The endodermis

Uclacyanin Proteins Are Required for Lignified Nanodomain Formation within Casparian Strips

Casparian 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 [2-5]. 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.

Are cuproproteins part of the multi-protein framework for making the Casparian strip?

Casparian strip (CS) is a lignified structure localized on the cell wall between adjacent root endodermal cells and functions as an apoplastic diffusion barrier in the root. The polarly localized, lignin-based CS is an excellent system for studying peptide signaling and position recognition. In this short review, we summarize advances in the past decade on the molecular mechanism governing CS development. In addition to the multi-protein framework underlying the CS membrane domain, we discuss recently observed participation of cell wall located cuproproteins in CS formation. These new discoveries shed light on a potential CS wall domain that coordinates with the membrane domain to provide bidirectional positional information for guiding precise CS development.

Aquaporins are main contributors to root hydraulic conductivity in pearl millet [Pennisetum glaucum (L) R. Br.]

Pearl millet is a key cereal for food security in arid and semi-arid regions but its yield is increasingly threatened by water stress. Physiological mechanisms relating to conservation of soil water or increased water use efficiency can alleviate that stress. Aquaporins (AQP) are water channels that mediate root water transport, thereby influencing plant hydraulics, transpiration and soil water conservation. However, AQP remain largely uncharacterized in pearl millet. Here, we studied AQP function in root water transport in two pearl millet lines contrasting for water use efficiency (WUE). We observed that these lines also contrasted for root hydraulic conductivity (Lpr) and AQP contribution to Lpr. The line with lower WUE showed significantly higher AQP contribution to Lpr. To investigate AQP isoforms contributing to Lpr, we developed genomic approaches to first identify the entire AQP family in pearl millet and secondly, characterize the plasma membrane intrinsic proteins (PIP) gene expression profile. We identified and annotated 33 AQP genes in pearl millet, among which ten encoded PIP isoforms. PgPIP1-3 and PgPIP1-4 were significantly more expressed in the line showing lower WUE, higher Lpr and higher AQP contribution to Lpr. Overall, our study suggests that the PIP1 AQP family are the main regulators of Lpr in pearl millet and may possibly be associated with mechanisms associated to whole plant water use. This study paves the way for further investigations on AQP functions in pearl millet hydraulics and adaptation to environmental stresses.

Morphological and anatomical adaptations to dry, shady environments in Adiantum reniforme var. sinense (Pteridaceae)

The natural distribution of the rare perennial fern Adiantum reniforme var. sinense (Pteridaceae), which is endemic to shady cliff environments, is limited to small areas of Wanzhou County, Chongqing, China. In this study, we used brightfield and epifluorescence microscopy to investigate the anatomical structures and histochemical features that may allow this species to thrive in shady, dry cliff environments. The A. reniforme var. sinense sporophyte had a primary structure and a dictyostele. The plants of this species had an endodermis, sclerenchyma layers and hypodermal sterome, reflecting an adaption to dry cliff environments. Blades had a thin cuticle and isolateral mesophyll, suggesting a tolerance of shady environments. These characteristics are similar to many sciophyte ferns such as Lygodium japonicum and Pteris multifida. Thus, the morphological and anatomical characteristics of A. reniforme var. sinense identified in this study are consistent with adaptations to shady, dry cliff environments.

Developmental programs interact with abscisic acid to coordinate root suberization in Arabidopsis

Suberin lamellae, which provide a hydrophobic protective barrier against biotic and abiotic stresses, are widely deposited in various cell types during plant development and in response to stress. However, it remains unclear how developmental programs interact with stress responses to direct the precise spatiotemporal pattern of suberin deposition. In this study, we found that SHORT-ROOT (SHR), together with its downstream factor MYB36, guided suberization specifically in the root endodermis. Despite a partial dependence on abscisic acid (ABA), the suberization mediated by SHR and MYB36 appeared to derive from a slow readout of developmental programs, which was in contrast to the rapid but transient suberization induced by ABA. Furthermore, we found the MYB39 transcription factor functioned as a common downstream hub of the SHR/MYB36 pathway and the ABA-triggered response. MYB39 could directly bind to the FAR5 (alcohol-forming fatty acyl-coenzyme A reductase) promoter to activate its expression. In addition, overexpression of MYB39 dramatically increased the amount of suberization in Arabidopsis roots. Our results provide important insights into the interaction between developmental programs and environmental stimuli in root suberization in Arabidopsis.

Co-incidence of Damage and Microbial Patterns Controls Localized Immune Responses in Roots

Recognition of microbe-associated molecular patterns (MAMPs) is crucial for the plant's immune response. How this sophisticated perception system can be usefully deployed in roots, continuously exposed to microbes, remains a mystery. By analyzing MAMP receptor expression and response at cellular resolution in Arabidopsis, we observed that differentiated outer cell layers show low expression of pattern-recognition receptors (PRRs) and lack MAMP responsiveness. Yet, these cells can be gated to become responsive by neighbor cell damage. Laser ablation of small cell clusters strongly upregulates PRR expression in their vicinity, and elevated receptor expression is sufficient to induce responsiveness in non-responsive cells. Finally, localized damage also leads to immune responses to otherwise non-immunogenic, beneficial bacteria. Damage-gating is overridden by receptor overexpression, which antagonizes colonization. Our findings that cellular damage can "switch on" local immune responses helps to conceptualize how MAMP perception can be used despite the presence of microbial patterns in the soil.

Laccase3-based extracellular domain provides possible positional information for directing Casparian strip formation in Arabidopsis

The Casparian strip (CS) is a tight junction-like structure formed by lignin impregnation on the walls of endodermal cells in plant roots. The CS membrane domain (CSD^M), demarked by the CASP proteins, is important for orienting lignification enzymes. Here, we report that an endodermis-expressed multicopper oxidase, LACCASE3 (LAC3) in Arabidopsis, locates to the interface between lignin domains and the cell wall during early CS development prior to CASP1 localizing to CSD^M and eventually flanks the mature CS. Pharmacological perturbation of LAC3 causes dispersed localization of CASP1 and compensatory ectopic lignification. These results support the existence of a LAC3-based CS wall domain which coordinates with CSD^M to provide bidirectional positional information that guides precise CS lignification.

