Cellular and physiological functions of SGR family in gravitropic response in higher plants

BACKGROUND

In plants, gravity directs bidirectional growth; it specifies upward growth of shoots and downward growth of roots. Due to gravity, roots establish robust anchorage and shoot, which enables to photosynthesize. It sets optimum posture and develops plant architecture to efficiently use resources like water, nutrients, CO2, and gaseous exchange. Hence, gravitropism is crucial for crop productivity as well as for the growth of plants in challenging climate. Some SGR members are known to affect tiller and shoot angle, organ size, and inflorescence stem in plants.

AIM OF REVIEW

Although the SHOOT GRAVITROPISM (SGR) family plays a key role in regulating the fate of shoot gravitropism, little is known about its function compared to other proteins involved in gravity response in plant cells and tissues. Moreover, less information on the SGR family's physiological activities and biochemical responses in shoot gravitropism is available. This review scrutinizes and highlights the recent developments in shoot gravitropism and provides an outlook for future crop development, multi-application scenarios, and translational research to improve agricultural productivity.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Plants have evolved multiple gene families specialized in gravitropic responses, of which the SGR family is highly significant. The SGR family regulates the plant's gravity response by regulating specific physiological and biochemical processes such as transcription, cell division, amyloplast sedimentation, endodermis development, and vacuole formation. Here, we analyze the latest discoveries in shoot gravitropism with particular attention to SGR proteins in plant cell biology, cellular physiology, and homeostasis. Plant cells detect gravity signals by sedimentation of amyloplast (starch granules) in the direction of gravity, and the signaling cascade begins. Gravity sensing, signaling, and auxin redistribution (organ curvature) are the three components of plant gravitropism. Eventually, we focus on the role of multiple SGR genes in shoot and present a complete update on the participation of SGR family members in gravity.

Raman developmental markers in root cell walls are associated with lodging tendency in tef

MAIN CONCLUSION

Using Raman micro-spectroscopy on tef roots, we could monitor cell wall maturation in lines with varied genetic lodging tendency. We describe the developing cell wall composition in root endodermis and cylinder tissue. Tef [Eragrostis tef (Zucc.) Trotter] is an important staple crop in Ethiopia and Eritrea, producing gluten-free and protein-rich grains. However, this crop is not adapted to modern farming practices due to high lodging susceptibility, which prevents the application of mechanical harvest. Lodging describes the displacement of roots (root lodging) or fracture of culms (stem lodging), forcing plants to bend or fall from their vertical position, causing significant yield losses. In this study, we aimed to understand the microstructural properties of crown roots, underlining tef tolerance/susceptibility to lodging. We analyzed plants at 5 and 10 weeks after emergence and compared trellised to lodged plants. Root cross sections from different tef genotypes were characterized by scanning electron microscopy, micro-computed tomography, and Raman micro-spectroscopy. Lodging susceptible genotypes exhibited early tissue maturation, including developed aerenchyma, intensive lignification, and lignin with high levels of crosslinks. A comparison between trellised and lodged plants suggested that lodging itself does not affect the histology of root tissue. Furthermore, cell wall composition along plant maturation was typical to each of the tested genotypes independently of trellising. Our results suggest that it is possible to select lines that exhibit slow maturation of crown roots. Such lines are predicted to show reduction in lodging and facilitate mechanical harvest.

N6-adenosine methylation of mRNA integrates multilevel auxin response and ground tissue development in Arabidopsis

N6-methyl adenosine (m6A) is a widespread internal mRNA modification impacting the expression of numerous genes. Here, we characterize auxin-related defects among the pleiotropic phenotypes of hypomorphic Arabidopsis thaliana mutants with impaired m6A status and reveal that they show strong resistance to exogenously applied auxin. By combining major published m6A datasets, we propose that among high-confidence target transcripts emerge those encoding the main components required for auxin signaling, including the TIR1/AFB auxin receptors and ARF transcriptional regulators. We also observe subtle changes in endogenous levels of indole-3-acetic acid metabolites in these hypomorphic lines, which correlate with the methylation status of indole-3-acetic acid amidohydrolase transcripts. In addition, we reveal that reduced m6A levels lead to defects in endodermal patterning in the primary root arising from impaired timing of periclinal cell divisions. These defects can be reverted by inhibition of auxin signaling. Together, our data underline that m6A likely affects auxin-dependent processes at multiple levels.

