Casparian Strip Research Articles

A redundant hydraulic function of root hairs in barley plants grown in hydroponics.

The root hair-less brb of Hordeum vulgare L. (bald root barley) mutant was used to assess the significance that root hairs have for the hydraulic properties of roots and response to a limited supply of mineral nutrients in plants grown on hydroponics. The barley brb mutant and its parent wild-type (H. vulgare cv. Pallas) were grown under nutrient sufficient control conditions, and under conditions of low supply of P and N. Plants were analysed when they were 14-18 days old. Root hydraulic conductivity (Lp) was determined for excised root systems and intact transpiring plants, and cell Lp was determined through cell pressure probe measurements. The formation of Casparian bands and suberin lamellae was followed through staining of cross-sections. The presence or absence of root hairs had no effect on the overall hydraulic response of plants to nutritional treatments. Root and cell Lp did not differ between the two genotypes. The most apparent difference between brb and wild-type plants was the consistently reduced formation of apoplastic barriers in brb plants. Any hydraulic function of root hairs can be redundant in barley, at least under the hydroponic conditions tested.

High-order mutants reveal an essential requirement for peroxidases but not laccases in Casparian strip lignification

Lignin has enabled plants to colonize land, grow tall, transport water within their bodies, and protect themselves against various stresses. Consequently, this polyphenolic polymer, impregnating cellulosic plant cell walls, is the second most abundant polymer on Earth. Yet, despite its great physiological, ecological, and economical importance, our knowledge of lignin biosynthesis in vivo, especially the polymerization steps within the cell wall, remains vague-specifically, the respective roles of the two polymerizing enzymes classes, laccases and peroxidases. One reason for this lies in the very high numbers of laccases and peroxidases encoded by 17 and 73 homologous genes, respectively, in Arabidopsis Here, we have focused on a specific lignin structure, the ring-like Casparian strips (CSs) within the root endodermis. By reducing candidate numbers using cellular resolution expression and localization data and by boosting stacking of mutants using CRISPR-Cas9, we mutated the majority of laccases in Arabidopsis in a nonuple mutant-essentially abolishing laccases with detectable endodermal expression. Yet, we were unable to detect even slight defects in CS formation. By contrast, we were able to induce a complete absence of CS formation in a quintuple peroxidase mutant. Our findings are in stark contrast to the strong requirement of xylem vessels for laccase action and indicate that lignin in different cell types can be polymerized in very distinct ways. We speculate that cells lignify differently depending on whether lignin is localized or ubiquitous and whether cells stay alive during and after lignification, as well as the composition of the cell wall.

Drought activates MYB41 orthologs and induces suberization of grapevine fine roots.

The permeability of roots to water and nutrients is controlled through a variety of mechanisms and one of the most conspicuous is the presence of the Casparian strips and suberin lamellae. Roots actively regulate the creation of these structures developmentally, along the length of the root, and in response to the environment, including drought. In the current study, we characterized the suberin composition along the length of grapevine fine roots during development and in response to water deficit, and in the same root systems we quantified changes in expression of suberin biosynthesis‐ and deposition‐related gene families (via RNAseq) allowing the identification of drought‐responsive suberin‐related genes. Grapevine suberin composition did not differ between primary and lateral roots, and was similar to that of other species. Under water deficit there was a global upregulation of suberin biosynthesis which resulted in an increase of suberin specific monomers, but without changes in their relative abundances, and this upregulation took place across all the developmental stages of fine roots. These changes corresponded to the upregulation of numerous suberin biosynthesis‐ and export‐related genes which included orthologs of the previously characterized AtMYB41 transcriptional factor. Functional validation of two grapevine MYB41 orthologs, VriMYB41 and VriMYB41‐like, confirmed their ability to globally upregulate suberin biosynthesis, export, and deposition. This study provides a detailed characterization of the developmental and water deficit induced suberization of grapevine fine roots and identifies important orthologs responsible for suberin biosynthesis, export, and its regulation in grape.