Integrative Multi-omics Analyses of Barley Rootzones under Salinity Stress Reveal Two Distinctive Salt Tolerance Mechanisms

The mechanisms underlying rootzone-localized responses to salinity during early stages of barley development remain elusive. In this study, we performed the analyses of multi-root-omes (transcriptomes, metabolomes, and lipidomes) of a domesticated barley cultivar (Clipper) and a landrace (Sahara) that maintain and restrict seedling root growth under salt stress, respectively. Novel generalized linear models were designed to determine differentially expressed genes (DEGs) and abundant metabolites (DAMs) specific to salt treatments, genotypes, or rootzones (meristematic Z1, elongation Z2, and maturation Z3). Based on pathway over-representation of the DEGs and DAMs, phenylpropanoid biosynthesis is the most statistically enriched biological pathway among all salinity responses observed. Together with histological evidence, an intense salt-induced lignin impregnation was found only at stelic cell wall of Clipper Z2, compared with a unique elevation of suberin deposition across Sahara Z2. This suggests two differential salt-induced modulations of apoplastic flow between the genotypes. Based on the global correlation network of the DEGs and DAMs, callose deposition that potentially adjusted symplastic flow in roots was almost independent of salinity in rootzones of Clipper, and was markedly decreased in Sahara. Taken together, we propose two distinctive salt tolerance mechanisms in Clipper (growth-sustaining) and Sahara (salt-shielding), providing important clues for improving crop plasticity to cope with deteriorating global soil salinization.

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

Background

Root 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.

Results

To 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.

Conclusions

This 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.

The barrier function of plant roots: biological bases for selective uptake and avoidance of soil compounds

The root is the main organ through which water and mineral nutrients enter the plant organism. In addition, root fulfils several other functions. Here, we propose that the root also performs the barrier function, which is essential not only for plant survival but for plant acclimation and adaptation to a constantly changing and heterogeneous soil environment. This function is related to selective uptake and avoidance of some soil compounds at the whole plant level. We review the toolkit of morpho-anatomical, structural, and other components that support this view. The components of the root structure involved in selectivity, permeability or barrier at a cellular, tissue, and organ level and their properties are discussed. In consideration of the arguments supporting barrier function of plant roots, evolutionary aspects of this function are also reviewed. Additionally, natural variation in selective root permeability is discussed which suggests that the barrier function is constantly evolving and is subject of natural selection.

SCHENGEN receptor module drives localized ROS production and lignification in plant roots

Production of reactive oxygen species (ROS) by NADPH oxidases (NOXs) impacts many processes in animals and plants, and many plant receptor pathways involve rapid, NOX‐dependent increases of ROS. Yet, their general reactivity has made it challenging to pinpoint the precise role and immediate molecular action of ROS. A well‐understood ROS action in plants is to provide the co‐substrate for lignin peroxidases in the cell wall. Lignin can be deposited with exquisite spatial control, but the underlying mechanisms have remained elusive. Here, we establish a kinase signaling relay that exerts direct, spatial control over ROS production and lignification within the cell wall. We show that polar localization of a single kinase component is crucial for pathway function. Our data indicate that an intersection of more broadly localized components allows for micrometer‐scale precision of lignification and that this system is triggered through initiation of ROS production as a critical peroxidase co‐substrate.

Exodermis and Endodermis Respond to Nutrient Deficiency in Nutrient-Specific and Localized Manner

The exodermis is a common apoplastic barrier of the outer root cortex, with high environmentally-driven plasticity and a protective function. This study focused on the trade-off between the protective advantages provided by the exodermis and its disadvantageous reduction of cortical membrane surface area accessible by apoplastic route, thus limiting nutrient acquisition from the rhizosphere. We analysed the effect of nutrient deficiency (N, P, K, Mg, Ca, K, Fe) on exodermal and endodermal differentiation in maize. To differentiate systemic and localized effects, nutrient deficiencies were applied in three different approaches: to the root system as a whole, locally to discrete parts, or on one side of a single root. Our study showed that the establishment of the exodermis was enhanced in low-N and low-P plants, but delayed in low-K plants. The split-root cultivation proved that the effect is non-systemic, but locally coordinated for individual roots. Within a single root, localized deficiencies didn't result in an evenly differentiated exodermis, in contrast to other stress factors. The maturation of the endodermis responded in a similar way. In conclusion, N, P, and K deficiencies strongly modulated exodermal differentiation. The response was nutrient specific and integrated local signals of current nutrient availability from the rhizosphere.

Modulatory Role of Reactive Oxygen Species in Root Development in Model Plant of Arabidopsis thaliana

Reactive oxygen species (ROS), a type of oxygen monoelectronic reduction product, have a higher chemical activity than O₂. Although ROS pose potential risks to all organisms via inducing oxidative stress, indispensable role of ROS in individual development cannot be ignored. Among them, the role of ROS in the model plant Arabidopsis thaliana is deeply studied. Mounting evidence suggests that ROS are essential for root and root hair development. In the present review, we provide an updated perspective on the latest research progress pertaining to the role of ROS in the precise regulation of root stem cell maintenance and differentiation, redox regulation of the cell cycle, and root hair initiation during root growth. Among the different types of ROS, O₂ ^•- and H₂O₂ have been extensively investigated, and they exhibit different gradient distributions in the roots. The concentration of O₂ ^•- decreases along a gradient from the meristem to the transition zone and the concentration of H₂O₂ decreases along a gradient from the differentiation zone to the elongation zone. These gradients are regulated by peroxidases, which are modulated by the UPBEAT1 (UPB1) transcription factor. In addition, multiple transcriptional factors, such as APP1, ABO8, PHB3, and RITF1, which are involved in the brassinolide signaling pathway, converge as a ROS signal to regulate root stem cell maintenance. Furthermore, superoxide anions (O₂ ^•-) are generated from the oxidation in mitochondria, ROS produced during plasmid metabolism, H₂O₂ produced in apoplasts, and catalysis of respiratory burst oxidase homolog (RBOH) in the cell membrane. Furthermore, ROS can act as a signal to regulate redox status, which regulates the expression of the cell-cycle components CYC2;3, CYCB1;1, and retinoblastoma-related protein, thereby controlling the cell-cycle progression. In the root maturation zone, the epidermal cells located in the H cell position emerge to form hair cells, and plant hormones, such as auxin and ethylene regulate root hair formation via ROS. Furthermore, ROS accumulation can influence hormone signal transduction and vice versa. Data about the association between nutrient stress and ROS signals in root hair development are scarce. However, the fact that ROBHC/RHD2 or RHD6 is specifically expressed in root hair cells and induced by nutrients, may explain the relationship. Future studies should focus on the regulatory mechanisms underlying root hair development via the interactions of ROS with hormone signals and nutrient components.

