Casparian Strip Research Articles

Correction for Hosmani et al., Dirigent domain-containing protein is part of the machinery required for formation of the lignin-based Casparian strip in the root

Correction for Hosmani et al., Dirigent domain-containing protein is part of the machinery required for formation of the lignin-based Casparian strip in the root

Process of aerenchyma formation and reactive oxygen species induced by waterlogging in wheat seminal roots

The development and regulation of aerenchyma in waterlogged conditions were studied in the seminal roots of wheat. Evans blue staining and the first cell death position indicated that the cortical cell death began at the root mid-cortex cells in flooding conditions. Continuous waterlogging treatment caused the spread of cell death from the mid-cortex to the neighboring cells and well-developed aerenchyma was formed after 72 h. Meanwhile, the formation of radial oxygen loss barrier was observed in the exodermis owing to the induction of Casparian bands and lignin deposition. Analysis of aerenchyma along the wheat root revealed that aerenchyma formed at 10 mm from the root tip, significantly increased toward the center of the roots, and decreased toward the basal region of the root. In situ detection of radial oxygen species (ROS) showed that ROS accumulation started in the mid-cortex cells, where cell death began indicating that cell death was probably accompanied by ROS production. Further waterlogging treatments resulted in the accumulation of ROS in the cortical cells, which were the zone for aerenchyma development. Accumulation and distribution of H₂O₂ at the subcellular level were revealed by ultracytochemical localization, which further verified the involvement of ROS in the cortical cell death process (i.e., aerenchyma formation). Furthermore, gene expression analysis indicated that ROS production might be the result of up-regulation of genes encoding for ROS-producing enzymes and the down-regulation of genes encoding for ROS-detoxifying enzymes. These results suggest that aerenchyma development in wheat roots starts in the mid-cortex cells and its formation is regulated by ROS.

Dirigent domain-containing protein is part of the machinery required for formation of the lignin-based Casparian strip in the root

The endodermis acts as a "second skin" in plant roots by providing the cellular control necessary for the selective entry of water and solutes into the vascular system. To enable such control, Casparian strips span the cell wall of adjacent endodermal cells to form a tight junction that blocks extracellular diffusion across the endodermis. This junction is composed of lignin that is polymerized by oxidative coupling of monolignols through the action of a NADPH oxidase and peroxidases. Casparian strip domain proteins (CASPs) correctly position this biosynthetic machinery by forming a protein scaffold in the plasma membrane at the site where the Casparian strip forms. Here, we show that the dirigent-domain containing protein, enhanced suberin1 (ESB1), is part of this machinery, playing an essential role in the correct formation of Casparian strips. ESB1 is localized to Casparian strips in a CASP-dependent manner, and in the absence of ESB1, disordered and defective Casparian strips are formed. In addition, loss of ESB1 disrupts the localization of the CASP1 protein at the casparian strip domain, suggesting a reciprocal requirement for both ESB1 and CASPs in forming the casparian strip domain.

A Spatio-Temporal Understanding of Growth Regulation during the Salt Stress Response in Arabidopsis

Plant environmental responses involve dynamic changes in growth and signaling, yet little is understood as to how progress through these events is regulated. Here, we explored the phenotypic and transcriptional events involved in the acclimation of the Arabidopsis thaliana seedling root to a rapid change in salinity. Using live-imaging analysis, we show that growth is dynamically regulated with a period of quiescence followed by recovery then homeostasis. Through the use of a new high-resolution spatio-temporal transcriptional map, we identify the key hormone signaling pathways that regulate specific transcriptional programs, predict their spatial domain of action, and link the activity of these pathways to the regulation of specific phases of growth. We use tissue-specific approaches to suppress the abscisic acid (ABA) signaling pathway and demonstrate that ABA likely acts in select tissue layers to regulate spatially localized transcriptional programs and promote growth recovery. Finally, we show that salt also regulates many tissue-specific and time point-specific transcriptional responses that are expected to modify water transport, Casparian strip formation, and protein translation. Together, our data reveal a sophisticated assortment of regulatory programs acting together to coordinate spatially patterned biological changes involved in the immediate and long-term response to a stressful shift in environment.