Revisiting the occurrence and evidence of endodermis in angiosperm shoots

A sampling of angiosperms across a broad spectrum of families and generas was done to determine if angiosperms from the ANA grade basal angiosperms to the Zingiberales of the monocots and Apiales of the eudicots had an endodermis with demonstrable Casparian bands in shoots because the literature has a paucity of demonstrable images, especially for key basal plants. In the Amborellales, Amborella had no stem endodermis, but the aquatic Trithuria of the Nymphaeales had a monostele in which a layer endodermis enclosed its stem axial vascular tissues. Cabomba, Brasenia, and Nuphar of the Nymphaeales had polysteles in which an endodermis of distinct Casparian bands encircled each axial vascular bundle; Nymphaea had either an atactostele or a reduced polystele with only a few axial vascular bundles surrounded by endodermis. Among the magnoliids, Saururus and Houttuynia had monosteles with distinct endodermis Casparian bands. In many monocots, from Acorus, Acorales, to Thalia, Zingiberales, there was a monostele with an endodermis of Casparian bands surrounding a central cylinder of scattered axial vascular bundles. At the base of the eudicots, Ceratophyllum had a vascular core without endodermis. Some members of the Ranunuculales had stem endodermis in a polystelic arrangement. Nelumbo, Proteales, had a monostele of Casparian bands only in stolons. Unusual endodermal and vascular patterns, siphonosteles, were found in Gunnera, Gunnerales. Most eudicots with stem endodermis usually had a monostele or one endodermal layer of Casparian bands only surrounding one ring of vascular bundles, e. g., Myriophyllum, Drosera, Hippuris, Alternanthera, Lysimachia, and Hydrocotyle, but a polystele was present in Justicia. Some leaf petioles and lamina, e. g., Gunnera, Myriophyllum, Hippuris, and Hydrocotyle, had endodermis around their individual vascular bundles. While a stem endodermis was a distinct feature of aquatic or wetland plants, derived from terrestrial ancestors, the presence of stem endodermis by plants at the bases of some major angiosperm lines suggests that it may have been present early in angiosperm evolution.

Differential cadmium translocation and accumulation between Nicotiana tabacum L. and Nicotiana rustica L. by transcriptome combined with chemical form analyses

Cadmium (Cd) is a severely toxic and carcinogenic heavy metal. Cigarette smoking is one of the major source of Cd exposure in humans. Nicotiana tabacum is primarily a leaf Cd accumulator, while Nicotiana rustica is a root Cd accumulator among Nicotiana species. However, little is known about the mechanisms of differential Cd translocation and accumulation in Nicotiana. To find the key factors, Cd concentration, Cd chemical forms, and transcriptome analysis were comparatively studied between N. tabacum and N. rustica under control or 10 μM Cd stress. The leaf/root Cd concentration ratio of N. tabacum was 2.26 and that of N. rustica was 0.14. The Cd concentration in xylem sap of N. tabacum was significantly higher than that of N. rustica. The root of N. tabacum had obviously higher proportion of ethanol extractable Cd (40%) and water extractable Cd (16%) than those of N. rustica (16% and 6%). Meanwhile the proportion of sodium chloride extracted Cd in N. rustica (71%) was significantly higher than that in N. tabacum (30%). A total of 30710 genes expressed differentially between the two species at control, while this value was 30,294 under Cd stress, among which 27,018 were collective genes, manifesting the two species existed enormous genetic differences. KEGG pathway analysis showed the phenylpropanoid biosynthesis pathway was overrepresented between the two species under Cd stress. Several genes associated with pectin methylesterase, suberin and lignin synthesis, and heavy metal transport were discovered to be differential expressed genes between two species. The results suggested that the higher accumulation of Cd in the leaf of N. tabacum depends on a comprehensive coordination of Cd transport, including less cell wall binding, weaker impediment by the Casparian strip, and efficient xylem loading.

Impact of long-term irrigation with municipal reclaimed wastewater on the uptake and degradation of organic contaminants in lettuce and leek

A two years drip irrigation of lettuce and leek crops with treated municipal wastewater without and with spiking with fourteen wastewater relevant contaminants at 10 μg/L concentration level was conducted under greenhouse cultivation conditions to investigate their potential accumulation in soil and leaves and to assess human health related risks. Lettuce and leek crops were selected as a worse-case scenario since leafy green vegetable has a high potential for organic contaminants uptake. The results revealed limited accumulation of contaminants in soil and plant leaves, their concentration levels being in the range of 1–30 ng/g and 1–660 ng/g range in soil and leaves, respectively. This was likely related to abiotic and biotic transformation or simply binding processes in soil, which limited contaminants plant uptake. This assumption was underpinned by studies of the enantiomeric fractionation of chiral compounds (e.g. climbazole and metoprolol) in soil as pieces of evidence of biodegradation and by the identification of transformation products or metabolites in leaves by means of liquid chromatography - high resolution - mass spectrometry using a suspect screening workflow. The high bioconcentration factors were not limited to compounds with intermediate Dow (100 to 1000) such as carbamazepine but also observed for hydrophilic compounds such as clarithromycin, hydrochlorothiazide and the food additives acesulfame and sucralose. This result assumed that accumulation was not only driven by passive processes (e.g. lipoidal diffusion through lipid bilayer cell membranes or Casparian strip) but might be supported by carrier-mediated transporters. As a whole, this study confirmed earlier reports on the a de minimis human health risk related to the consumption of raw leafy green vegetable irrigated with domestic TWW containing organic contaminants residues.

Plant Biology: Journey to the Center of the Casparian Strip

Diffusion barriers in roots play an important role in regulating the movement of compounds between the soil environment and the vasculature. A new study provides new mechanistic insights into how a pair of copper-binding proteins facilitate the formation of a lignified nanodomain within Casparian strips.

Comparative vegetative anatomy of Neotropical Goodyerinae Klotzsch (Orchidaceae Juss.: Orchidoideae Lindl.)

ABSTRACT The Neotropical genera Aspidogyne and Microchilus (Goodyerinae, Cranichideae, Orchidaceae) comprise ca. 200 rainforests terrestrial species. Although species of Goodyerinae are described with similar anatomy to other taxa of Cranichideae, some anatomical characteristics appear to be specific to the subtribe. Our goal was to characterise the anatomical structure of the vegetative organs of Aspidogyne and Microchilus to identify specific characters of Goodyerinae. Root, stem and leaf samples from eleven species were analysed using light and scanning electron microscopy. The leaves are hypostomatic and glabrous with predominantly anisocytic or tetracytic stomata, thin cuticle, homogeneous mesophyll with chromoplast and raphides, and collateral vascular bundles with parenchyma sheath and collenchyma in the midrib. Spiranthosomes were cofirmed for all species and some specific characteristics were identified in the root, such as the presence of collenchyma in both the cortex and the stele, vascular tissue with fibre-tracheids in the centre of the vascular cylinder, and the presence of a true endodermis with Casparian strips in the stem. Therefore, the root and stem were the organs that showed more taxon-specific characteristics for Goodyerinae, which can be used to better delimited the subtribe.

Responses of physiological activities and lateral root anatomical structure of Cercis glabra to waterlogging stress

Cercis glabra is a colour-leaf tree with excellent ornamental value, whereas its physiological and morphological responses to waterlogging stress are still unclear. A potted study was conducted to determine the effects of waterlogging stress on antioxidative enzymes (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)), lipid peroxidation (in terms of malondialdehyde (MDA) content), relative electric conductivity, and osmotic substance (free proline) of leaves and aerenchyma, lignification, suberization and Casparian strip of lateral roots of C. glabra. The result showed that the SOD, POD, and CAT activity and free proline content of C. glabra were significantly increased by the different degrees of waterlogging stress compared with the non-waterlogged treatment at 8 and 12 days, and the MDA content and relative electric conductivity of C. glabra leaves were significantly increased under the different degrees of waterlogging stress compared to the non-waterlogged treatment at 16 days, and the degrees of change increased among treatments was ranked as total waterlogged > semi-waterlogged > shallow waterlogged. The lateral roots of C. glabra not only formed developed aerenchyma in the cortex but also formed suberization and Casparian strip in the endodermis under semi-waterlogged treatment at 16 days. These results implied that C. glabra had a certain tolerance to waterlogging stress, which was associated with the increasing antioxidant enzyme activity and osmotic adjustment substance content, and with the formation of aerenchyma, suberization and Casparian strip in the lateral root to adapt to the waterlogged environment.

Uclacyanin Proteins Are Required for Lignified Nanodomain Formation within Casparian Strips

SummaryCasparian strips (CSs) are cell wall modifications of vascular plants restricting extracellular free diffusion into and out of the vascular system [1]. This barrier plays a critical role in controlling the acquisition of nutrients and water necessary for normal plant development [2, 3, 4, 5]. CSs are formed by the precise deposition of a band of lignin approximately 2 μm wide and 150 nm thick spanning the apoplastic space between adjacent endodermal cells [6, 7]. Here, we identified a copper-containing protein, Uclacyanin1 (UCC1), that is sub-compartmentalized within the CS. UCC1 forms a central CS nanodomain in comparison with other CS-located proteins that are found to be mainly accumulated at the periphery of the CS. We found that loss-of-function of two uclacyanins (UCC1 and UCC2) reduces lignification specifically in this central CS nanodomain, revealing a nano-compartmentalized machinery for lignin polymerization. This loss of lignification leads to increased endodermal permeability and, consequently, to a loss of mineral nutrient homeostasis.

Photosynthetically active radiation impacts significantly on root and cell hydraulics in barley (Hordeum vulgare L.).

Photosynthetically active radiation (PAR) affects transpirational water loss, yet we do not know through which mechanisms root water uptake is adjusted in parallel. Here, we exposed hydroponically grown barley plants to three levels of PAR [Normal (control), Low, High] and focused on the role which aquaporins (AQPs), apoplastic barriers (Casparian bands, suberin lamellae) and root morphology play in the adjustment of root hydraulic conductivity (Lp). Plants were analyzed when they were 14-18 days (d) old. Root and cell Lp, which involves AQP activity, was determined through exudation and cell pressure probe measurements, respectively. Gene expression of AQPs was analyzed through qPCR. The formation of apoplastic barriers was studied through staining of cross-sections. The rate of transpirational water loss per plant and unit leaf area increased in response to high-PAR and decreased in response to low-PAR treatments, both during day and night. Hydraulic conductivity in roots decreased significantly at organ and cell level in response to Low-PAR, and increased (organ) or did not change (cell level) in response to High-PAR. The formation of apoplastic barriers was little affected by PAR. Gene expression of AQPs tended to be highest in the Low-PAR treatment. Lateral roots, showing few apoplastic barriers, contributed the least in Low- and the most to root surface area in High-PAR plants. It is concluded that barley plants which experience changes in shoot transpirational water loss in response to PAR adjust root water uptake through changes in root Lp, and that these changes are mediated through altered AQP activity and root morphology.

Chloride allocation in the euhalophyte Suaeda salsa from different habitats in field and controlled saline conditions

Abstract Suaeda salsa L. is an annual euhalophyte with high salt tolerance which occurs in intertidal zone and inland saline soils. It is not clear how the two populations deal with high Cl−. In the current study, seedlings of two S. salsa populations were cultured with 0 and 300 mM NaCl. The results revealed that the content of Cl− in the roots of S. salsa from an intertidal zone was higher than those of the inland population in both field and controlled saline conditions, and SsCLCc may be related to this process. Generally, the expression of Ssperoxidase64 and SsCASP related to Casparian strip formation was higher in the intertidal population compared to the inland population in saline conditions. The formation of Casparian strip as observed under confocal microscopy in the roots of the intertidal population was more distinct compared to the inland population. The intertidal population of S. salsa appeared to well regulate Cl− allocation in the roots and leaves compared to the inland population under saline conditions, and the Casparian strip may play a role in this process.

Constitutive Activation of Leucine-Rich Repeat Receptor Kinase Signaling Pathways by BAK1-INTERACTING RECEPTOR-LIKE KINASE3 Chimera.

Receptor kinases with extracellular leucine-rich repeat domains (LRR-RKs) form the largest group of membrane signaling proteins in plants. LRR-RKs can sense small molecule, peptide, or protein ligands and may be activated by ligand-induced interaction with a shape complementary SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE (SERK) coreceptor kinase. We have previously shown that SERKs can also form constitutive, ligand-independent complexes with the LRR ectodomains of BAK1-INTERACTING RECEPTOR-LIKE KINASE3 (BIR3) receptor pseudokinases, negative regulators of LRR-RK signaling. Here, we report that receptor chimera in which the extracellular LRR domain of BIR3 is fused to the cytoplasmic kinase domains of the SERK-dependent LRR-RKs BRASSINOSTEROID INSENSITIVE1, HAESA and ERECTA form tight complexes with endogenous SERK coreceptors in the absence of ligand stimulus. Expression of these chimeras under the control of the endogenous promoter of the respective LRR-RK leads to strong gain-of-function brassinosteroid, floral abscission, and stomatal patterning phenotypes, respectively. Importantly, a BIR3-GASSHO1 (GSO1)/SCHENGEN3 (SGN3) chimera can partially complement sgn3 Casparian strip formation phenotypes, suggesting that SERK proteins also mediate GSO1/SGN3 receptor activation. Collectively, our protein engineering approach may be used to elucidate the physiological functions of orphan LRR-RKs and to identify their receptor activation mechanism in single transgenic lines.

Anatomical investigations of the Turkish critically endangered species: Achillea sivasica Çelik et Akpulat (Asteraceae)

In this study, the root, stem, leaf midrib and leaf lamina anatomy and achene micromorphology of the Turkish critically endangered endemic Achillea sivasica were investigated for the first time. In this study, the root was found in late primary growth and in early secondary growth stage. It has a large cortex layer consisting of 12-16 cell rows beneath the periderm. Secretory ducts formed by 5-12 secretory cells embedded in the cortex and located near the vascular bundle were found at the root, which was in the early stage of secondary development. The stem was circular-pentagonal in cross-section. There was lamellar collenchyma beneath epidermis of pentagon corners, and cortex parenchyma between corners. Secretory ducts located near the phloem, between the cortex and endodermis on the interfascicular region, were also observed. An endodermis layer was evident and its cells have indentations and protrusions where they touch adjacent endodermis cells, which strengthens the connection between them. In addition, casparian strips were conspicuous in many endodermis cells. The leaf midrib area had a triangular cross section. There were secretory ducts, consisting of 4-5 secretory cells observed on both sides of the sclerenchymatous fibers that accompany the xylem. The leaf lamina was amphistomatic and stomata type was anomocytic. Mesophyll layer was equifacial. There was a large secretory duct and its diameter is bigger than the nearest main lamina vascular bundle. Achene shape of A. sivasica was lanceolate-oblong and its surface was ribbed and glabrous.

A comparative study of root cadmium radial transport in seedlings of two wheat (Triticum aestivum L.) genotypes differing in grain cadmium accumulation

The radial transport of cadmium (Cd) is essential for Cd influx in roots. The role of radial transport pathway on the Cd translocation from root to shoot among wheat genotypes are still poorly understood. This study explored the role of apoplastic and symplastic pathway on root Cd uptake and root-to-shoot translocation in Zhenmai 10 (ZM10, high Cd in grains) and Aikang 58 (AK58, low Cd in grains). Under Cd treatment, the deposition of Casparian strips (CSs) and suberin lamellae (SL) initiated closer to the root apex in ZM10 than that in AK58, which resulted in the lower Cd concentration in apoplastic fluid of ZM10. Simultaneously, Cd-induced expression levels of genes related to Cd uptake in roots were significantly higher in AK58 by contrast with ZM10, contributing to the symplastic Cd accumulation in AK58 root. Moreover, the addition of metabolic inhibitor CCCP noticeably decreased the Cd accumulation in root of both genotypes. Intriguingly, compared to ZM10, greater amounts of Cd were sequestrated in the cell walls and vacuoles in roots of AK58, limiting the translocation of Cd from root to shoot. Furthermore, the elevated TaHMA2 expression in ZM10 indicates that ZM10 had a higher capacity of xylem loading Cd than AK58. All of these results herein suggest that the radial transport is significant for Cd accumulation in roots, but it cannot explain the difference in root-to-shoot translocation of Cd in wheat genotypes with contrast Cd accumulation in grains.

Casparian strip membrane domain proteins in Gossypium arboreum: genome-wide identification and negative regulation of lateral root growth

BackgroundRoot systems are critical for plant growth and development. The Casparian strip in root systems is involved in stress resistance and maintaining homeostasis. Casparian strip membrane domain proteins (CASPs) are responsible for the formation of Casparian strips.ResultsTo investigate the function of CASPs in cotton, we identified and characterized 48, 54, 91 and 94 CASPs from Gossypium arboreum, Gossypium raimondii, Gossypium barbadense and Gossypium hirsutum, respectively, at the genome-wide level. However, only 29 common homologous CASP genes were detected in the four Gossypium species. A collinearity analysis revealed that whole genome duplication (WGD) was the primary reason for the expansion of the genes of the CASP family in the four cotton species. However, dispersed duplication could also contribute to the expansion of the GaCASPs gene family in the ancestors of G. arboreum. Phylogenetic analysis was used to cluster a total of 85 CASP genes from G. arboreum and Arabidopsis into six distinct groups, while the genetic structure and motifs of CASPs were conserved in the same group. Most GaCASPs were expressed in diverse tissues, with the exception of that five GaCASPs (Ga08G0113, Ga08G0114, Ga08G0116, Ga08G0117 and Ga08G0118) that were highly expressed in root tissues. Analyses of the tissue and subcellular localization suggested that GaCASP27 genes (Ga08G0117) are membrane protein genes located in the root. In the GaCASP27 silenced plants and the Arabidopsis mutants, the lateral root number significantly increased. Furthermore, GaMYB36, which is related to root development was found to regulate lateral root growth by targeting GaCASP27.ConclusionsThis study provides a fundamental understanding of the CASP gene family in cotton and demonstrates the regulatory role of GaCASP27 on lateral root growth and development.

Suberized transport barriers in plant roots: the effect of silicon.

Plant roots are the major organs that take up water and dissolved nutrients. It has been widely shown that apoplastic barriers such as Casparian bands and suberin lamellae in the endo- and exodermis of roots have an important effect on regulating radial water and nutrient transport. Furthermore, it has been described that silicon can promote plant growth and survival under different conditions. However, the potential effects of silicon on the formation and structure of apoplastic barriers are controversial. A delayed as well as an enhanced suberization of root apoplastic barriers with silicon has been described in the literature. Here we review the effects of silicon on the formation of suberized apoplastic barriers in roots, and present results of the effect of silicon treatment on the formation of endodermal suberized barriers on barley seminal roots under control conditions and when exposed to osmotic stress. Chemical analysis confirmed that osmotic stress enhanced barley root suberization. While a supplementation with silicon in both, control conditions and osmotic stress, did not enhanced barley root suberization. These results suggest that enhanced stress tolerance of plants after silicon treatment is due to other responses.

<i>De novo</i> transcriptome analysis reveals an unperturbed transcriptome under high cadmium conditions in the Cd-hypertolerant fern <i>Athyrium yokoscense</i>

Athyrium yokoscense shows strong tolerance to cadmium exposure, even at levels that are many times greater than the toxic levels in ordinary plants. To determine the mechanism of Cd tolerance in A. yokoscense, we grew these plants under high Cd conditions and observed the tissue-specific accumulation of Cd and generation of reactive oxygen species, which is one of the major physiological responses to Cd stress. Fuchsin staining indicated the existence of a casparian strip in A. yokoscense roots, which may participate in Cd hypertolerance in A. yokoscense. Moreover, we performed RNA-seq of RNA samples from A. yokoscense plants treated with or without Cd exposure and obtained comprehensive RNA sequences as well as the Cd-responsive expression patterns of individual genes. Through de novo transcriptome assembly and gene expression analysis, we found that A. yokoscense showed normal features with no significant change in the expression levels of any transporter genes, even under high Cd exposure conditions. Our results demonstrate that A. yokoscense has an unusual mechanism that allows the invading Cd to partition into the distal roots, thus avoiding translocation of Cd into the xylem.

Silicon reduces the iron uptake in rice and induces iron homeostasis related genes

Gramineous plants take up silicon (Si) that enhances the formation of exodermal Casparian bands (CBs) in the roots of rice (Oryza sativa L.). Furthermore, it is known that Si supply reduces the concentration of Fe in rice shoots. We hypothesized that the Si-enhanced CB formation in the exodermis reduces in the flux of Fe in the apoplast and the uptake of Fe loaded deoxymugineic acid. Thus, the effect of silicic acid supply at varied Fe concentrations and Fe forms was investigated in nutrient solution. The Fe concentrations in the shoot and apoplastic Fe concentrations in the root were determined and an Affymetrix GeneChip experiment was carried out together with qRT-PCR measurements for observation of transcriptomic reactions. Additionally, the Fe uptake of an overexpression mutant of OsABCG25 with an enhanced exodermal CB formation was investigated. The application of silicic acid reduced the Fe concentrations in shoot DM independently of the supplied Fe concentration and Fe form. As a reaction to the Fe shortage, the full cascade of Fe-homeostasis-related genes in the roots was upregulated. Silicic acid supply also decreased the apoplastic Fe concentrations in roots. In addition, an overexpression mutant of OsABCG25 with an enhanced CB formation showed a reduced uptake of Fe in excess Fe conditions. The results suggest that the Si-induced CB formation in the exodermis hampers the flux of Fe into the apoplast of the cortex and, thus, Fe uptake of rice grown in nutrient solution which is reflected in the upregulation of Fe homeostasis-related genes.

Low calcium-induced delay in development of root apoplastic barriers enhances Cd uptake and accumulation in Sedum alfredii

Mineral nutrients play an important role in heavy metal uptake and accumulation in plant. However, the effects of calcium (Ca) supply level on apoplastic transport in roots and consequences for uptake of cadmium (Cd) in hyperaccumulators are poorly understood. Here, we investigated how Ca regulated the development of apoplastic barriers in the roots of two ecotypes of Sedum alfredii and assessed its effects on Cd uptake. Results of correlation analysis indicated that Ca content was positively correlated with the development of Casparian strips (CSs) and suberin lamellae (SL) in the absence or presence of Cd. Simultaneously, low Ca supply was proven to delay the formation of endodermis CSs and suberin accumulation by decreasing the relative expressions of genes associated with CSs localization and lignin/suberin synthesis. Moreover, Cd in apoplastic fluid and cell walls (regarding the apoplastic transport) and symplastic fractions were elevated by low Ca supply. Contrary to high Ca supply, the expression levels of genes related to Cd influx and xylem loading were increased upon low Ca addition in roots of both ecotypes. All the results above suggested that low Ca supply promotes root Cd uptake via apoplastic pathway by delaying apoplastic barriers development and also regulating Cd transport to the xylem in S. alfredii.