vessels or tracheids. Each species has a characteristic combination of types of transfer cell. The variations encountered in the survey are classified. Consistent differences in the frequency and form of ingrowths are to be found between the different types of transfer cell of a single species, and between different species in respect to a particular type of transfer cell. The functional significance of transfer cells in minor veins is discussed in relation to the loading and unloading of the conducting elements and to the retrieval of extra-cytoplasmic solutes from the mesophyll and the transpiration stream.

The distribution and time course of development of transfer cells in the hypocotyl region of lettuce (Lactuca sativa L.) and groundsel (Senecio vulgaris L.) are examined by light microscopy of serial sections through a sequence of ages of hypocotyls. Investments of xylem transfer cells occur in departing traces to the cotyledons and, later, in the traces to foliage leaves; phloem transfer cells are widely distributed but particularly prominent in those bands of protophloem in the plumule vasculature which lie alongside xylem of the cotyledonary traces. Both classes of transfer cell are well endowed with wall ingrowths before differentiation of xylem and perforation of stomata occurs in the plumule. Autoradiographic evidence is obtained of a transport pathway from cotyledonary trace xylem elements to xylem transfer cell to plumule, and analyses of xylem sap collected from above or below the zones of transfer cells in the hypocotyl show that certain materials can be removed from the xylem sap by transfer cells as it moves towards the cotyledons. From these findings it is concluded that the seedling transfer cells play an important role in nutrition of the young plumule, particularly before the latter has become adequately connected with the vascular systems of cotyledons and root. Experiments on the experimental modification of transfer cell development in the hypocotyl suggest that both photosynthetic fixation of carbon dioxide and a transpirational loss of water by a cotyledon must take place before the presumptive xylem transfer cells in its traces can develop normal sets of wall ingrowths. Discussion is extended to the general role of transfer cells in the nodal regions of stems. Possible functions envisaged are, the general nutrition of young tissues of the apical region, the abstraction of assimilates for local storage, the transfer of assimilates to axillary buds released from apical dominance, and the interchange of assimilates between adjacent vascular traces running through the node.

Plant cells possessing ingrowths of wall material, and hence having protoplasts with unusually high surface-to-volume ratios, may be found in a wide variety of anatomical situations and in most of the major taxa of multicellular plants. They are termed here “transfer cells”. Their function relates to any of four categories of trans membrane flux: 1. Absorption of solutes from the external environment (e.g. epidermis of submerged leaves), 2. Secretion of solutes to the external environment (e.g. nectaries and other glands), 3. Absorption of solutes from an internal, extracytoplasmic compartment (e.g. vascular parenchyma, haustorial-type connections, embryo sacs and embryos), 4. Secretion of solutes into an internal extracytoplasmic compartment (e.g. tapetum of anther, pericycle of root nodule). An overall assessment of their occurrence, structure, development and role in the plant is presented taking account of published information and new observations.