Cell-Type-Dependent but CME-Independent Polar Localization of Silicon Transporters in Rice

Silicon (Si) is an important nutrient required for sustainable and high production of rice and its uptake is mediated by a pair of influx (OsLsi1)-efflux (OsLsi2) transporters showing polar localization. However, the mechanisms underlying their polarity are unknown. Here, we revealed that the polarity of the Si transporters depends on cell types. The polar localization of both OsLsi1 and OsLsi2 was not altered by Si supply, but their protein abundance was reduced. Double immunostaining showed that localization of OsLsi1 and OsLsi2 was separated at the edge of the lateral polar domain by Casparian strips in the endodermis, whereas they were slightly overlapped at the transversal side of the exodermis. When OsLsi1 was ectopically expressed in the shoots, it showed polar localization at the xylem parenchyma cells of the basal node and leaf sheath, but not at the phloem companion cells. Ectopic expression of non-polar Si transporters, barley HvLsi2 and maize ZmLsi2 in rice, resulted in their polar localization at the proximal side. The polar localization of OsLsi1 and OsLsi2 was not altered by inhibition of clathrin-mediated endocytosis (CME) by dominant-negative induction of dynamin-related protein1A and knockout of mu subunit of adaptor protein 2 complex, although the knockout mutants of OsAP2M gene showed dwarf phenotype. These results indicate that CME is not required for the polar localization of Si transporters. Taken together, our results indicate that CME-independent machinery controls the polar localization of Si transporters in exodermis, endodermis of root cells and xylem parenchyma cells.

Building and breaking of a barrier: Suberin plasticity and function in the endodermis

Plant cells coated with hydrophobic compounds constitute a protective barrier to control movement of materials through plant tissues. In roots, the endodermis develops two barriers: the Casparian strips establish an apoplastic barrier and suberin lamellae prevent diffusion through the plasma membrane. Suberin is a complex biopolymer and its deposition is highly responsive to the environment. While the enzymatic framework involved in suberin biosynthesis is well characterized, subsequent steps in suberin formation and regulation remained elusive. Recent publications, studying suberin from a cell biological perspective, have enriched our knowledge on suberin transport and polymerization in the cell wall. These studies have also elucidated the molecular mechanisms controlling suberin biosynthesis and regulation as well as its physiological role in plant abiotic and biotic interactions.

Visualizing polymeric components that define distinct root barriers across plant lineages

Hydrophobic cell wall depositions in roots play a key role in plant development and interaction with the soil environment, as they generate barriers that regulate bidirectional nutrient flux. Techniques to label the respective polymers are emerging, but are efficient only in thin roots or sections. Moreover, simultaneous imaging of the barrier constituents lignin and suberin remains problematic owing to their similar chemical compositions. Here, we describe a staining method compatible with single- and multiphoton confocal microscopy that allows for concurrent visualization of primary cell walls and distinct secondary depositions in one workflow. This protocol permits efficient separation of suberin- and lignin-specific signals with high resolution, enabling precise dissection of barrier constituents. Our approach is compatible with imaging of fluorescent proteins, and can thus complement genetic markers or aid the dissection of barriers in biotic root interactions. We further demonstrate applicability in deep root tissues of plant models and crops across phylogenetic lineages. Our optimized toolset will significantly advance our understanding of root barrier dynamics and function, and of their role in plant interactions with the rhizospheric environment.

Confocal Analysis of Arabidopsis Root Cell Divisions in 3D: A Focus on the Endodermis

The development of multicellular organisms requires coordinated cell divisions for the production of diverse cell types and body plan elaboration and growth. There are two main types of cell divisions: proliferative or symmetric divisions, which produce more cells of a given type, and formative or asymmetric divisions, which produce cells of different types. Because plant cells are surrounded by cell walls, the orientation of plant cell divisions is particularly important in cell fate specification and tissue or organ morphology. The cellular organization of the Arabidopsis thaliana root makes an excellent tool to study how oriented cell division contributes to tissue patterning during organ development. To understand how division plane orientation in a specific genotype or growth condition may impact organ or tissue development, a detailed characterization of cell division orientation is required. Here we describe a confocal microscopy-based, live imaging method for Arabidopsis root tips to examine the 3D orientations of cell division planes and quantify formative, proliferative, and atypical endodermal cell divisions.