Groups of multi-cellular passage cells in the root exodermis of Echinochloa crus-galli varieties lack not only suberin lamellae but also lignin deposits

Passage cells are frequently found in the exodermis and the endodermis of the roots. Because passage cells lack an apoplastic diffusion barrier, they are thought to provide pathways for the transport of nutrients and the entrance of endomycorrhizal fungi. Exodermal passage cells possess Casparian strips but not suberin lamellae. So far, exodermal passage cells have not been associated with a particular internal structure. In some wetland plants, the outer part of the root (i.e., epidermis, exodermis, and sclerenchyma) of emerging lateral root primordia has an oxygen leaky zone called a window. The exodermis at the window site also lacks suberin lamellae, but it remains unclear whether the exodermis at the window site also lacks Casparian strips. Here, we report that several of the exodermal cells in the window of Echinochloa crus-galli grown under aerated or deoxygenated stagnant agar nutrient solution also lack lignin, which is a major constituent of Casparian strips. The sclerenchyma cells that form part of the window also lacked lignin deposits. Sites at which lateral root primordia developed were highly permeable to an apoplastic tracer (periodic acid). These observations indicate that windows consist of a novel type of passage cell at the exodermis that lacks lignin as well as suberin lamellae.

Apoplastic barriers, aquaporin gene expression and root and cell hydraulic conductivity in phosphate-limited sheepgrass plants

Mineral nutrient supply can affect the hydraulic property of roots. The aim of the present work on sheepgrass (Leymus chinensis L.) plants was to test whether any changes in root hydraulic conductivity (Lp; exudation analyses) in response to a growth-limiting supply of phosphate (P) are accompanied by changes in (1) cell Lp via measuring the cell pressure, (2) the aquaporin (AQP) gene expression by performing qPCR and (3) the formation of apoplastic barriers, by analyzing suberin lamella and Casparian bands via cross-sectional analyses in roots. Plants were grown hydroponically on complete nutrient solution, containing 250 µM P, until they were 31-36 days old, and then kept for 2-3 weeks on either complete solution, or transferred on solution containing 2.5 µM (low-P) or no added P (no-P). Phosphate treatments caused significant decreases in root and cell-Lp and AQP gene expression, while the formation of apoplastic barriers increased, particularly in lateral roots. Experiments using the AQP inhibitor mercury (Hg) suggested that a significant portion of radial root water uptake in sheepgrass occurs along a path involving AQPs, and that the Lp of this path is reduced under low- and no-P. It is concluded that a growth-limiting supply of phosphate causes parallel changes in (1) cell Lp and aquaporin gene expression (decrease) and (2) apoplastic barrier formation (increase), and that the two may combine to reduce root Lp. The reduction in root Lp, in turn, facilitates an increased root-to-shoot surface area ratio, which allocates resources to the root, sourcing the limiting nutrient.

OsCASP1 forms complexes with itself and OsCASP2 in rice

OsCASP1 (Casparian strip domain protein 1) was recently identified to function in Casparian strip (CS) formation at the endodermal cells in rice roots, which was required for selective mineral uptake in rice. Here, we further investigate the functional form of OsCASP1 in vivo. Expression analysis shows that OsCASP1, OsCASP2, OsCASP3, and OsCASP5 were expressed in roots apart from OsCASP4. A yeast two-hybrid (Y2H) assay revealed that OsCASP1 can interact with itself and OsCASP2, but not with OsCASP3 and OsCASP5. These interactions of OsCASP1 with itself and OsCASP2 at the plasma membrane were confirmed using bimolecular fluorescence complementation (BiFC) in rice protoplasts. These results indicated that OsCASP1 can form complexes with itself and OsCASP2 in rice roots.

The Induction and Roles Played by Phi Thickenings in Orchid Roots

Phi thickenings are specialised secondary wall thickenings present in the root cortex of many plant species, including both angiosperms and gymnosperms. While environmental stresses induce phi thickenings, their role(s) in the root remain unclear. Suggested functions include regulation of transport through the apoplast in a manner similar to the Casparian strip, limiting fungal infections, and providing mechanical support to the root. We investigated phi thickening induction and function in Miltoniopsis sp., an epiphytic orchid. As movement of a fluorescent tracer through the apoplast was not blocked by phi thickenings, and as phi thickenings developed in the roots of sterile cultures in the absence of fungus and did not prevent fungal colonisation of cortical cells, the phi thickenings in Miltoniopsis did not function as a barrier. Phi thickenings, absent in roots grown on agar, remained absent when plants were transplanted to moist soil, but were induced when plants were transplanted to well-drained media, and by the application of water stress. We suggest that it is likely that phi thickenings stabilise to the root during water stress. Nevertheless, the varied phi thickening induction responses present in different plant species suggest that the phi thickenings may play multiple adaptive roles depending on species.

Polarized transport across root epithelia

Plant roots explore the soil to acquire water and nutrients which are often available at concentrations that drastically differ from the plant's actual need for growth and development. This stark difference between availability and requirement can be dealt with owing to the root's architecture as an inverted gut. In roots, the two epithelial characteristics (selective acquisition and diffusion barrier) are split between two cell layers: the epidermis at the root periphery and the endodermis as the innermost cortical cell layer around the vasculature. Polarized transport of nutrients across the root epithelium can be achieved through different pathways: apoplastic, symplastic, or coupled transcellular. This review highlights different features of the root that allow this polarized transport. Special emphasis is placed on the coupled transcellular pathway, facilitated by polarized nutrient carriers along root cell layers but barred by suberin lamellae in endodermal cells.

Cortex cell hydraulic conductivity, endodermal apoplastic barriers and root hydraulics change in barley (Hordeum vulgare L.) in response to a low supply of N and P

BACKGROUND

Mineral nutrient limitation affects the water flow through plants. We wanted to test on barley whether any change in root-to-shoot ratio in response to low supply of nitrogen and phosphate is accompanied by changes in root and cell hydraulic properties and involves changes in aquaporin (AQP) gene expression and root apoplastic barriers (suberin lamellae, Casparian bands).