The Endodermis

A Casparian strip-bearing endodermis is a feature that has been invariably present in the roots of ferns and angiosperms for approximately 400 million years. As the innermost cortical layer that surrounds the central vasculature of roots, the endodermis acts as a barrier to the free diffusion of solutes from the soil into the stele. Based on an enormous body of anatomical and physiological work, the protective endodermal diffusion barrier is thought to be of major importance for many aspects of root biology, reaching from efficient water and nutrient transport to defense against soil-borne pathogens. Until recently, however, we were ignorant about the genes and mechanisms that drive the differentiation of this intricately structured barrier. Recent work in Arabidopsis has now identified the first major players in Casparian strip formation. A mechanistic understanding of endodermal differentiation will finally allow us to specifically interfere with endodermal barrier function and study the effects on plant growth and survival under various stress conditions. Here, I critically review the major findings and models related to endodermal structure and function from other plant species and assess them in light of recent molecular data from Arabidopsis, pointing out where the older, descriptive work can provide a framework and inspiration for further molecular dissection.

Anatomy and Development of the Endodermis and Phellem ofQuercus suberL. Roots

Quercus suber L. has been investigated with special attention to the stem bark and its cork formation layer, but excluding the roots. Roots are the location of infection by pathogens such as Phytophthora cinnamomi responsible for the tree's sudden death. It is widely accepted that suberin establishes boundaries within tissues, serves as a barrier against free water and ion passage, and works as a shield against pathogen attacks. We followed the suberization of young secondary roots of cork oak. The first suberin deposition detectable by transmission electron microscopy (TEM) and neutral red (NR) was in the endoderm Casparian strips. Casparian strips are not detected by Sudan red 7B and Fluorol yellow (FY) that specifically stain lamellae suberin. Reaction to Sudan was verified in the endodermis and later on in phellem cells that resulted from the phellogen. Under TEM, the Sudan and FY-stained cells showed clear suberin lamellae while the newer formed phellem cells displayed a distinct NR signal compared to the outermost phellem cells. We concluded that suberin chemical components are arranged differently in the cell wall according to the physiological role or maturation stage of a given tissue.

Polarized Radicals

Establishing polarized surfaces enables cells to carry out specialized tasks. In this issue, Lee etal. present a mechanism for cell polarization in which localized peroxidases are used to position the Casparian strip, a diffusion barrier deposited between endodermal cells in plant roots.

A Mechanism for Localized Lignin Deposition in the Endodermis

The precise localization of extracellular matrix and cell wall components is of critical importance for multicellular organisms. Lignin is a major cell wall modification that often forms intricate subcellular patterns that are central to cellular function. Yet the mechanisms of lignin polymerization and the subcellular precision of its formation remain enigmatic. Here, we show that the Casparian strip, a lignin-based, paracellular diffusion barrier in plants, forms as a precise, median ring by the concerted action of a specific, localized NADPH oxidase, brought into proximity of localized peroxidases through the action of Casparian strip domain proteins (CASPs). Our findings in Arabidopsis provide a simple mechanistic model of how plant cells regulate lignin formation with subcellular precision. We speculate that scaffolding of NADPH oxidases to the downstream targets of the reactive oxygen species (ROS) that they produce might be a widespread mechanism to ensure specificity and subcellular precision of ROS action within the extracellular matrix.

Apoplastic diffusion barriers in Arabidopsis.

During the development of Arabidopsis and other land plants, diffusion barriers are formed in the apoplast of specialized tissues within a variety of plant organs. While the cuticle of the epidermis is the primary diffusion barrier in the shoot, the Casparian strips and suberin lamellae of the endodermis and the periderm represent the diffusion barriers in the root. Different classes of molecules contribute to the formation of extracellular diffusion barriers in an organ- and tissue-specific manner. Cutin and wax are the major components of the cuticle, lignin forms the early Casparian strip, and suberin is deposited in the stage II endodermis and the periderm. The current status of our understanding of the relationships between the chemical structure, ultrastructure and physiological functions of plant diffusion barriers is discussed. Specific aspects of the synthesis of diffusion barrier components and protocols that can be used for the assessment of barrier function and important barrier properties are also presented.

The pea stem

The Casparian strip is commonly observed in the endodermis of roots of vascular plants and, in some cases, also in the stems. Pea stems develop the Casparian strip, and its development has been reported to be regulated by blue light. In addition, for the purpose of photobiological studies, pea stems provide a unique experimental system for other physiological studies of the development of the Casparian strip. In this article, I have briefly summarized (1) the effects of environmental factors on the development of the Casparian strip, (2) the advantage of using pea stems for physiological studies of the development of the Casparian strip, and (3) cellular events indicated to be involved in the development of the Casparian strip, focusing on the studies using pea stems as well as other recent studies.

Transfer of rare earth elements (REE) from natural soil to plant systems: implications for the environmental availability of anthropogenic REE

Rare Earth Elements (REE) are widely used to trace natural geochemical processes. They are also increasingly used by man (electronics industry, medicine, agriculture) and therefore considered as emerging pollutants. The present study documents REE mobility in non-polluted natural soil-plant systems in order to characterize their environmental availability for future anthropogenic pollution. The study is based on a field approach in non-polluted natural sites with contrasting geological environments (limestone, granite, and carbonatite) and highly variable REE contents. REE concentrations in soils do not directly reflect bedrock concentrations, but depend largely on pedogenetic processes and on the mineralogy of bedrock and soil. The soils of all sites are with respect to bedrock enriched in heavy REE. The REE uptake by plants is not primarily controlled by the plant itself, but depends on the concentration and the speciation in the soil and the adsorbed soil water pool. REE uptake by plant roots are linked with those of Fe. Roots absorb preferentially the light REE. Before translocation, REE are retained by the Casparian strip leading to much lower concentrations in the aerial parts. The transport of the REE within the xylem is associated with the general nutrient flux.

A strategy of Ca2+ alleviating Na+ toxicity in salt-treated Cyclocarya paliurus seedlings: photosynthetic and nutritional responses

Cyclocarya paliurus seedlings were subjected to 85 mM NaCl and 0, 6, 12 or 18 mM Ca(NO3)2 treatments to study changes in plant growth, photosynthetic parameters and distribution and/or accumulation of organic and inorganic solutes. Na+ toxicity symptoms were observed in plants non-treated with Ca(NO3)2, while 12 mM Ca(NO3)2 supplementation produced a significant promotion of shoot growth; meanwhile chlorophyll content, photosynthetic rate and optimum quantum yield of photosystem II (PSII), represented by the Fv/Fm ratio and pigments content as well as proline and soluble sugars, significantly increased. Ca(NO3)2 supply increased K+ and Ca2+ concentration, whereas the Na+ transport to the shoot was inhibited. There was a strong increase in the K+/Na+ ratio in shoot of Ca(NO3)2-treated plants. X-Ray microanalysis of roots showed that K+, Ca2+ and Na+ accumulated mainly in the epidermal cells and cortical cells of roots with 12 mM Ca(NO3)2 supply, and low accumulation was observed in stelar parenchyma, indicating exogenous Ca2+ possibly induced or strengthened effects of Casparian bands on ion transport. These results suggest that Ca(NO3)2 supplement increased inorganic and organic solutes accumulation in shoot and leaf, and restricted Na+ transport to the shoot by reinforcing barrier effects for attenuating salt injuries in plants, which could be a strategy of Ca2+ alleviating Na+ toxicity in C. paliurus seedlings subjected to salt stress.

Casparian strip diffusion barrier in Arabidopsis is made of a lignin polymer without suberin

Casparian strips are ring-like cell-wall modifications in the root endodermis of vascular plants. Their presence generates a paracellular barrier, analogous to animal tight junctions, that is thought to be crucial for selective nutrient uptake, exclusion of pathogens, and many other processes. Despite their importance, the chemical nature of Casparian strips has remained a matter of debate, confounding further molecular analysis. Suberin, lignin, lignin-like polymers, or both, have been claimed to make up Casparian strips. Here we show that, in Arabidopsis, suberin is produced much too late to take part in Casparian strip formation. In addition, we have generated plants devoid of any detectable suberin, which still establish functional Casparian strips. In contrast, manipulating lignin biosynthesis abrogates Casparian strip formation. Finally, monolignol feeding and lignin-specific chemical analysis indicates the presence of archetypal lignin in Casparian strips. Our findings establish the chemical nature of the primary root-diffusion barrier in Arabidopsis and enable a mechanistic dissection of the formation of Casparian strips, which are an independent way of generating tight junctions in eukaryotes.

Anatomical differences of poplar (Populus × euramericana clone I-214) roots exposed to zinc excess

Poplar is one of the suitable candidates for phytoremediation due to extensive root system, fast growth rate, easy propagation and high biomass production. Zinc (Zn) is an essential element, but at high concentration becomes toxic to plants, similarly like cadmium (Cd). In order to evaluate the effect of Zn on root tissue development we conducted experiments with poplar (Populus × euramericana clone I-214) grown in hydroponics. Plants were treated with low (control) and excess level of Zn (1 mM). Changes in the development of apoplasmic barriers — Casparian bands and suberin lamellae in endodermis, as well as lignification of xylem vessels have been investigated. We found that both apoplasmic barriers developed closer to the root apex in higher Zn-treated root when compared with control root. Similar changes were observed in lignification of xylem vessels. For localization of Zn within root tissues, cryo-SEM/EDXMA analyses were used. Most of Zn was localized in the cortical tissues and four-time less Zn was determined in the inner part of the root below the endodermis. This indicates that endodermis serves as efficient barrier of apoplasmic Zn transport across the poplar root.

Silicon modifies root anatomy, and uptake and subcellular distribution of cadmium in young maize plants.

Silicon (Si) has been shown to ameliorate the negative influence of cadmium (Cd) on plant growth and development. However, the mechanism of this phenomenon is not fully understood. Here we describe the effect of Si on growth, and uptake and subcellular distribution of Cd in maize plants in relation to the development of root tissues. Young maize plants (Zea mays) were cultivated for 10 d hydroponically with 5 or 50 µm Cd and/or 5 mm Si. Growth parameters and the concentrations of Cd and Si were determined in root and shoot by atomic absorption spectrometry or inductively coupled plasma mass spectroscopy. The development of apoplasmic barriers (Casparian bands and suberin lamellae) and vascular tissues in roots were analysed, and the influence of Si on apoplasmic and symplasmic distribution of (109)Cd applied at 34 nm was investigated between root and shoot. Si stimulated the growth of young maize plants exposed to Cd and influenced the development of Casparian bands and suberin lamellae as well as vascular tissues in root. Si did not affect the distribution of apoplasmic and symplasmic Cd in maize roots, but considerably decreased symplasmic and increased apoplasmic concentration of Cd in maize shoots. Differences in Cd uptake of roots and shoots are probably related to the development of apoplasmic barriers and maturation of vascular tissues in roots. Alleviation of Cd toxicity by Si might be attributed to enhanced binding of Cd to the apoplasmic fraction in maize shoots.

Formation of Endodermis-like Cells with Casparian Strip and Thick Wall Cells Derived from Pericycle in the Roots of Feijoa sellowiana (Myrtaceae)

Two anatomical features of cells derived from the pericycle of the roots of Feijoa sellowiana are reported: (1) appearance of endodermis-like cell layer with a Casparian strip, which was confirmed by histological examination; (2) occurrence of a thick wall cell between endodermis-like cells. These features were considered to develop as follows: two tangential cell walls appeared successively in the pericycle cells, resulting in the formation of three cell layers. The walls of the cells of the outer layer, adjacent to the endodermis, became thicker on the interior side of the cell and heavily encrusted with lignin. The walls of the cells in the middle layer derived from the pericycle became suberized and developed into Casparian strips. These cells subsequently developed into endodermis-like cells. The innermost cell layer derived from the pericycle retained their original morphology for a while, but again divided tangentially and developed as mentioned above. Consequently, in aged roots, there were several layers, in which endodermis-like cells with Casparian strips alternated with cells with thick inner walls. The tangential width of endodermis-like cells reduced in sequence from the outer to inner layer. It is quite important to know how the anatomical features of feijoa trees appear under different soil conditions.

Short‐term control of maize cell and root water permeability through plasma membrane aquaporin isoforms

Although it is widely accepted that aquaporins are involved in the regulation of root water uptake, the role of specific isoforms in this process is poorly understood. The mRNA expression and protein level of specific plasma membrane intrinsic proteins (PIPs) were analysed in Zea mays in relation to cell and root hydraulic conductivity. Plants were analysed during the day/night period, under different growth conditions (aeroponics/hydroponics) and in response to short-term osmotic stress applied through polyethylene glycol (PEG). Higher protein levels of ZmPIP1;2, ZmPIP2;1/2;2, ZmPIP2;5 and ZmPIP2;6 during the day coincided with a higher water permeability of root cortex cells during the day compared with night period. Similarly, plants which were grown under aeroponic conditions and which developed a hypodermis ('exodermis') with Casparian bands, effectively forcing more water along a membranous uptake path across roots, showed increased levels of ZmPIP2;5 and ZmPIP1;2 in the rhizodermis and exodermis. When PEG was added to the root medium (2-8 h), expression of PIPs and cell water permeability in roots increased. These data support a role of specific PIP isoforms, in particular ZmPIP1;2 and ZmPIP2;5, in regulating root water uptake and cortex cell hydraulic conductivity in maize.

The endodermis—development and differentiation of the plant’s inner skin

Controlling external compound entrance is essential for plant survival. To set up an efficient and selective sorting of nutrients, free diffusion via the apoplast in vascular plants is blocked at the level of the endodermis. Although we have learned a lot about endodermal specification in the last years, information regarding its differentiation is still very limited. A differentiated endodermal cell can be defined by the presence of the "Casparian strip" (CS), a cell wall modification described first by Robert Caspary in 1865. While the anatomical description of CS in many vascular plants has been very detailed, we still lack molecular information about the establishment of the Casparian strips and their actual function in roots. The recent isolation of a novel protein family, the CASPs, that localizes precisely to a domain of the plasma membrane underneath the CS represents an excellent point of entry to explore CS function and formation. In addition, it has been shown that the endodermis contains transporters that are localized to either the central (stele-facing) or peripheral (soil-facing) plasma membranes. These features suggest that the endodermis functions as a polar plant epithelium.

Development of the Casparian strip is delayed by blue light in pea stems.

To understand the regulatory mechanisms involved in tissue development by light, the kinetics of regulation of Casparian strip (CS) development in garden pea stems was studied. We found that short-term irradiation with white light delayed the development of the CS and used this delay to assess the quantitative effect of light on CS development. We examined the effect of the duration and fluence rates of white light treatment on CS development and observed a significant relationship between fluence and the delay in CS development indicating that the Bunsen-Roscoe law of reciprocity holds for this response. The effect of white light irradiation was not inhibited in the presence of a photosynthetic inhibitor, DCMU, or a carotenoid biosynthesis inhibitor, Norflurazon, indicating that the delay in CS development by light is a photomorphogenetic response rather than a subsidiary effect mediated by photosynthetic activity. An action spectrum for the response displayed a major peak in the blue-light region, suggesting a dominant role for blue-light receptors. A minor peak in the red-light region also suggested the possible involvement of phytochromes. Although phytochromes are known to contribute to blue-light responses, phytochrome-deficient mutants showed a normal delay of CS development in response to blue light, indicating that the response is not mediated by phytochrome and suggesting a role for one or more specific blue-light receptors.

A novel protein family mediates Casparian strip formation in the endodermis

Polarized epithelia are fundamental to multicellular life. In animal epithelia, conserved junctional complexes establish membrane diffusion barriers, cellular adherence and sealing of the extracellular space. Plant cellular barriers are of independent evolutionary origin. The root endodermis strongly resembles a polarized epithelium and functions in nutrient uptake and stress resistance. Its defining features are the Casparian strips, belts of specialized cell wall material that generate an extracellular diffusion barrier. The mechanisms localizing Casparian strips are unknown. Here we identify and characterize a family of transmembrane proteins of previously unknown function. These 'CASPs' (Casparian strip membrane domain proteins) specifically mark a membrane domain that predicts the formation of Casparian strips. CASP1 displays numerous features required for a constituent of a plant junctional complex: it forms complexes with other CASPs; it becomes immobile upon localization; and it sediments like a large polymer. CASP double mutants display disorganized Casparian strips, demonstrating a role for CASPs in structuring and localizing this cell wall modification. To our knowledge, CASPs are the first molecular factors that are shown to establish a plasma membrane and extracellular diffusion barrier in plants, and represent a novel way of epithelial barrier formation in eukaryotes.