Multiple effects of silicon on alleviation of nickel toxicity in young maize roots

Although beneficial metalloid silicon (Si) has been shown to alleviate the toxicity of various heavy metals, there is a lack of knowledge about the role of Si in possible alleviation of phytotoxicity caused by excess of essential nickel (Ni). In the present study we investigated the growth and biomass production, reactive oxygen species (ROS) formation and activities of selected antioxidants, as well as combined effect of Ni and Si on the integrity of cell membranes and electrolyte leakage in young maize roots treated for 24, 48 and 72 h with excess of Ni and/or Si. By histochemical methods we also visualized Ni distribution in root tissues and compared the uptake of Ni and Si with the development of root apoplasmic barriers. Ni enhanced the root lignification and suberization and shifted the development of apoplasmic barriers towards the root tip. Similarly, localization of Ni correlated with lignin and suberin deposition in root endodermis, further supporting the barrier role of this tissue in Ni uptake. Si reversed the negative impact of Ni on root anatomy. Additionally, improved cell membrane integrity, and enhanced ascorbate-based antioxidant system might be the mechanisms how Si partially mitigates the deleterious effects of Ni excess in maize plants.

Endogenous nutrients are concentrated in specific tissues in the Zea mays seedling

K, P, Cl, and Ca are distributed in tissue-specific patterns in Zea mays seedlings. These elements were mapped and analyzed using a relatively simple semi-quantitative technique, i.e., fast freezing, followed by freeze fracturing, then freeze drying, and finally scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS). In the radicle, endogenously derived (i.e., from seed) K and P transition from being homogenous in the apical meristem to tissue-specific in older regions. At 3 mm from the radicle apex, K concentration is approximately 40 mM in mid-cortex and decreases by approximately 50% at 15 mm. From 3 to 55 mm, P concentration in pericycle is approximately twice that found in adjacent regions. Ca is not detectable in younger portions of the radicle by SEM/EDS, but in older regions, it is present at 13 mM in mid-cortex. K concentration values of entire radicles analyzed with inductively coupled plasma optical emission spectrometry (ICP-OES) exceeded the SEM/EDS values. For Ca, the reverse was true. But, SEM/EDS analysis did not include several vascular tissues that contained high concentrations of K and low concentrations of Ca. The inception of lateral root primordia was accompanied by a localized decrease in Ca in cortical regions that were centrifugal to the primordium tip. A region of O-rich cells in endosperm was identified centripetal to the aleurone. These results indicate that (1) outer, mid-, and inner cortical regions, as well as the adjacent tissues, have distinct ion accumulation properties, and (2) ions are concentrated in some radicle tissues prior to development of Casparian strips.

A Novel Signaling Pathway Required for Arabidopsis Endodermal Root Organization Shapes the Rhizosphere Microbiome

The Casparian strip (CS) constitutes a physical diffusion barrier to water and nutrients in plant roots, which is formed by the polar deposition of lignin polymer in the endodermis tissue. The precise pattern of lignin deposition is determined by the scaffolding activity of membrane-bound Casparian Strip domain proteins (CASPs), but little is known of the mechanism(s) directing this process. Here, we demonstrate that Endodermis-specific Receptor-like Kinase 1 (ERK1) and, to a lesser extent, ROP Binding Kinase1 (RBK1) are also involved in regulating CS formation, with the former playing an essential role in lignin deposition as well as in the localization of CASP1. We show that ERK1 is localized to the cytoplasm and nucleus of the endodermis and that together with the circadian clock regulator, Time for Coffee (TIC), forms part of a novel signaling pathway necessary for correct CS organization and suberization of the endodermis, with their single or combined loss of function resulting in altered root microbiome composition. In addition, we found that other mutants displaying defects in suberin deposition at the CS also display altered root exudates and microbiome composition. Thus, our work reveals a complex network of signaling factors operating within the root endodermis that establish both the CS diffusion barrier and influence the microbial composition of the rhizosphere.

3D Analysis of Mitosis Distribution Pattern in the Plant Root Tip with iRoCS Toolbox

The protocol allows to define and characterize mitosis distribution patterns in the plant root meristem. The method does not require genetic markers, which makes it applicable to plants of different non-transgenic genotypes, including ecotypes, mutants, and non-model plant species. Computer analysis of the mitosis distribution in three dimensions with iRoCS Toolbox identifies statistically significant changes in proliferation activity within specific root tissues and cell lineages.

Structural modifications of plant organs and tissues by metals and metalloids in the environment: A review

At the dawn of the industrial revolution, the exorbitant use of heavy metals and toxic elements by mankind unfurls a powerful and complex web of hazard all around the world that significantly contributed to unprecedented trends in environmental degradation. Plants as sessile organisms, that cannot escape from the stress directly, have adapted to this environment via concurrent configurations of several traits. Among them the anatomy has been identified as much more advanced field of research that brought the explosion of interest among the expertise and its prodigious importance in stress physiology is unavoidable. In conjunction with various other disciplines, like physiology, biochemistry, genomics and metabolomics, the plant anatomy provides a large data sets that are paving the way towards a comprehensive and holistic understanding of plant growth, development, defense and productivity under heavy metal and toxic element stress. Present paper advances our recent knowledge about structural alterations of plant tissues induced by metals and metalloids, like antimony (Sb), arsenic (As), aluminium (Al), copper (Cu), cadmium (Cd), chromium (Cr), lead (Pb), manganese (Mn), mercury (Hg), nickel (Ni) and zinc (Zn) and points on essential role of plant anatomy and its understanding for plant growth and development in changing environment. Understanding of anatomical adaptations of various plant organs and tissues to heavy metals and metalloids could greatly contribute to integral and modern approach for investigation of plants in changing environmental conditions. These findings are necessary for understanding of the whole spectra of physiological and biochemical reactions in plants and to maintain the crop productivity worldwide. Moreover, our holistic perception regarding the processes underlying the plant responses to metal(loids) at anatomical level are needed for improving crop management and breeding techniques.

Unique lignin modifications pattern the nucleation of silica in sorghum endodermis

Silicon dioxide in the form of hydrated silica is a component of plant tissues that can constitute several percent by dry weight in certain taxa. Nonetheless, the mechanism of plant silica formation is mostly unknown. Silicon (Si) is taken up from the soil by roots in the form of monosilicic acid molecules. The silicic acid is carried in the xylem and subsequently polymerizes in target sites to silica. In roots of sorghum (Sorghum bicolor), silica aggregates form in an orderly pattern along the inner tangential cell walls of endodermis cells. Using Raman microspectroscopy, autofluorescence, and scanning electron microscopy, we investigated the structure and composition of developing aggregates in roots of sorghum seedlings. Putative silica aggregation loci were identified in roots grown under Si starvation. These micrometer-scale spots were constructed of tightly packed modified lignin, and nucleated trace concentrations of silicic acid. Substantial variation in cell wall autofluorescence between Si+ and Si- roots demonstrated the impact of Si on cell wall chemistry. We propose that in Si- roots, the modified lignin cross-linked into the cell wall and lost its ability to nucleate silica. In Si+ roots, silica polymerized on the modified lignin and altered its structure. Our work demonstrates a high degree of control over lignin and silica deposition in cell walls.

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.

SCARECROW promoter-driven expression of a bacterial mercury transporter MerC in root endodermal cells enhances mercury accumulation in Arabidopsis shoots

Mercury accumulation in Arabidopsis shoots is accelerated by endodermis specific expression of fusion proteins of a bacterial mercury transporter MerC and a plant SNARE SYP121 under control of SCARECROW promoter. We previously demonstrated that the CaMV 35S RNA promoter (p35S)-driven ubiquitous expression of a bacterial mercury transporter MerC, fused with SYP121, an Arabidopsis SNARE protein increases mercury accumulation of Arabidopsis. To establish an improved fine-tuned mercury transport system in plants for phytoremediation, the present study generated and characterized transgenic Arabidopsis plants expressing MerC-SYP121 specifically in the root endodermis, which is a crucial cell type for root element uptake. We generated four independent transgenic Arabidopsis lines expressing a transgene encoding mCherry-MerC-SYP121 under the control of the endodermis-specific SCARECROW promoter (hereafter pSCR lines). Quantitative real-time PCR analysis showed that expression levels of the transgene in roots of the pSCR lines were 3-23% of the p35S driven-overexpressing line. Confocal microscopy analysis showed that mCherry-MerC-SYP121 was dominantly expressed in the endodermis of the meristematic zone as well as in the mature zone of the pSCR roots. Mercury accumulation in shoots of the pSCR lines exposed to inorganic mercury was overall higher than the wild-type and comparable to the p35S over-expressing line. These results suggest that endodermis-specific expression of the MerC-SYP121 fusion proteins in plant roots sufficiently enhances mercury uptake and accumulation into shoots, which would be an ideal phenotype for phytoremediation of mercury-contaminated environments.

SHR overexpression induces the formation of supernumerary cell layers with cortex cell identity in rice

The number of root cortex cell layers varies among plants, and many species have several cortical cell layers. We recently demonstrated that the two rice orthologs of the Arabidopsis SHR gene, OsSHR1 and OsSHR2, could complement the A. thaliana shr mutant. Moreover, OsSHR1 and OsSHR2 expression in A. thaliana roots induced the formation of extra root cortical cell layers. In this article, we demonstrate that the overexpression of AtSHR and OsSHR2 in rice roots leads to plants with wide and short roots that contain a high number of extra cortical cell layers. We hypothesize that SHR genes share a conserved function in the control of cortical cell layer division and the number of ground tissue cell layers in land plants.

Stem development through vascular tissues: EPFL-ERECTA family signaling that bounces in and out of phloem

Plant cells communicate with each other using a variety of signaling molecules. Recent studies have revealed that various types of secreted peptides, as well as phytohormones known since long ago, mediate cell-cell communication in diverse contexts of plant life. These peptides affect cellular activities, such as proliferation and cell fate decisions, through their perception by cell surface receptors located on the plasma membrane of target cells. ERECTA (ER), an Arabidopsis thaliana receptor kinase gene, was first identified as a stem growth regulator, and since then an increasing number of studies have shown that ER is involved in a wide range of developmental and physiological processes. In particular, molecular functions of ER have been extensively studied in stomatal patterning. Furthermore, the importance of ER signaling in vascular tissues of inflorescence stems, especially in phloem cells, has recently been highlighted. In this review article, first we briefly summarize the history of ER research including studies on stomatal development, then introduce ER functions in vascular tissues, and discuss its interactions with phytohormones and other receptor kinase signaling pathways. Future questions and challenges will also be addressed.

N availability modulates the role of NPF3.1, a gibberellin transporter, in GA-mediated phenotypes in Arabidopsis

AtNPF3.1 gene expression is promoted by limiting nitrogen nutrition. Atnpf3.1 mutants are affected in hypocotyl elongation and seed germination under conditions of low-nitrate availability. The NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER (NPF) family encodes nitrate or peptides transporters, some of which are also able to transport hormones. AtNPF3.1 has been described as a nitrate/nitrite/gibberellin transporter. Until now only its gibberellins (GAs) transport capacity have been proven in planta. We further analyzed its substrate specificity towards different GA species using a yeast heterologous system which revealed that (1) NPF3.1 transported not only bioactive GAs but also their precursors and metabolites and (2) the GAs' import activity of NPF3.1 was not affected by the presence of exogenous nitrate. Gene expression analysis along with germination assays and hypocotyl length measurements of loss of function mutants was used to understand the in planta role of NPF3.1. GUS staining revealed that this gene is expressed mainly in the endodermis of roots and hypocotyls, in shoots, stamens, and dry seeds. Germination assays in the presence of paclobutrazol, a GA biosynthesis inhibitor, revealed that the germination rate of npf3.1 mutants was lower compared to wild type when GA was added at the same time. Likewise, hypocotyl length measurements showed that the npf3.1 mutants were less sensitive to exogenous GA addition in the presence of paclobutrazol, compared to wild type. