Transfer cell wall architecture: a contribution towards understanding localized wall deposition

ZmDRR206 Regulates Nutrient Accumulation in Endosperm through Its Role in Cell Wall Biogenesis during Maize Kernel Development

Dirigent proteins (DIRs) contribute to plant fitness by dynamically reorganizing the cell wall and/or by generating defense compounds during plant growth, development, and interactions with environmental stresses. ZmDRR206 is a maize DIR, it plays a role in maintaining cell wall integrity during seedling growth and defense response in maize, but its role in regulating maize kernel development is unclear. Association analysis of candidate genes indicated that the natural variations of ZmDRR206 were significantly associated with maize hundred-kernel weight (HKW). ZmDRR206 plays a dominant role in storage nutrient accumulation in endosperm during maize kernel development, ZmDRR206 overexpression resulted in small and shrunken maize kernel with significantly reduced starch content and significantly decreased HKW. Cytological characterization of the developing maize kernels revealed that ZmDRR206 overexpression induced dysfunctional basal endosperm transfer layer (BETL) cells, which were shorter with less wall ingrowth, and defense response was constitutively activated in developing maize kernel at 15 and 18 DAP by ZmDRR206 overexpression. The BETL-development-related genes and auxin signal-related genes were down-regulated, while cell wall biogenesis-related genes were up-regulated in developing BETL of the ZmDRR206-overexpressing kernel. Moreover, the developing ZmDRR206-overexpressing kernel had significantly reduced contents of the cell wall components such as cellulose and acid soluble lignin. These results suggest that ZmDRR206 may play a regulatory role in coordinating cell development, storage nutrient metabolism, and stress responses during maize kernel development through its role in cell wall biogenesis and defense response, and provides new insights into understanding the mechanisms of kernel development in maize.

Immunocytochemical Analysis of the Wall Ingrowths in the Digestive Gland Transfer Cells in Aldrovanda vesiculosa L. (Droseraceae)

Carnivorous plants are unique due to their ability to attract small animals or protozoa, retain them in specialized traps, digest them, and absorb nutrients from the dissolved prey material; however, to this end, these plants need a special secretion-digestive system (glands). A common trait of the digestive glands of carnivorous plants is the presence of transfer cells. Using the aquatic carnivorous species Aldrovanda vesiculosa, we showed carnivorous plants as a model for studies of wall ingrowths/transfer cells. We addressed the following questions: Is the cell wall ingrowth composition the same between carnivorous plant glands and other plant system models? Is there a difference in the cell wall ingrowth composition between various types of gland cells (glandular versus endodermoid cells)? Fluorescence microscopy and immunogold electron microscopy were employed to localize carbohydrate epitopes associated with major cell wall polysaccharides and glycoproteins. The cell wall ingrowths were enriched with arabinogalactan proteins (AGPs) localized with the JIM8, JIM13, and JIM14 epitopes. Both methylesterified and de-esterified homogalacturonans (HGs) were absent or weakly present in the wall ingrowths in transfer cells (stalk cells and head cells of the gland). Both the cell walls and the cell wall ingrowths in the transfer cells were rich in hemicelluloses: xyloglucan (LM15) and galactoxyloglucan (LM25). There were differences in the composition between the cell wall ingrowths and the primary cell walls in A. vesiculosa secretory gland cells in the case of the absence or inaccessibility of pectins (JIM5, LM19, JIM7, LM5, LM6 epitopes); thus, the wall ingrowths are specific cell wall microdomains. Even in the same organ (gland), transfer cells may differ in the composition of the cell wall ingrowths (glandular versus endodermoid cells). We found both similarities and differences in the composition of the cell wall ingrowths between the A. vesiculosa transfer cells and transfer cells of other plant species.

Molecular mechanisms of maize endosperm transfer cell development

Endosperm transfer cells function as the nutrient transporter, antimicrobic barrier, and signal mediator between filial and maternal tissues. Sugar supply of maternal tissues, sugar demand of filial tissues, and requirement for defence against pathogens are three elemental factors inducing differentiation of endosperm transfer cells. Epigenetic factors, especially MEG1, moderate the key genetic factor ZmMRP-1 to activate endosperm transfer cell-specific genes that control the flange wall ingrowth formation and defensin-like protein secretion in maize. Auxin and cytokinin are primary hormones involved in development of maize endosperm transfer cells. Crosstalk between glucose and hormone signaling regulates endosperm transfer cell development via modifying ZmMRP-1 expression. This review summarizes the current knowledge on maize endosperm transfer cell development, and discusses its potential molecular mechanisms. It is expected to strengthen the theoretical basis for structural and functional optimization of endosperm transfer cells, and yield improvement of kernels in maize.

ENB1 encodes a cellulose synthase 5 that directs synthesis of cell wall ingrowths in maize basal endosperm transfer cells

Development of the endosperm is strikingly different in monocots and dicots: it often manifests as a persistent tissue in the former and transient tissue in the latter. Little is known about the controlling mechanisms responsible for these different outcomes. Here we characterized a maize (Zea mays) mutant, endosperm breakdown1 (enb1), in which the typically persistent endosperm (PE) was drastically degraded during kernel development. ENB1 encodes a cellulose synthase 5 that is predominantly expressed in the basal endosperm transfer layer (BETL) of endosperm cells. Loss of ENB1 function caused a drastic reduction in formation of flange cell wall ingrowths (ingrowths) in BETL cells. Defective ingrowths impair nutrient uptake, leading to premature utilization of endosperm starch to nourish the embryo. Similarly, developing wild-type kernels cultured in vitro with a low level of sucrose manifested early endosperm breakdown. ENB1 expression is induced by sucrose via the BETL-specific Myb-Related Protein1 transcription factor. Overexpression of ENB1 enhanced development of flange ingrowths, facilitating sucrose transport into BETL cells and increasing kernel weight. The results demonstrated that ENB1 enhances sucrose supply to the endosperm and contributes to a PE in the kernel.

Earliest record of transfer cells in Lower Devonian plants

Key sources of information on the nature of early terrestrial ecosystems are the fossilized remains of plants and associated organic encrustations, which are interpreted as either biofilms, biological soil crusts or lichens. The hypothesis that some of these encrustations might be the remains of the thalloid gametophytes of embryophytes provided the stimulus for this investigation. Fossils preserved in charcoal were extracted from Devonian Period (Lochkovian Stage, c. 410-419 Myr old) sediments at a geological site in Shropshire (UK). Scanning electron micrographs (SEMs) of the fossils were compared with new and published SEMs of extant bryophytes and tracheophytes, respectively. One specimen was further prepared and imaged by transmission electron microscopy. Fossils of thalloid morphology were composed almost entirely of cells with labyrinthine ingrowths; these also were present in fossils of axial morphology where they were associated with putative food-conducting cells. Comparison with modern embryophytes demonstrates that these distinctive cells are transfer cells (TCs). Our fossils provide by far the earliest geological evidence of TCs. They also show that some organic encrustations are the remains of thalloid land plants and that these are possibly part of the life cycle of a newly recognized group of plants called the eophytes.