METHODS

Plants were grown hydroponically on complete nutrient solution or on solution containing only 3.3 % or 2.5 % of the control level of nutrient. Plants were analysed when they were 14-18 d old.

RESULTS

Nutrient-limited plants adjusted water flow to an increased root-to-shoot surface area ratio through a reduction in root hydraulic conductivity (Lp) as determined through exudation analyses. Cortex cell Lp (cell pressure probe analyses) decreased in the immature but not the mature region of the main axis of seminal roots and in primary lateral roots. The aquaporin inhibitor HgCl2 reduced root Lp most in nutrient-sufficient control plants. Exchange of low-nutrient for control media caused a rapid (20-80 min) and partial recovery in Lp, though cortex cell Lp did not increase in any of the root regions analysed. The gene expression level (qPCR analyses) of five plasma membrane-localized AQP isoforms did not change in bulk root extracts, while the formation of apoplastic barriers increased considerably along the main axis of root and lateral roots in low-nutrient treatments.

CONCLUSIONS

Decrease in root and cortex cell Lp enables the adjustment of root water uptake to increased root-to-shoot area ratio in nutrient-limited plants. Aquaporins are the prime candidate to play a key role in this response. Modelling of water flow suggests that some of the reduction in root Lp is due to increased formation of apoplastic barriers.

Fine control of aerenchyma and lateral root development through AUX/IAA- and ARF-dependent auxin signaling

Lateral roots (LRs) are derived from a parental root and contribute to water and nutrient uptake from the soil. Auxin/indole-3-acetic acid protein (AUX/IAA; IAA) and auxin response factor (ARF)-mediated signaling are essential for LR formation. Lysigenous aerenchyma, a gas space created by cortical cell death, aids internal oxygen transport within plants. Rice (Oryza sativa) forms lysigenous aerenchyma constitutively under aerobic conditions and increases its formation under oxygen-deficient conditions; however, the molecular mechanisms regulating constitutive aerenchyma (CA) formation remain unclear. LR number is reduced by the dominant-negative effect of a mutated AUX/IAA protein in the iaa13 mutant. We found that CA formation is also reduced in iaa13 We have identified ARF19 as an interactor of IAA13 and identified a lateral organ boundary domain (LBD)-containing protein (LBD1-8) as a target of ARF19. IAA13, ARF19, and LBD1-8 were highly expressed in the cortex and LR primordia, suggesting that these genes function in the initiation of CA and LR formation. Restoration of LBD1-8 expression recovered aerenchyma formation and partly recovered LR formation in the iaa13 background, in which LBD1-8 expression was reduced. An auxin transport inhibitor suppressed CA and LR formation, and a natural auxin stimulated CA formation in the presence of the auxin transport inhibitor. Our findings suggest that CA and LR formation are both regulated through AUX/IAA- and ARF-dependent auxin signaling. The initiation of CA formation lagged that of LR formation, which indicates that the formation of CA and LR are regulated differently by auxin signaling during root development in rice.

Phi thickenings in roots: novel secondary wall structures responsive to biotic and abiotic stresses

Phi thickenings are specialized secondary walls found in root cortical cells. Despite their widespread occurrence throughout the plant kingdom, these specialized thickenings remain poorly understood. First identified by Van Tieghem in 1871, phi thickenings are a lignified and thickened cell wall band that is deposited inside the primary wall, as a ring around the cells' radial walls. Phi thickenings can, however, display structural variations including a fine, reticulate network of wall thickenings extending laterally from the central lignified band. While phi thickenings have been proposed to mechanically strengthen roots, act as a permeability barrier to modulate solute movement, and regulate fungal interactions, these possibilities remain to be experimentally confirmed. Furthermore, since temporal and spatial development of phi thickenings varies widely between species, thickenings may perform diverse roles in different species. Phi thickenings can be induced by abiotic stresses in different species; they can, for example, be induced by heavy metals in the Zn/Cd hyperaccumulator Thlaspi caerulescens, and in a cultivar-specific manner by water stress in Brassica. This latter observation provides an experimental platform to probe phi thickening function, and to identify genetic pathways responsible for their formation. These pathways might be expected to differ from those involved in secondary wall formation in xylem, since phi thickening deposition in not linked to programmed cell death.

Plant Leucine-Rich Repeat Receptor Kinase (LRR-RK): Structure, Ligand Perception, and Activation Mechanism

In recent years, secreted peptides have been recognized as essential mediators of intercellular communication which governs plant growth, development, environmental interactions, and other mediated biological responses, such as stem cell homeostasis, cell proliferation, wound healing, hormone sensation, immune defense, and symbiosis, among others. Many of the known secreted peptide ligand receptors belong to the leucine-rich repeat receptor kinase (LRR-RK) family of membrane integral receptors, which contain more than 200 members within Arabidopsis making it the largest family of plant receptor kinases (RKs). Genetic and biochemical studies have provided valuable data regarding peptide ligands and LRR-RKs, however, visualization of ligand/LRR-RK complex structures at the atomic level is vital to understand the functions of LRR-RKs and their mediated biological processes. The structures of many plant LRR-RK receptors in complex with corresponding ligands have been solved by X-ray crystallography, revealing new mechanisms of ligand-induced receptor kinase activation. In this review, we briefly elaborate the peptide ligands, and aim to detail the structures and mechanisms of LRR-RK activation as induced by secreted peptide ligands within plants.

Plant Leucine-Rich Repeat Receptor Kinase (LRR-RK): Structure, Ligand Perception, and Activation Mechanism

In recent years, secreted peptides have been recognized as essential mediators of intercellular communication which governs plant growth, development, environmental interactions, and other mediated biological responses, such as stem cell homeostasis, cell proliferation, wound healing, hormone sensation, immune defense, and symbiosis, among others. Many of the known secreted peptide ligand receptors belong to the leucine-rich repeat receptor kinase (LRR-RK) family of membrane integral receptors, which contain more than 200 members within Arabidopsis making it the largest family of plant receptor kinases (RKs). Genetic and biochemical studies have provided valuable data regarding peptide ligands and LRR-RKs, however, visualization of ligand/LRR-RK complex structures at the atomic level is vital to understand the functions of LRR-RKs and their mediated biological processes. The structures of many plant LRR-RK receptors in complex with corresponding ligands have been solved by X-ray crystallography, revealing new mechanisms of ligand-induced receptor kinase activation. In this review, we briefly elaborate the peptide ligands, and aim to detail the structures and mechanisms of LRR-RK activation as induced by secreted peptide ligands within plants.

Plant Biology: Building Barriers… in Roots

The Casparian strip is an important barrier regulating water and nutrient uptake into root tissues. New research reveals two peptide signals and their co-receptors play critical roles patterning and maintaining barrier integrity.

Abscisic acid-mediated modifications of radial apoplastic transport pathway play a key role in cadmium uptake in hyperaccumulator Sedum alfredii

Abscisic acid (ABA) is a key phytohormone underlying plant resistance to toxic metals. However, regulatory effects of ABA on apoplastic transport in roots and consequences for uptake of metal ions are poorly understood. Here, we demonstrate how ABA regulates development of apoplastic barriers in roots of two ecotypes of Sedum alfredii and assess effects on cadmium (Cd) uptake. Under Cd treatment, increased endogenous ABA level was detected in roots of nonhyperaccumulating ecotype (NHE) due to up-regulated expressions of ABA biosynthesis genes (SaABA2, SaNCED), but no change was observed in hyperaccumulating ecotype (HE). Simultaneously, endodermal Casparian strips (CSs) and suberin lamellae (SL) were deposited closer to root tips of NHE compared with HE. Interestingly, the vessel-to-CSs overlap was identified as an ABA-driven anatomical trait. Results of correlation analyses and exogenous applications of ABA/Abamine indicate that ABA regulates development of both types of apoplastic barriers through promoting activities of phenylalanine ammonialyase, peroxidase, and expressions of suberin-related genes (SaCYP86A1, SaGPAT5, and SaKCS20). Using scanning ion-selected electrode technique and PTS tracer confirmed that ABA-promoted deposition of CSs and SL significantly reduced Cd entrance into root stele. Therefore, maintenance of low ABA levels in HE minimized deposition of apoplastic barriers and allowed maximization of Cd uptake via apoplastic pathway.

Surveillance of cell wall diffusion barrier integrity modulates water and solute transport in plants

The endodermis is a key cell layer in plant roots that contributes to the controlled uptake of water and mineral nutrients into plants. In order to provide such functionality the endodermal cell wall has specific chemical modifications consisting of lignin bands (Casparian strips) that encircle each cell, and deposition of a waxy-like substance (suberin) between the wall and the plasma membrane. These two extracellular deposits provide control of diffusion enabling the endodermis to direct the movement of water and solutes into and out of the vascular system in roots. Loss of integrity of the Casparian strip-based apoplastic barrier is sensed by the leakage of a small peptide from the stele into the cortex. Here, we report that such sensing of barrier integrity leads to the rebalancing of water and mineral nutrient uptake, compensating for breakage of Casparian strips. This rebalancing involves both a reduction in root hydraulic conductivity driven by deactivation of aquaporins, and downstream limitation of ion leakage through deposition of suberin. These responses in the root are also coupled to a reduction in water demand in the shoot mediated by ABA-dependent stomatal closure.

The Root Cap Cuticle: A Cell Wall Structure for Seedling Establishment and Lateral Root Formation

The root cap surrounding the tip of plant roots is thought to protect the delicate stem cells in the root meristem. We discovered that the first layer of root cap cells is covered by an electron-opaque cell wall modification resembling a plant cuticle. Cuticles are polyester-based protective structures considered exclusive to aerial plant organs. Mutations in cutin biosynthesis genes affect the composition and ultrastructure of this cuticular structure, confirming its cutin-like characteristics. Strikingly, targeted degradation of the root cap cuticle causes a hypersensitivity to abiotic stresses during seedling establishment. Furthermore, lateral root primordia also display a cuticle that, when defective, causes delayed outgrowth and organ deformations, suggesting that it facilitates lateral root emergence. Our results show that the previously unrecognized root cap cuticle protects the root meristem during the critical phase of seedling establishment and promotes the efficient formation of lateral roots.

Sieve Plate Pores in the Phloem and the Unknowns of Their Formation

Sieve pores of the sieve plates connect neighboring sieve elements to form the conducting sieve tubes of the phloem. Sieve pores are critical for phloem function. From the 1950s onwards, when electron microscopes became increasingly available, the study of their formation had been a pillar of phloem research. More recent work on sieve elements instead has largely focused on sieve tube hydraulics, phylogeny, and eco-physiology. Additionally, advanced molecular and genetic tools available for the model species Arabidopsis thaliana helped decipher several key regulatory mechanisms of early phloem development. Yet, the downstream differentiation processes which form the conductive sieve tube are still largely unknown, and our understanding of sieve pore formation has only moderately progressed. Here, we summarize our current knowledge on sieve pore formation and present relevant recent advances in related fields such as sieve element evolution, physiology, and plasmodesmata formation.

Modulation of the Root Microbiome by Plant Molecules: The Basis for Targeted Disease Suppression and Plant Growth Promotion

Plants host a mesmerizing diversity of microbes inside and around their roots, known as the microbiome. The microbiome is composed mostly of fungi, bacteria, oomycetes, and archaea that can be either pathogenic or beneficial for plant health and fitness. To grow healthy, plants need to surveil soil niches around the roots for the detection of pathogenic microbes, and in parallel maximize the services of beneficial microbes in nutrients uptake and growth promotion. Plants employ a palette of mechanisms to modulate their microbiome including structural modifications, the exudation of secondary metabolites and the coordinated action of different defence responses. Here, we review the current understanding on the composition and activity of the root microbiome and how different plant molecules can shape the structure of the root-associated microbial communities. Examples are given on interactions that occur in the rhizosphere between plants and soilborne fungi. We also present some well-established examples of microbiome harnessing to highlight how plants can maximize their fitness by selecting their microbiome. Understanding how plants manipulate their microbiome can aid in the design of next-generation microbial inoculants for targeted disease suppression and enhanced plant growth.

A Missing Link in Radial Ion Transport: Ion Transporters in the Endodermis

In higher plants, roots have important functions, such as the acquisition of water and ions, as well as transportation into the aerial parts of the plant via the xylem vessels. Radial ion transport in the root is strongly regulated in the endodermis, which is characterized by the presence of the Casparian strip (CS) and suberin lamellae. Although tremendous progress has been made with regard to the ion transporters and endodermal cells, little is known about the relationship between the ion transporters in the endodermis and ion homeostasis in aboveground tissues. This review summarizes the current knowledge about the location of the ion transporters (or channels) in the endodermis. We mainly discuss the effects of mutants related to the CS and/or suberin formation on the role of endodermal ion transporters in ion homeostasis. We also wish to emphasize the critical role of endodermal ion transporters in the regulation of radial ion transport in the root.

Root and cell hydraulic conductivity, apoplastic barriers and aquaporin gene expression in barley (Hordeum vulgare L.) grown with low supply of potassium

BACKGROUND AND AIMS

Limited supply of mineral nutrients often reduces plant growth and transpirational water flow while increasing the ratio of water-absorbing root to water-losing shoot surface. This could potentially lead to an imbalance between water uptake (too much) and water loss (too little). The aim of the present study was to test whether, as a countermeasure, the hydraulic properties (hydraulic conductivity, Lp) of roots decrease at organ and cell level and whether any decreases in Lp are accompanied by decreases in the gene expression level of aquaporins (AQPs) or increases in apoplastic barriers to radial water movement.

METHODS

Barley plants were grown hydroponically on complete nutrient solution, containing 2 mm K+ (100 %), or on low-K solution (0.05 mm K+; 2.5 %), and analysed when they were 15-18 d old. Transpiration, fresh weight, surface area, shoot water potential (ψ), K and Ca concentrations, root (exudation) and cortex cell Lp (cell pressure probe), root anatomy (cross-sections) and AQP gene expression (qPCR) were analysed.

KEY RESULTS

The surface area ratio of root to shoot increased significantly in response to low K. This was accompanied by a small decrease in the rate of water loss per unit shoot surface area, but a large (~50 %) and significant decrease in Lp at root and cortex cell levels. Aquaporin gene expression in roots did not change significantly, due to some considerable batch-to-batch variation in expression response, though HvPIP2;5 expression decreased on average by almost 50 %. Apoplastic barriers in the endodermis did not increase in response to low K.

CONCLUSIONS

Barley plants that are exposed to low K adjust to an increased ratio of root (water uptake) to shoot (water loss) surface primarily through a decrease in root and cell Lp. Reduced gene expression of HvPIP2;5 may contribute to the decrease in Lp.

Plant epithelia: What is the role of the mortar in the wall?

In a growing plant root, the inner vascular system is sealed off by an epithelium, the endodermis. The space between all of the cells in the endodermal layer is filled with an impermeable mass called the Casparian strip, which closes the spaces between cells in the endodermal layer. The role of the Casparian strip has been proposed to prevent backflow of water and nutrients into the soil, but as mutant plants lacking the Casparian strip only have weak phenotypes, the view that it serves an essential function in plants has been challenged. In an accompanying paper, it is shown that loss of the Casparian strip impacts the ability of the plant to take up ammonium and allocate it to the shoots.

What is the function of the Casparian strip, a plant structure first described in 1865? This Primer explores the implications of a new study which reports that loss of the Casparian strip in root endodermis affects the ability of the plant to take up ammonium and allocate it to the shoots.

The Casparian strip-one ring to bring cell biology to lignification?

Lignin research has long been motivated by the outstanding importance of wood for human societies. The annual, non-woody Arabidopsis thaliana, has nevertheless contributed greatly to our understanding of lignification, due to its unrivalled genetic resources. Arabidopsis is also great for cell and developmental biology, allowing precise imaging and tracking of cell types. Root endodermis differentiation involves the precise lignification of the Casparian Strip, as an apoplastic barrier; while barrier damage triggers a less localized, compensatory lignification. Transcriptional reprogramming and peptide-induced signalling emerge as promising tools for the study of endodermal lignification. We argue that endodermis lignification is an attractive model complementary to equally powerful, cellular xylem differentiation systems, as it might better represent the restricted - often localized - lignification seen in non-vascular cells.

Divide and Rule: Plant Plasma Membrane Organization

Since the publication of the fluid mosaic as a relevant model for biological membranes, accumulating evidence has revealed the outstanding complexity of the composition and organization of the plant plasma membrane (PM). Powerful new methodologies have uncovered the remarkable multiscale and multicomponent heterogeneity of PM subcompartmentalization, and this is emerging as a general trait with different features and properties. It is now evident that the dynamics of such a complex organization are intrinsically related to signaling pathways that regulate key physiological processes. Listing and linking recent progress in precisely qualifying these heterogeneities will help to draw an integrated picture of the plant PM. Understanding the key principles governing such a complex dynamic organization will contribute to deciphering the crucial role of the PM in cell physiology.

Plant Biology: The Making of an Epithelium

The endodermis of plants is surrounded by an impermeable belt, the casparian strip, making it a highly selective barrier for nutrient uptake. Successful reprogramming of non-endodermal cells to form a casparian strip requires two signals produced by the vasculature.

Feeding the Walls: How Does Nutrient Availability Regulate Cell Wall Composition?

Nutrients are critical for plants to grow and develop, and nutrient depletion severely affects crop yield. In order to optimize nutrient acquisition, plants adapt their growth and root architecture. Changes in growth are determined by modifications in the cell walls surrounding every plant cell. The plant cell wall, which is largely composed of complex polysaccharides, is essential for plants to attain their shape and to protect cells against the environment. Within the cell wall, cellulose strands form microfibrils that act as a framework for other wall components, including hemicelluloses, pectins, proteins, and, in some cases, callose, lignin, and suberin. Cell wall composition varies, depending on cell and tissue type. It is governed by synthesis, deposition and remodeling of wall components, and determines the physical and structural properties of the cell wall. How nutrient status affects cell wall synthesis and organization, and thus plant growth and morphology, remains poorly understood. In this review, we aim to summarize and synthesize research on the adaptation of root cell walls in response to nutrient availability and the potential role of cell walls in nutrient sensing.

Regulation of Division and Differentiation of Plant Stem Cells

A major challenge in developmental biology is unraveling the precise regulation of plant stem cell maintenance and the transition to a fully differentiated cell. In this review, we highlight major themes coordinating the acquisition of cell identity and subsequent differentiation in plants. Plant cells are immobile and establish position-dependent cell lineages that rely heavily on external cues. Central players are the hormones auxin and cytokinin, which balance cell division and differentiation during organogenesis. Transcription factors and miRNAs, many of which are mobile in plants, establish gene regulatory networks that communicate cell position and fate. Small peptide signaling also provides positional cues as new cell types emerge from stem cell division and progress through differentiation. These pathways recruit similar players for patterning different organs, emphasizing the modular nature of gene regulatory networks. Finally, we speculate on the outstanding questions in the field and discuss how they may be addressed by emerging technologies.

Minimum requirements for changing and maintaining endodermis cell identity in the Arabidopsis root

Changes in gene regulation during differentiation are governed by networks of transcription factors. The Arabidopsis root endodermis is a tractable model to address how transcription factors contribute to differentiation. We used a bottom-up approach to understand the extent to which transcription factors that are required for endodermis differentiation can confer endodermis identity to a non-native cell type. Our results show that the transcription factors SHORTROOT and MYB36 alone have limited ability to induce ectopic endodermal features in the absence of additional cues. The stele-derived signalling peptide CIF2 stabilizes SHORTROOT-induced endodermis identity acquisition. The outcome is a partially impermeable barrier deposited in the subepidermal cell layer, which has a transcriptional signature similar to the endodermis. These results demonstrate that other root cell types can be forced to differentiate into the endodermis and highlight a previously unappreciated role for receptor kinase signalling in maintaining endodermis identity.

Root cell wall solutions for crop plants in saline soils

The root growth of most crop plants is inhibited by soil salinity. Roots respond by modulating metabolism, gene expression and protein activity, which results in changes in cell wall composition, transport processes, cell size and shape, and root architecture. Here, we focus on the effects of salt stress on cell wall modifying enzymes, cellulose microfibril orientation and non-cellulosic polysaccharide deposition in root elongation zones, as important determinants of inhibition of root elongation, and highlight cell wall changes linked to tolerance to salt stressed and water limited roots. Salt stress induces changes in the wall composition of specific root cell types, including the increased deposition of lignin and suberin in endodermal and exodermal cells. These changes can benefit the plant by preventing water loss and altering ion transport pathways. We suggest that binding of Na⁺ ions to cell wall components might influence the passage of Na⁺ and that Na⁺ can influence the binding of other ions and hinder the function of pectin during cell growth. Naturally occurring differences in cell wall structure may provide new resources for breeding crops that are more salt tolerant.

Diffusible repression of cytokinin signalling produces endodermal symmetry and passage cells

In vascular plants, the root endodermis surrounds the central vasculature as a protective sheath that is analogous to the polarized epithelium in animals, and contains ring-shaped Casparian strips that restrict diffusion. After an initial lag phase, individual endodermal cells suberize in an apparently random fashion to produce 'patchy' suberization that eventually generates a zone of continuous suberin deposition. Casparian strips and suberin lamellae affect paracellular and transcellular transport, respectively. Most angiosperms maintain some isolated cells in an unsuberized state as so-called 'passage cells', which have previously been suggested to enable uptake across an otherwise-impermeable endodermal barrier. Here we demonstrate that these passage cells are late emanations of a meristematic patterning process that reads out the underlying non-radial symmetry of the vasculature. This process is mediated by the non-cell-autonomous repression of cytokinin signalling in the root meristem, and leads to distinct phloem- and xylem-pole-associated endodermal cells. The latter cells can resist abscisic acid-dependent suberization to produce passage cells. Our data further demonstrate that, during meristematic patterning, xylem-pole-associated endodermal cells can dynamically alter passage-cell numbers in response to nutrient status, and that passage cells express transporters and locally affect the expression of transporters in adjacent cortical cells.

Exodermis and endodermis are the sites of xanthone biosynthesis in Hypericum perforatum roots

Xanthones are specialized metabolites with antimicrobial properties, which accumulate in roots of Hypericum perforatum. This medicinal plant provides widely taken remedies for depressive episodes and skin disorders. Owing to the array of pharmacological activities, xanthone derivatives attract attention for drug design. Little is known about the sites of biosynthesis and accumulation of xanthones in roots. Xanthone biosynthesis is localized at the transcript, protein, and product levels using in situ mRNA hybridization, indirect immunofluorescence detection, and high lateral and mass resolution mass spectrometry imaging (AP-SMALDI-FT-Orbitrap MSI), respectively. The carbon skeleton of xanthones is formed by benzophenone synthase (BPS), for which a cDNA was cloned from root cultures of H. perforatum var. angustifolium. Both the BPS protein and the BPS transcripts are localized to the exodermis and the endodermis of roots. The xanthone compounds as the BPS products are detected in the same tissues. The exodermis and the endodermis, which are the outermost and innermost cell layers of the root cortex, respectively, are not only highly specialized barriers for controlling the passage of water and solutes but also preformed lines of defence against soilborne pathogens and predators.

Bundle sheath lignification mediates the linkage of leaf hydraulics and venation

The lignification of the leaf vein bundle sheath (BS) has been observed in many species and would reduce conductance from xylem to mesophyll. We hypothesized that lignification of the BS in lower-order veins would provide benefits for water delivery through the vein hierarchy but that the lignification of higher-order veins would limit transport capacity from xylem to mesophyll and leaf hydraulic conductance (K_leaf ). We further hypothesized that BS lignification would mediate the relationship of K_leaf to vein length per area. We analysed the dependence of K_leaf , and its light response, on the lignification of the BS across vein orders for 11 angiosperm tree species. Eight of 11 species had lignin deposits in the BS of the midrib, and two species additionally only in their secondary veins, and for six species up to their minor veins. Species with lignification of minor veins had a lower hydraulic conductance of xylem and outside-xylem pathways and lower K_leaf . K_leaf could be strongly predicted by vein length per area and highest lignified vein order (R²  = .69). The light-response of K_leaf was statistically independent of BS lignification. The lignification of the BS is an important determinant of species variation in leaf and thus whole plant water transport.

You Shall Not Pass: Root Vacuoles as a Symplastic Checkpoint for Metal Translocation to Shoots and Possible Application to Grain Nutritional Quality

Plant nutrient uptake is performed mostly by roots, which have to acquire nutrients while avoiding excessive amounts of essential and toxic elements. Apoplastic barriers such as the casparian strip and suberin deposition block free diffusion from the rhizosphere into the xylem, making selective plasma membrane transporters able to control elemental influx into the root symplast, efflux into the xylem and therefore shoot translocation. Additionally, transporters localized to the tonoplast of root cells have been demonstrated to regulate the shoot ionome, and may be important for seed elemental translocation. Here we review the role of vacuolar transporters in the detoxification of elements such as zinc (Zn), manganese (Mn), cadmium (Cd), cobalt (Co) and nickel (Ni) that are co-transported with iron (Fe) during the Fe deficiency response in Arabidopsis thaliana, and the possible conservation of this mechanism in rice (Oryza sativa). We also discuss the evidence that vacuolar transporters are linked to natural variation in shoot ionome in Arabidopsis and rice, indicating that vacuolar storage might be amenable to genetic engineering without strong phenotypical changes. Finally, we discuss the possible use of root's vacuolar transporters to increase the nutritional quality of crop grains.

Continuous root xylem formation and vascular acclimation to water deficit involves endodermal ABA signalling via miR165

The plant root xylem comprises a specialized tissue for water distribution to the shoot. Despite its importance, its potential morphological plasticity in response to environmental conditions such as limited water availability has not been thoroughly studied. Here, we identify a role for the phytohormone abscisic acid (ABA) for proper xylem development and describe how ABA signalling-mediated effects on core developmental regulators are employed to alter xylem morphology under limited water availability in Arabidopsis. Plants with impaired ABA biosynthesis and reduced ABA signalling in the cell layer surrounding the vasculature displayed defects in xylem continuity, suggesting that non-cell autonomous ABA signalling is required for proper xylem development. Conversely, upon external ABA application or under limited water availability extra xylem strands were formed. The observed xylem developmental alterations were dependent on adequate endodermal ABA signalling, which activated MIR165A. This resulted in increased miR165 levels that repress target class III HD-ZIP transcription factors in the stele. We conclude that a pathway known to control core developmental features is employed as a means of modifying plant xylem morphology under conditions of environmental stress.

Exploring peptide hormones in plants: identification of four peptide hormone-receptor pairs and two post-translational modification enzymes

The identification of hormones and their receptors in multicellular organisms is one of the most exciting research areas and has lead to breakthroughs in understanding how their growth and development are regulated. In particular, peptide hormones offer advantages as cell-to-cell signals in that they can be synthesized rapidly and have the greatest diversity in their structure and function. Peptides often undergo post-translational modifications and proteolytic processing to generate small oligopeptide hormones. In plants, such small post-translationally modified peptides constitute the largest group of peptide hormones. We initially explored this type of peptide hormone using bioassay-guided fractionation and later by in silico gene screening coupled with biochemical peptide detection, which led to the identification of four types of novel peptide hormones in plants. We also identified specific receptors for these peptides and transferases required for their post-translational modification. This review summarizes how we discovered these peptide hormone-receptor pairs and post-translational modification enzymes, and how these molecules function in plant growth, development and environmental adaptation.

In vivo cytological and chemical analysis of Casparian strips using stimulated Raman scattering microscopy

The Casparian strip, a barrier to the apoplastic movement of solutes from the cortex to the stele, is essential for the exclusion of salts, selective nutrient uptake, and many other processes. To date, extensive studies have focused on the physiological functions of endodermal Casparian strips. However, the chemical deposition nature of Casparian strips, as well as its relevance with respect to diffusion barrier functions, remains to be further elucidated. Here, we revealed three developmental stages of Casparian strips in maize primary roots using a traditional fluorescent staining method. Apoplastic permeability tests demonstrated that the barrier function of Casparian strips is largely related to their developmental stage and the pattern of lignin and suberin deposits. Fourier transform infrared (FTIR) analysis showed that the Casparian strips from the roots exhibited significant absorption bands characteristic of lignin and suberin, implying that the Casparian strips in maize primary roots consist largely of lignin and suberin. Furthermore, we developed a new method for label-free, in vivo structural, and biochemical analysis of Casparian strips based on stimulated Raman scattering (SRS) microscopy. Using SRS microscopy, we found that lignin and suberin accumulate simultaneously during the Casparian strip formation process. Based on these results, we propose a potential application of SRS for the chemical composition analysis of plant Casparian strips in situ.

Spatial Expression and Functional Analysis of Casparian Strip Regulatory Genes in Endodermis Reveals the Conserved Mechanism in Tomato

Casparian strip (CS) is an impregnation of endodermal cell wall, forming an apoplastic diffusion barrier which forces the symplastic and selective transport of nutrients across endodermis. This extracellular structure can be found in the roots of all higher plants and is thought to provide the protection of vascular tissues. In Arabidopsis, a genetic toolbox regulating the formation of Casparian strips has emerged recently. However, Arabidopsis has the stereotypical root which is much simpler than most other plant species. To understand the Casparian strip formation in a more complex root system, we examined CS regulatory pathways in tomato. Our results reveal a spatiotemporally conserved expression pattern of most essential components of CS machinery in tomato. Further functional analyses verify the role of homologous CS genes in the Casparian strip formation in tomato, indicating the functional conservation of CS regulatory cascade in tomato.

Getting to the Roots: A Developmental Genetic View of Root Anatomy and Function From Arabidopsis to Lycophytes

Roots attach plants to the ground and ensure efficient and selective uptake of water and nutrients. These functions are facilitated by the morphological and anatomical structures of the root, formed by the activity of the root apical meristem (RAM) and consecutive patterning and differentiation of specific tissues with distinct functions. Despite the importance of this plant organ, its evolutionary history is not clear, but fossils suggest that roots evolved at least twice, in the lycophyte (clubmosses and their allies) and in the euphyllophyte (ferns and seed plants) lineages. Both lycophyte and euphyllophyte roots grow indeterminately by the action of an apical meristem, which is protected by a root cap. They produce root hairs, and in most species the vascular stele is guarded by a specialized endodermal cell layer. Hence, most of these traits must have evolved independently in these lineages. This raises the question if the development of these apparently analogous tissues is regulated by distinct or homologous genes, independently recruited from a common ancestor of lycophytes and euphyllophytes. Currently, there are few studies of the genetic and molecular regulation of lycophyte and fern roots. Therefore, in this review, we focus on key regulatory networks that operate in root development in the model angiosperm Arabidopsis. We describe current knowledge of the mechanisms governing RAM maintenance as well as patterning and differentiation of tissues, such as the endodermis and the vasculature, and compare with other species. We discuss the importance of comparative analyses of anatomy and morphology of extant and extinct species, along with analyses of gene regulatory networks and, ultimately, gene function in plants holding key phylogenetic positions to test hypotheses of root evolution.