Maize Endosperm Development: Tissues, Cells, Molecular Regulation and Grain Quality Improvement

Maize endosperm plays important roles in human diet, animal feed and industrial applications. Knowing the mechanisms that regulate maize endosperm development could facilitate the improvement of grain quality. This review provides a detailed account of maize endosperm development at the cellular and histological levels. It features the stages of early development as well as developmental patterns of the various individual tissues and cell types. It then covers molecular genetics, gene expression networks, and current understanding of key regulators as they affect the development of each tissue. The article then briefly considers key changes that have occurred in endosperm development during maize domestication. Finally, it considers prospects for how knowledge of the regulation of endosperm development could be utilized to enhance maize grain quality to improve agronomic performance, nutrition and economic value.

What makes a molecule a pre- or a post-herbicide - how valuable are physicochemical parameters for their design?

Pre-emergence herbicides are taken up by seeds before germination and by roots, hypocotyls, cotyledons, coleoptiles or leaves before emergence, whereas post-emergence herbicides are taken up primarily by foliage and stems. Most modern pre-emergence herbicides are lipophilic, but post-emergence herbicides may be lipophilic or hydrophilic. The metabolic conversion of herbicides to inactive or active metabolites after plant uptake is of major importance for some compound classes. Several herbicides are proherbicides as for example some acetyl-coenzyme A carboxylase (ACCase)-inhibitors. The physicochemical characteristics of proherbicides and herbicides are usually unrelated. A major role can be attributed to the site of action at a cellular level. A great number of herbicides such as photosystem II (PS II)-inhibitors, protoporphyrinogen oxidase (PPO)-inhibitors or carotenoid biosynthesis inhibitors require light for activity. Others, such as cellulose-biosynthesis and mitotic inhibitors seem to be primarily active in belowground organs. Several lipophilic barriers against the uptake of xenobiotics exist in aboveground and belowground plant parts. The relevance of these barriers needs, however, further clarification. Uptake and translocation models are valuable tools for the explanation of the potential movement of compounds. Many factors other than uptake and translocation have, however, to be considered for the design of herbicides. For post-emergence herbicides, ultraviolet (UV) light stability, stability in formulations, and mixability with other agrochemicals have to be kept in mind while, in addition to the aforementioned factors soil interaction plays a major role for pre-emergence herbicides. In our opinion, general physicochemical characteristics of pre- or post-emergence herbicides do, unfortunately not exist yet. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Transfer cells: what regulates the development of their intricate wall labyrinths?

Transfer cells (TCs) support high nutrient rates into, or at symplasmic discontinuities within, the plant body. Their transport capacity is conferred by an amplified plasma membrane surface area, enriched in nutrient transporters, supported on an intricately invaginated wall labyrinth (WL). Thus, development of the WL is at the heart of TC function. Enquiry has shifted from describing WL architecture and formation to discovering mechanisms regulating WL assembly. Experimental systems used to examine these phenomena are critiqued. Considerable progress has been made in identifying master regulators that commit stem cells to a TC fate (e.g. the maize Myeloblastosis (MYB)-related R1-type transcription factor) and signals that induce differentiated cells to undergo trans-differentiation to a TC phenotype (e.g. sugar, auxin and ethylene). In addition, signals that provide positional information for assembly of the WL include apoplasmic hydrogen peroxide and cytosolic Ca²⁺ plumes. The former switches on, and specifies the intracellular site for WL construction, while the latter creates subdomains to direct assembly of WL invaginations. Less is known about macromolecule species and their spatial organization essential for WL assembly. Emerging evidence points to a dependency on methyl-esterified homogalacturonan accumulation, unique patterns of cellulose and callose deposition and spatial positioning of arabinogalactan proteins.

A new insight on structural and some functional aspects of peri-endodermal thickenings, a specific layer in Noccaea caerulescens roots

BACKGROUND AND AIMS

Cell walls of the peri-endodermis, a layer adjacent to the endodermis in alpine pennycress (Noccaea caerulescens) roots, form C-shaped peri-endodermal thickenings (PETs). Despite its specific position close to the endodermis, the assumed similarity of PETs to phi thickenings in many other species, and the fact that N. caerulescens is a well-studied heavy-metal-hyperaccumulating plant, the PET as a root trait is still not understood.

METHODS

Here, we characterized PET cell walls by histochemical techniques, Raman spectroscopy, immunolabelling and electron microscopy. Moreover, a role of PETs in solute transport was tested and compared with Arabidopsis thaliana plants, which do not form PETs in roots.

KEY RESULTS

Cell walls with PETs have a structured relief mainly composed of cellulose and lignin. Suberin, typical of endodermal cells, is missing but pectins are present on the inner surface of the PET. Penetrating dyes are not able to cross PETs either by the apoplasmic or the symplasmic pathway, and a significantly higher content of metals is found in root tissues outside of PETs than in innermost tissues.

CONCLUSIONS

Based on their development and chemical composition, PETs are different from the endodermis and closely resemble phi thickenings. Contrarily, the different structure and dye impermeability of PETs, not known in the case of phi thickenings, point to an additional barrier function which makes the peri-endodermis with PETs a unique and rare layer.

Barley HISTIDINE KINASE 1 (HvHK1) coordinates transfer cell specification in the young endosperm

Cereal endosperm represents the most important source of the world's food; nevertheless, the molecular mechanisms underlying cell and tissue differentiation in cereal grains remain poorly understood. Endosperm cellularization commences at the maternal-filial intersection of grains and generates endosperm transfer cells (ETCs), a cell type with a prominent anatomy optimized for efficient nutrient transport. Barley HISTIDINE KINASE1 (HvHK1) was identified as a receptor component with spatially restricted expression in the syncytial endosperm where ETCs emerge. Here, we demonstrate its function in ETC fate acquisition using RNA interference-mediated downregulation of HvHK1. Repression of HvHK1 impairs cell specification in the central ETC region and the development of transfer cell morphology, and consecutively defects differentiation of adjacent endosperm tissues. Coinciding with reduced expression of HvHK1, disturbed cell plate formation and fusion were observed at the initiation of endosperm cellularization, revealing that HvHK1 triggers initial cytokinesis of ETCs. Cell-type-specific RNA sequencing confirmed loss of transfer cell identity, compromised cell wall biogenesis and reduced transport capacities in aberrant cells and elucidated two-component signaling and hormone pathways that are mediated by HvHK1. Gene regulatory network modeling was used to specify the direct targets of HvHK1; this predicted non-canonical auxin signaling elements as the main regulatory links governing cellularization of ETCs, potentially through interaction with type-B response regulators. This work provides clues to previously unknown molecular mechanisms directing ETC specification, a process with fundamental impact on grain yield in cereals.

Development of Embryo Suspensors for Five Genera of Crassulaceae with Special Emphasis on Plasmodesmata Distribution and Ultrastructure

The suspensor in the majority of angiosperms is an evolutionally conserved embryonic structure functioning as a conduit that connects ovule tissues with the embryo proper for nutrients and growth factors flux. This is the first study serving the purpose of investigating the correlation between suspensor types and plasmodesmata (PD), by the ultrastructure of this organ in respect of its full development. The special attention is paid to PD in representatives of Crassulaceae genera: Sedum, Aeonium, Monanthes, Aichryson and Echeveria. The contribution of the suspensor in transporting nutrients to the embryo was confirmed by the basal cell structure of the suspensor which produced, on the micropylar side of all genera investigated, a branched haustorium protruding into the surrounding ovular tissue and with wall ingrowths typically associated with cell transfer. The cytoplasm of the basal cell was rich in endoplasmic reticulum, mitochondria, dictyosomes, specialized plastids, microtubules, microbodies and lipid droplets. The basal cell sustained a symplasmic connection with endosperm and neighboring suspensor cells. Our results indicated the dependence of PD ultrastructure on the type of suspensor development: (i) simple PD are assigned to an uniseriate filamentous suspensor and (ii) PD with an electron-dense material are formed in a multiseriate suspensor. The occurrence of only one or both types of PD seems to be specific for the species but not for the genus. Indeed, in the two tested species of Sedum (with the distinct uniseriate/multiseriate suspensors), a diversity in the structure of PD depends on the developmental pattern of the suspensor. In all other genera (with the multiseriate type of development of the suspensor), the one type of electron-dense PD was observed.

Enzymes contributing to the hydrogen peroxide signal dynamics that regulate wall labyrinth formation in transfer cells

Transfer cells are characterized by an amplified plasma membrane area supported on a wall labyrinth composed of a uniform wall layer (UWL) from which wall ingrowth (WI) papillae arise. Adaxial epidermal cells of developing Vicia faba cotyledons, when placed in culture, undergo a rapid (hours) trans-differentiation to a functional epidermal transfer cell (ETC) phenotype. The trans-differentiation event is controlled by a signalling cascade comprising auxin, ethylene, apoplasmic reactive oxygen species (apoROS), and cytosolic Ca2+. Apoplasmic hydrogen peroxide (apoH2O2) was confirmed as the apoROS regulating UWL and WI papillae formation. Informed by an ETC-specific transcriptome, a pharmacological approach identified a temporally changing cohort of H2O2 biosynthetic enzymes. The cohort contained a respiratory burst oxidase homologue, polyamine oxidase, copper amine oxidase, and a suite of class III peroxidases. Collectively these generated two consecutive bursts in apoH2O2 production. Spatial organization of biosynthetic/catabolic enzymes was deduced from responses to pharmacologically blocking their activities on the cellular and subcellular distribution of apoH2O2. The findings were consistent with catalase activity constraining the apoH2O2 signal to the outer periclinal wall of the ETCs. Strategic positioning of class III peroxidases in this outer domain shaped subcellular apoH2O2 signatures that differed during assembly of the UWL and WI papillae.

Node-controlled allocation of mineral elements in Poaceae

Mineral elements taken up by the roots will be delivered to different organs and tissues depending on their requirements. In Poaceae, this selective distribution is mainly mediated in the nodes, which have highly developed and fully organized vascular systems. Inter-vascular transfer of mineral elements from enlarged vascular bundles to diffuse vascular bundles is required for their preferential distribution to developing tissues and reproductive organs. A number of transporters involved in this inter-vascular transfer processes have been identified mainly in rice. They are localized at the different cell layers and form an efficient machinery within the node. Furthermore, some these transporters show rapid response to the environmental changes of mineral elements at the protein level. In addition to the node-based transporters, distinct nodal structures including enlarged xylem area, folded plasma membrane of xylem transfer cells and presence of an apoplastic barrier are also required for the efficient inter-vascular transfer. Manipulation of node-based transporters will provide a novel breeding target to improve nutrient use efficiency, productivity, nutritional value and safety in cereal crops.

Transfer cells in trichomatous nectary in Adenocalymma magnificum (Bignoniaceae)

ABSTRACT Transfer cells are specialized cells that play an important role where there are high energy costs due to facilitation of transmembrane flow of solutes. This study aimed to investigate the ontogenesis, histochemistry and ultrastructure of glandular trichomes focusing on stalk cells and their possible transfer function. Samples of prophylls of axillary buds, calyces of flowers in anthesis, and flower buds in different stages of development were collected, fixed and processed according to the common methods of microscopy. The glandular trichomes are composed of a secretory head with its cells in columnar format. The stalk is formed by two layers of cells, with the upper layer composed of cuboidal cells where the wall starts to thicken at the beginning of the pre-secretory phase. The secretion is heterogeneous, releasing glucose, other carbohydrates, lipids and phenolic compounds, with two types of release - eccrine and granulocrine. These trichomes are functionally termed as nectaries. The stalk cells appear as transfer cells since they have a thicker anticlinal wall with irregular ingrowths. The presence of transfer cells in the nectaries suggests a high specialization because it improves transport capacity of nectar and compensation in the high energy expenditure for its production and release.

Localization of sesquiterpene lactone biosynthesis in cells of capitate glandular trichomes of Helianthus annuus (Asteraceae)

Capitate glandular trichomes (CGT) of sunflower, Helianthus annuus, synthesize bioactive sesquiterpene lactones (STLs) within a short period of only a few days during trichome development. In the current project, the subcellular localization of H. annuus germacrene A monooxygenase (HaGAO), a key enzyme of the STL biosynthesis in sunflower CGT, was investigated. A polyclonal antibody raised against this enzyme was used for immunolabelling. HaGAO was found in secretory and stalk cells of CGT. This correlated with the appearance of smooth endoplasmic reticulum in both cell types. Stalk cells and secretory cells differed in form, size and types of plastids, but both had structures necessary for secretion. No HaGAO-specific immunoreaction was found in sunflower leaf tissue outside of CGT or in developing CGT before the secretory phase had started. Our results indicated that not only secretory cells but also nearly all cells of the CGT were involved in the biosynthesis of STL and that this process was not linked to the presence or absence of a specific type of plastid.

Arabinogalactan proteins are involved in root hair development in barley

The arabinogalactan proteins (AGPs) are involved in a range of plant processes, including cell differentiation and expansion. Here, barley root hair mutants and their wild-type parent cultivars were used, as a model system, to reveal the role of AGPs in root hair development. The treatment of roots with different concentrations of βGlcY (a reagent which binds to all classes of AGPs) inhibited or totally suppressed the development of root hairs in all of the cultivars. Three groups of AGP (recognized by the monoclonal antibodies LM2, LM14, and MAC207) were diversely localized in trichoblasts and atrichoblasts of root hair-producing plants. The relevant epitopes were present in wild-type trichoblast cell walls and cytoplasm, whereas in wild-type atrichoblasts and in all epidermal cells of a root hairless mutant, they were only present in the cytoplasm. In all of cultivars the higher expression of LM2, LM14, and MAC207 was observed in trichoblasts at an early stage of development. Additionally, the LM2 epitope was detected on the surface of primordia and root hair tubes in plants able to generate root hairs. The major conclusion was that the AGPs recognized by LM2, LM14, and MAC207 are involved in the differentiation of barley root epidermal cells, thereby implying a requirement for these AGPs for root hair development in barley.

Observation and investigation of three endosperm transport tissues in sorghum caryopses

Endosperm transport tissues in sorghum caryopses include endosperm transfer cells, endosperm conducting cells, and the embryo surrounding region. To elucidate the structural changes of these tissues and their relationship with the caryopsis development, sorghum caryopses were analyzed at different days after pollination using light, fluorescence, and electron microscopy. The following results were obtained: post-phloem maternal tissues included the placentochalaza and the nucellar projection-like nucellus. Well-developed endosperm transfer cells exhibited very evident flange-type wall ingrowths. Very few wall ingrowths were present in the initially developed endosperm transfer cells when the level of sucrose from the initially developed vascular system was low. At the middle stage of caryopsis development, the level of sucrose from the well-developed vascular system was high. Endosperm transfer cells increased in both area and layer amount, and their wall ingrowths increased in both length and density. Later in caryopsis development, the level of sucrose from the degenerated vascular system was low and wall ingrowths distorted in the degenerated endosperm transfer cells. Endosperm conducting cells primarily occupied the most part of endosperm, but decreased gradually because the upper part transformed into the starchy endosperm and the lower part degenerated to give space to the embryo growth. Although the embryo surrounding region initially enveloped the small embryo, it rapidly degenerated and finally disappeared. Our data showed that (1) the caryopsis vascular system influenced the differentiation of endosperm transfer cells by controlling the sugar levels (2) and configuration of endosperm transport tissues were probably altered to favor the growth of filial tissues.

The node, a hub for mineral nutrient distribution in graminaceous plants

Mineral elements, including both essential and toxic elements, are delivered to different tissues after they are taken up from the roots, but the mechanism (or mechanisms) underlying the distribution remains poorly understood. In graminaceous plants, this distribution occurs in nodes, which have a complex, well-organized vascular system. A transfer of mineral elements between different vascular bundles is required, especially for preferential distribution to developing tissues that have low transpiration but high nutrient requirements. This intervascular transfer is mediated by various transporters localized at different cells in the node. In this opinion article, we propose four modes of distribution for different mineral elements: xylem-switch, phloem-tropic, phloem-kickback, and minimum-shift, based on specific molecular transport processes identified in the nodes mainly of rice (Oryza sativa). We also discuss the prospects for future studies on mineral nutrient distribution in the nodes.

Adaptive expansion of the maize maternally expressed gene (Meg) family involves changes in expression patterns and protein secondary structures of its members

BACKGROUND

The Maternally expressed gene (Meg) family is a locally-duplicated gene family of maize which encodes cysteine-rich proteins (CRPs). The founding member of the family, Meg1, is required for normal development of the basal endosperm transfer cell layer (BETL) and is involved in the allocation of maternal nutrients to growing seeds. Despite the important roles of Meg1 in maize seed development, the evolutionary history of the Meg cluster and the activities of the duplicate genes are not understood.

RESULTS

In maize, the Meg gene cluster resides in a 2.3 Mb-long genomic region that exhibits many features of non-centromeric heterochromatin. Using phylogenetic reconstruction and syntenic alignments, we identified the pedigree of the Meg family, in which 11 of its 13 members arose in maize after allotetraploidization ~4.8 mya. Phylogenetic and population-genetic analyses identified possible signatures suggesting recent positive selection in Meg homologs. Structural analyses of the Meg proteins indicated potentially adaptive changes in secondary structure from α-helix to β-strand during the expansion. Transcriptomic analysis of the maize endosperm indicated that 6 Meg genes are selectively activated in the BETL, and younger Meg genes are more active than older ones. In endosperms from B73 by Mo17 reciprocal crosses, most Meg genes did not display parent-specific expression patterns.

CONCLUSIONS

Recently-duplicated Meg genes have different protein secondary structures, and their expressions in the BETL dominate over those of older members. Together with the signs of positive selections in the young Meg genes, these results suggest that the expansion of the Meg family involves potentially adaptive transitions in which new members with novel functions prevailed over older members.

On the track of transfer cell formation by specialized plant-parasitic nematodes

Transfer cells are ubiquitous plant cells that play an important role in plant development as well as in responses to biotic and abiotic stresses. They are highly specialized and differentiated cells playing a central role in the acquisition, distribution and exchange of nutrients. Their unique structural traits are characterized by augmented ingrowths of invaginated secondary wall material, unsheathed by an amplified area of plasma membrane enriched in a suite of solute transporters. Similar morphological features can be perceived in vascular root feeding cells induced by sedentary plant-parasitic nematodes, such as root-knot and cyst nematodes, in a wide range of plant hosts. Despite their close phylogenetic relationship, these obligatory biotrophic plant pathogens engage different approaches when reprogramming root cells into giant cells or syncytia, respectively. Both nematode feeding-cells types will serve as the main source of nutrients until the end of the nematode life cycle. In both cases, these nematodes are able to remarkably maneuver and reprogram plant host cells. In this review we will discuss the structure, function and formation of these specialized multinucleate cells that act as nutrient transfer cells accumulating and synthesizing components needed for survival and successful offspring of plant-parasitic nematodes. Plant cells with transfer-like functions are also a renowned subject of interest involving still poorly understood molecular and cellular transport processes.

Development of endosperm transfer cells in barley

Endosperm transfer cells (ETCs) are positioned at the intersection of maternal and filial tissues in seeds of cereals and represent a bottleneck for apoplasmic transport of assimilates into the endosperm. Endosperm cellularization starts at the maternal-filial boundary and generates the highly specialized ETCs. During differentiation barley ETCs develop characteristic flange-like wall ingrowths to facilitate effective nutrient transfer. A comprehensive morphological analysis depicted distinct developmental time points in establishment of transfer cell (TC) morphology and revealed intracellular changes possibly associated with cell wall metabolism. Embedded inside the grain, ETCs are barely accessible by manual preparation. To get tissue-specific information about ETC specification and differentiation, laser microdissection (LM)-based methods were used for transcript and metabolite profiling. Transcriptome analysis of ETCs at different developmental stages by microarrays indicated activated gene expression programs related to control of cell proliferation and cell shape, cell wall and carbohydrate metabolism reflecting the morphological changes during early ETC development. Transporter genes reveal distinct expression patterns suggesting a switch from active to passive modes of nutrient uptake with the onset of grain filling. Tissue-specific RNA-seq of the differentiating ETC region from the syncytial stage until functionality in nutrient transfer identified a high number of novel transcripts putatively involved in ETC differentiation. An essential role for two-component signaling (TCS) pathways in ETC development of barley emerged from this analysis. Correlative data provide evidence for abscisic acid and ethylene influences on ETC differentiation and hint at a crosstalk between hormone signal transduction and TCS phosphorelays. Collectively, the data expose a comprehensive view on ETC development, associated pathways and identified candidate genes for ETC specification.

Role of callose synthases in transfer cell wall development in tocopherol deficient Arabidopsis mutants

Tocopherols (vitamin E) are lipid-soluble antioxidants produced by all plants and algae, and many cyanobacteria, yet their functions in these photosynthetic organisms are still not fully understood. We have previously reported that the vitamin E deficient 2 (vte2) mutant of Arabidopsis thaliana is sensitive to low temperature (LT) due to impaired transfer cell wall (TCW) development and photoassimilate export associated with massive callose deposition in transfer cells of the phloem. To further understand the roles of tocopherols in LT induced TCW development we compared the global transcript profiles of vte2 and wild-type leaves during LT treatment. Tocopherol deficiency had no significant impact on global gene expression in permissive conditions, but significantly affected expression of 77 genes after 48 h of LT treatment. In vte2 relative to wild type, genes associated with solute transport were repressed, while those involved in various pathogen responses and cell wall modifications, including two members of callose synthase gene family, GLUCAN SYNTHASE LIKE 4 (GSL4) and GSL11, were induced. However, introduction of gsl4 or gsl11 mutations individually into the vte2 background did not suppress callose deposition or the overall LT-induced phenotypes of vte2. Intriguingly, introduction of a mutation disrupting GSL5, the major GSL responsible for pathogen-induced callose deposition, into vte2 substantially reduced vascular callose deposition at LT, but again had no effect on the photoassimilate export phenotype of LT-treated vte2. These results suggest that GSL5 plays a major role in TCW callose deposition in LT-treated vte2 but that this GSL5-dependent callose deposition is not the primary cause of the impaired photoassimilate export phenotype.

Evolution and development of cell walls in cereal grains

The composition of cell walls in cereal grains and other grass species differs markedly from walls in seeds of other plants. In the maternal tissues that surround the embryo and endosperm of the grain, walls contain higher levels of cellulose and in many cases are heavily lignified. This may be contrasted with walls of the endosperm, where the amount of cellulose is relatively low, and the walls are generally not lignified. The low cellulose and lignin contents are possible because the walls of the endosperm perform no load-bearing function in the mature grain and indeed the low levels of these relatively intractable wall components are necessary because they allow rapid degradation of the walls following germination of the grain. The major non-cellulosic components of endosperm walls are usually heteroxylans and (1,3;1,4)-β-glucans, with lower levels of xyloglucans, glucomannans, and pectic polysaccharides. Pectic polysaccharides and xyloglucans are the major non-cellulosic wall constituents in most dicot species, in which (1,3;1,4)-β-glucans are usually absent and heteroxylans are found at relatively low levels. Thus, the "core" non-cellulosic wall polysaccharides in grain of the cereals and other grasses are the heteroxylans and, more specifically, arabinoxylans. The (1,3;1,4)-β-glucans appear in the endosperm of some grass species but are essentially absent from others; they may constitute from zero to more than 45% of the cell walls of the endosperm, depending on the species. It is clear that in some cases these (1,3;1,4)-β-glucans function as a major store of metabolizable glucose in the grain. Cereal grains and their constituent cell wall polysaccharides are centrally important as a source of dietary fiber in human societies and breeders have started to select for high levels of non-cellulosic wall polysaccharides in grain. To meet end-user requirements, it is important that we understand cell wall biology in the grain both during development and following germination.

Lignification of developing maize (Zea mays L.) endosperm transfer cells and starchy endosperm cells

Endosperm transfer cells in maize have extensive cell wall ingrowths that play a key role in kernel development. Although the incorporation of lignin would support this process, its presence in these structures has not been reported in previous studies. We used potassium permanganate staining combined with transmission electron microscopy - energy dispersive X-ray spectrometry as well as acriflavine staining combined with confocal laser scanning microscopy to determine whether the most basal endosperm transfer cells (MBETCs) contain lignified cell walls, using starchy endosperm cells for comparison. We investigated the lignin content of ultrathin sections of MBETCs treated with hydrogen peroxide. The lignin content of transfer and starchy cell walls was also determined by the acetyl bromide method. Finally, the relationship between cell wall lignification and MBETC growth/flange ingrowth orientation was evaluated. MBETC walls and ingrowths contained lignin throughout the period of cell growth we monitored. The same was true of the starchy cells, but those underwent an even more extensive growth period than the transfer cells. Both the reticulate and flange ingrowths were also lignified early in development. The significance of the lignification of maize endosperm cell walls is discussed in terms of its impact on cell growth and flange ingrowth orientation.

The plant cell wall in the feeding sites of cyst nematodes

Plant parasitic cyst nematodes (genera Heterodera and Globodera) are serious pests for many crops. They enter the host roots as migratory second stage juveniles (J2) and migrate intracellularly toward the vascular cylinder using their stylet and a set of cell wall degrading enzymes produced in the pharyngeal glands. They select an initial syncytial cell (ISC) within the vascular cylinder or inner cortex layers to induce the formation of a multicellular feeding site called a syncytium, which is the only source of nutrients for the parasite during its entire life. A syncytium can consist of more than hundred cells whose protoplasts are fused together through local cell wall dissolutions. While the nematode produces a cocktail of cell wall degrading and modifying enzymes during migration through the root, the cell wall degradations occurring during syncytium development are due to the plants own cell wall modifying and degrading proteins. The outer syncytial cell wall thickens to withstand the increasing osmotic pressure inside the syncytium. Furthermore, pronounced cell wall ingrowths can be formed on the outer syncytial wall at the interface with xylem vessels. They increase the surface of the symplast-apoplast interface, thus enhancing nutrient uptake into the syncytium. Processes of cell wall degradation, synthesis and modification in the syncytium are facilitated by a variety of plant proteins and enzymes including expansins, glucanases, pectate lyases and cellulose synthases, which are produced inside the syncytium or in cells surrounding the syncytium.

Rhinanthus serotinus (Schönheit) Oborny (Scrophulariaceae): immunohistochemical and ultrastructural studies of endosperm chalazal haustorium development

Chalazal endosperm haustorium in Rhinanthus serotinus consists of a single large binucleate cell. It originates from the primary endosperm cell dividing transversely into two unequal cells: a smaller micropylar cell and a larger chalazal cell. The chalazal cell undergoes a single mitotic division, then lengthens significantly during development and functions as a chalazal endosperm haustorium. In this paper, immunofluorescent techniques, rhodamine phalloidin assay, and electron microscopy were used to examine the actin and tubulin cytoskeleton during the development of the chalazal haustorium. During the differentiation stage, numerous longitudinally oriented bundles of microfilaments ran along the axis of transvacuolar strands in haustorium. Microtubules formed intensely fluorescent areas near the nuclear envelope and also formed radial perinuclear microtubule arrays. In the fully differentiated haustorium cell, the actin cytoskeleton formed dense clusters of microfilaments on the chalazal and micropylar poles of the haustorium. Numerous microfilament bundles occurred near wall ingrowths on the chalazal wall. There were numerous clusters of microfilaments and microtubules around the huge lobed polytenic haustorial nuclei. The microfilaments were oriented longitudinally to the long axis of the haustorium cell and surrounded both nuclei. The microtubules formed radial perinuclear systems which were appeared to radiate from the surface of the nuclear envelope. The early stage of degeneration of the chalazal haustorium was accompanied by the degradation of microtubules and disruption of the parallel orientation of microtubules in the chalazal area of the cell. The degree of vacuolization increased, autophagous vacuoles appeared and the number of vesicles decreased.

Intersection of transfer cells with phloem biology-broad evolutionary trends, function, and induction

Transfer cells (TCs) are ubiquitous throughout the plant kingdom. Their unique ingrowth wall labyrinths, supporting a plasma membrane enriched in transporter proteins, provides these cells with an enhanced membrane transport capacity for resources. In certain plant species, TCs have been shown to function to facilitate phloem loading and/or unloading at cellular sites of intense resource exchange between symplasmic/apoplasmic compartments. Within the phloem, the key cellular locations of TCs are leaf minor veins of collection phloem and stem nodes of transport phloem. In these locations, companion and phloem parenchyma cells trans-differentiate to a TC morphology consistent with facilitating loading and re-distribution of resources, respectively. At a species level, occurrence of TCs is significantly higher in transport than in collection phloem. TCs are absent from release phloem, but occur within post-sieve element unloading pathways and particularly at interfaces between generations of developing Angiosperm seeds. Experimental accessibility of seed TCs has provided opportunities to investigate their inductive signaling, regulation of ingrowth wall formation and membrane transport function. This review uses this information base to explore current knowledge of phloem transport function and inductive signaling for phloem-associated TCs. The functional role of collection phloem and seed TCs is supported by definitive evidence, but no such information is available for stem node TCs that present an almost intractable experimental challenge. There is an emerging understanding of inductive signals and signaling pathways responsible for initiating trans-differentiation to a TC morphology in developing seeds. However, scant information is available to comment on a potential role for inductive signals (auxin, ethylene and reactive oxygen species) that induce seed TCs, in regulating induction of phloem-associated TCs. Biotic phloem invaders have been used as a model to speculate on involvement of these signals.

Development of flange and reticulate wall ingrowths in maize (Zea mays L.) endosperm transfer cells

Maize (Zea mays L.) endosperm transfer cells are essential for kernel growth and development so they have a significant impact on grain yield. Although structural and ultrastructural studies have been published, little is known about the development of these cells, and prior to this study, there was a general consensus that they contain only flange ingrowths. We characterized the development of maize endosperm transfer cells by bright field microscopy, transmission electron microscopy, and confocal laser scanning microscopy. The most basal endosperm transfer cells (MBETC) have flange and reticulate ingrowths, whereas inner transfer cells only have flange ingrowths. Reticulate and flange ingrowths are mostly formed in different locations of the MBETC as early as 5 days after pollination, and they are distinguishable from each other at all stages of development. Ingrowth structure and ultrastructure and cellulose microfibril compaction and orientation patterns are discussed during transfer cell development. This study provides important insights into how both types of ingrowths are formed in maize endosperm transfer cells.

Differentiation of endosperm transfer cells of barley: a comprehensive analysis at the micro-scale

Barley endosperm cells differentiate into transfer cells (ETCs) opposite the nucellar projection. To comprehensively analyse ETC differentiation, laser microdissection-based transcript and metabolite profiles were obtained from laser microdissected tissues and cell morphology was analysed. Flange-like secondary-wall ingrowths appeared between 5 and 7 days after pollination within the three outermost cell layers. Gene expression analysis indicated that ethylene-signalling pathways initiate ETC morphology. This is accompanied by gene activity related to cell shape control and vesicle transport, with abundant mitochondria and endomembrane structures. Gene expression analyses indicate predominant formation of hemicelluloses, glucuronoxylans and arabinoxylans, and transient formation of callose, together with proline and 4-hydroxyproline biosynthesis. Activation of the methylation cycle is probably required for biosynthesis of phospholipids, pectins and ethylene. Membrane microdomains involving sterols/sphingolipids and remorins are potentially involved in ETC development. The transcriptional activity of assimilate and micronutrient transporters suggests ETCs as the main uptake organs of solutes into the endosperm. Accordingly, the endosperm grows maximally after ETCs are fully developed. Up-regulated gene expression related to amino acid catabolism, C:N balances, carbohydrate oxidation, mitochondrial activity and starch degradation meets high demands for respiratory energy and carbohydrates, required for cell proliferation and wall synthesis. At 10 days after pollination, ETCs undergo further differentiation, potentially initiated by abscisic acid, and metabolism is reprogrammed as shown by activated storage and stress-related processes. Overall, the data provide a comprehensive view of barley ETC differentiation and development, and identify candidate genes and associated pathways.

Unique cell wall abnormalities in the putative phosphoinositide phosphatase mutant AtSAC9

SAC9 is a putative phosphoinositide phosphatase in Arabidopsis thaliana involved in phosphoinositide signaling. sac9-1 plants have a constitutively stressed phenotype with shorter roots which notably accumulate phosphatidylinositol 4,5-bisphosphate and its hydrolysis product inositol trisphosphate. We investigated the primary roots of sac9-1 seedlings at the cytological and ultrastructural level to determine the structural basis for this altered growth. Despite the normal appearance of organelles and cytoplasmic elements, our studies reveal extreme abnormalities of cell wall and membrane structures in sac9-1 primary root cells, regardless of cell type, position within the meristematic area, and plane of section. Cell wall material was deposited locally and in a range of abnormal shapes, sometimes completely fragmenting the cell. Simple protuberances, broad flanges, diffuse patches, elaborate folds, irregular loops and other complex three-dimensional structures were found to extend randomly from the pre-existing cell wall. Abundant vesicles and excessive membrane material were associated with these irregular wall structures. We argue that a perturbed phosphoinositide metabolism most likely induces these observed abnormalities and hypothesize that a disorganized cytoskeleton and excessive membrane trafficking mediate the cell wall defects.

Contrast observation and investigation of wheat endosperm transfer cells and nucellar projection transfer cells

In cereal seed, there are no symplastic connections between the maternal tissues and the endosperm. In order to facilitate solute transport, both the nucellar projection and its opposite endosperm epithelial cells in wheat caryopsis differentiate into transfer cells. In this paper, we did contrast observation and investigation of wheat endosperm transfer cells (ETC) and nucellar projection transfer cells (NPTC). The experimental results showed that there were some similarities and differences between ETC and NPTC. ETC and NPTC almost developed synchronously. Wall ingrowths of ETC and NPTC formed firstly in the first layer nearest to the endosperm cavity, and formed later in the inner layer further from the endosperm cavity. The mature ETC were mainly three layers and the mature NPTC were mainly four layers. Wall ingrowths of ETC were flange type and wall ingrowths of NPTC were reticulate type. NPTC were not nutrient-storing cells, but the first layer of ETC had aleurone cell features, and the second layer and third layer of ETC accumulated starch granules and protein bodies.

Evidence for the role of transfer cells in the evolutionary increase in seed and fiber biomass yield in cotton

Transfer cells (TCs) are specialized cells exhibiting invaginated wall ingrowths (WIs), thereby amplifying their plasma membrane surface area (PMSA) and hence the capacity to transport nutrients. However, it remains unknown as to whether TCs play a role in biomass yield increase during evolution or domestication. Here, we examine this issue from a comparative evolutionary perspective. The cultivated tetraploid AD genome species of cotton and its A and D genome diploid progenitors displayed high, medium, and low seed and fiber biomass yield, respectively. In all three species, cells of the innermost layer of the seed coat juxtaposed to the filial tissues trans-differentiated to a TC morphology. Electron microscopic analyses revealed that these TCs are characterized by sequential formation of flange and reticulate WIs during the phase of rapid increase in seed biomass. Significantly, TCs from the tetraploid species developed substantially more flange and reticulate WIs and exhibited a higher degree of reticulate WI formation than their progenitors. Consequently, the estimated PMSA of TCs of the tetraploid species was about 4 and 70 times higher than that of TCs of the A and D genome progenitors, respectively, which correlates positively with seed and fiber biomass yield. Further, TCs with extensive WIs in the tetraploid species had much stronger expression of sucrose synthase, a key enzyme involved in TC WI formation and function, than those from the A and D progenitors. The analyses provide a set of novel evidence that the development of TC WIs may play an important role in the increase of seed and fiber biomass yield through polyploidization during evolution.

Current opinions on endosperm transfer cells in maize

Endosperm transfer cells (ETC) mainly occur in the endosperm epithelial layer near the pedicel. They transport the nutrient unloaded by the maternal vascular tissue to filial tissues. Wall ingrowths of ETC can facilitate solute transportation. Sugar, especially glucose, is found to modulate the promoter activity of ZmMRP-1, a determinant of transfer cell-specific expression. The ZmMRP-1-encoded protein can transactivate the promoters of transfer cell-specific genes. Signalling and early events leading to wall ingrowth formation depend upon gene expression. Sucrose synthase and the cytoskeleton probably play a primary role in the wall ingrowth formation. The major solutes transferred by ETC are amino acids, sucrose, and monosaccharides, which is consistent with the expression of their transporters and transport-associated genes. In this paper, we review current opinions on the differentiation, wall ingrowth formation, and function of ETC in maize. According to the experimental materials provided by predecessors, we also give some speculations about the differentiation mechanisms of ETC and process of wall ingrowth formation.

GIGANTEA is a component of a regulatory pathway determining wall ingrowth deposition in phloem parenchyma transfer cells of Arabidopsis thaliana

Transfer cells are specialised transport cells containing invaginated wall ingrowths that generate an amplified plasma membrane surface area with high densities of transporter proteins. They trans-differentiate from differentiated cells at sites at which enhanced rates of nutrient transport occur across apo/symplasmic boundaries. Despite their physiological importance, little is known of the molecular mechanisms regulating construction of their intricate wall ingrowths. We investigated the genetic control of wall ingrowth formation in phloem parenchyma transfer cells of leaf minor veins in Arabidopsis thaliana. Wall ingrowth development in these cells is substantially enhanced upon exposing plants to high-light or cold treatments. A hierarchical bioinformatic analysis of public microarray datasets derived from the leaves of plants subjected to these treatments identified GIGANTEA (GI) as one of 46 genes that are commonly up-regulated twofold or more under both high-light and cold conditions. Histological analysis of the GI mutants gi-2 and gi-3 showed that the amount of phloem parenchyma containing wall ingrowths was reduced 15-fold compared with wild-type. Discrete papillate wall ingrowths were formed in gi-2 plants but failed to develop into branched networks. Wall ingrowth development in gi-2 was not rescued by exposing these plants to high-light or cold conditions. In contrast, over-expression of GI in the gi-2 background restored wall ingrowth deposition to wild-type levels. These results indicate that GI regulates the ongoing development of wall ingrowth networks at a point downstream of inputs from environmental signals.

Wall ingrowth formation in transfer cells: novel examples of localized wall deposition in plant cells

The formation of wall ingrowths increases plasma membrane surface areas of transfer cells involved in membrane transport of nutrients in plants. Construction of these ingrowths provides intriguing and diverse examples of localized wall deposition. Flange wall ingrowths resemble secondary wall thickenings of tracheary elements in morphology and probable mechanisms of deposition. By contrast, reticulate wall ingrowths, deposited as discrete papillate projections, branch and fuse to create a fenestrated wall labyrinth representing a novel form of localized wall deposition. Papillate wall ingrowths are initiated as patches of disorganized cellulosic material and are compositionally similar to primary walls, except for a surrounding layer of callose and enhanced levels of arabinogalactan proteins at the ingrowth/membrane interface. How this unusual form of localized wall deposition is constructed is unknown but may involve constraining cellulose-synthesizing rosette complexes at their growing tips.

Embryology of Ceratopteris richardii (Pteridaceae, tribe Ceratopterideae), with emphasis on placental development

This comprehensive study of early embryology in Ceratopteris richardii combines light microscopy with the first ultrastructural evaluation of any pteridophyte embryo. Emphasis is placed on ontogeny of the foot and placental transfer cells. The embryology of C. richardii shares many similarities with that of other polypodiacious ferns while exhibiting distinctive division patterns. Formative embryonic stages have been reconstructed into three-dimensional models for ease of interpretation. The zygote divides perpendicular to the gametophyte plane and anterioposterior axis. This division establishes a prone embryological habit that maximizes rapid independent establishment of a leaf-root axis in a cordate gametophyte. After the formation of a globular eight-celled stage, initials of the first leaf, and root and shoot apical meristems are defined early by discrete formative divisions. Concomitantly, the foot expands and differentiates to transport nutrients from the gametophyte for the developing embryonic organs. Transfer cell wall ingrowth deposition begins in the gametophyte placental cells before the adjacent sporophyte cells just after the eight-celled stage. These observations provide an anatomical framework for future comparative developmental genetic studies of embryogenesis in free-sporing plants.

Characteristic thickened cell walls of the bracts of the 'eternal flower' Helichrysum bracteatum

BACKGROUND AND AIMS

Helichrysum bracteatum is called an 'eternal flower' and has large, coloured, scarious bracts. These maintain their aesthetic value without wilting or discoloration for many years. There have been no research studies of cell death or cell morphology of the scarious bract, and hence the aim of this work was to elucidate these characteristics for the bract of H. bracteatum.

METHODS

DAPI (4'6-diamidino-2-phenylindol dihydrochloride) staining and fluorescence microscopy were used for observation of cell nuclei. Light microscopy (LM), transmission electron microscopy (TEM) and polarized light microscopy were used for observation of cells, including cell wall morphology.

KEY RESULTS

Cell death occurred at the bract tip during the early stage of flower development. The cell wall was the most prominent characteristic of H. bracteatum bract cells. Characteristic thickened secondary cell walls on the inside of the primary cell walls were observed in both epidermal and inner cells. In addition, the walls of all cells exhibited birefringence. Characteristic thickened secondary cell walls have orientated cellulose microfibrils as well as general secondary cell walls of the tracheary elements. For comparison, these characters were not observed in the petal and bract tissues of Chrysanthemum morifolium.

CONCLUSIONS

Bracts at anthesis are composed of dead cells. Helichrysum bracteatum bracts have characteristic thickened secondary cell walls that have not been observed in the parenchyma of any other flowers or leaves. The cells of the H. bracteatum bract differ from other tissues with secondary cell walls, suggesting that they may be a new cell type.

Tocopherols modulate extraplastidic polyunsaturated fatty acid metabolism in Arabidopsis at low temperature

Tocopherols (vitamin E) are synthesized in plastids and have long been assumed to have essential functions restricted to these organelles. We previously reported that the vitamin e-deficient2 (vte2) mutant of Arabidopsis thaliana is defective in transfer cell wall development and photoassimilate transport at low temperature (LT). Here, we demonstrate that LT-treated vte2 has a distinct composition of polyunsaturated fatty acids (PUFAs): lower levels of linolenic acid (18:3) and higher levels of linoleic acid (18:2) compared with the wild type. Enhanced 18:3 oxidation was not involved, as indicated by the limited differences in oxidized lipid species between LT-treated vte2 and the wild type and by a lack of impact on the LT-induced vte2 phenotype in a vte2 fad3 fad7 fad8 quadruple mutant deficient in 18:3. PUFA changes in LT-treated vte2 occur primarily in phospholipids due to reduced conversion of dienoic to trienoic fatty acids in the endoplasmic reticulum (ER) pathway. Introduction of the ER fatty acid desaturase mutation, fad2, and to a lesser extent the plastidic fad6 mutation into the vte2 background suppressed the LT-induced vte2 phenotypes, including abnormal transfer cell wall development. These results provide biochemical and genetic evidence that plastid-synthesized tocopherols modulate ER PUFA metabolism early in the LT adaptation response of Arabidopsis.

Research note: Deposition patterns of cellulose microfibrils in flange wall ingrowths of transfer cells indicate clear parallels with those of secondary wall thickenings

The arrangement of cellulose microfibrils and cortical microtubules in transfer cells depositing flange wall ingrowths have been determined with field emission scanning electron microscopy and immunofluorescence confocal microscopy. In xylem transfer cells of wheat (Triticum aestivum) stem nodes and transfer cells of corn (Zea mays) endosperm tissue, cellulose microfibrils were aligned in parallel bundles to form the linear wall ingrowths characteristic of flange ingrowth morphology. In both cell types, linear bundles of cellulose microfibrils were deposited over an underlying wall composed of randomly arranged microfibrils. Acid extraction of wheat xylem transfer cells established that flange ingrowths were composed of crystalline cellulose. Immunofluorescence labelling of microtubules in wheat xylem transfer cells showed that bundles of microtubules were positioned directly below and parallel with developing flange ingrowths, whereas more mature ingrowths were flanked by bundles of microtubules. These results show that the parallel organisation of cellulose microfibrils in flange wall ingrowths is similar to those in secondary wall thickenings in xylem elements, and that deposition of these structures in transfer cells is also likely to involve bundling of parallel arrays of microtubules. Our observations are discussed in terms of the possible role of microtubules in building flange-type wall ingrowths and the consequences in terms of predicted mechanisms required to build the fundamentally different reticulate-type wall ingrowths.

Wall ingrowths in epidermal transfer cells of Vicia faba cotyledons are modified primary walls marked by localized accumulations of arabinogalactan proteins

Despite the importance of transfer cells in enhancing nutrient transport in plants, little is known about how deposition of the complex morphology of their wall ingrowths is regulated. We probed thin sections of mature cotyledon epidermal transfer cells of Vicia faba with affinity probes and antibodies specific to polysaccharides and glycoproteins, to determine the distribution of these components in their walls. Walls of these transfer cells consist of the pre-existing primary wall, a uniformly deposited wall layer and wall ingrowths which are comprised of two regions; an electron-opaque inner region and an electron-translucent outer region. The primary wall reacted strongly with antibodies against esterified pectin, xyloglucan, the side chains of rhamnogalaturonan-1 and a cellulase-gold affinity probe. The electron-opaque inner region of wall ingrowths displayed a similar labeling pattern to that of the primary wall, showing strong cross-reactivity with all antibodies tested, except those reacting against highly de-esterified pectins. The electron-opaque outer layer of developmentally more mature wall ingrowths reacted strongly with anti-callose monoclonal and polyclonal antibodies, but showed no reaction for pectin or xyloglucan antibodies or the cellulase-gold affinity probe. The plasma membrane-wall interface was labeled strongly with anti-arabinogalactan protein (AGP) antibodies, with some AGP-reactive antibodies also labeling the electron-translucent zone. Nascent wall ingrowths were labeled specifically with AGPs but not anti-callose. A reduction in wall ingrowth density was observed when developing transfer cells were exposed to beta-d-glucosyl Yariv reagent compared with controls. Our results indicate that wall ingrowths of transfer cells are primary wall-like in composition and probably require AGPs for localized deposition.

Cellulose synthesis is required for deposition of reticulate wall ingrowths in transfer cells

Despite the recognized physiological importance of transfer cells, little is known about how these specialized cells achieve localized deposition of cell wall material, leading to amplification of plasma membrane surface area and enhanced membrane transport capacity. This study establishes that cellulose synthesis is a key early factor in the construction of 'reticulate' wall ingrowths, an elaborate but common form of localized wall deposition characteristic of most transfer cells. Using field emission scanning electron microscopy, wall ingrowths were first visible in epidermal transfer cells of Faba bean cotyledons as raised 'patches' of disorganized and tangled cellulosic material, and, from these structures, ingrowths emerged via further deposition of wall material. The cellulose biosynthesis inhibitors 2,6-dichlorobenzonitrile and isoxaben both caused dramatic reductions in the number of cells depositing wall ingrowths, altered wall ingrowth morphology and visibly disrupted microfibril structure. The restriction of cellulose deposition to discrete patches suggests a novel mechanism for cellulose synthesis in this circumstance. Overall, these results implicate a central role for cellulose synthesis in reticulate wall ingrowth morphology, especially at the initial stage of ingrowth formation, possibly by providing a template for the self-assembly of wall polymers.

Transfer cells: cells specialized for a special purpose

Transfer cells are plant cells with secondary wall ingrowths. These cells are ubiquitous, occurring in all plant taxonomic groups and in algae and fungi. Transfer cells form from differentiated cells across developmental windows and in response to stress. They are considered to play a central role in nutrient distribution by facilitating high rates of transport at bottlenecks for apo-/symplasmic solute exchange. These properties are conferred by their unique structural features--an invaginated secondary wall ensheathed by an amplified area of plasma membrane enriched in a suite of solute transporters. Recent development of transfer cell experimental systems, combined with technologies to image the three-dimensional structure of wall ingrowths, is allowing identification of inductive and regulatory signals, discovery of sequential processes involved in their differentiation, and a search for transfer cell identity genes. A model of key events in differentiation of a transfer cell is presented to highlight areas for future investigation.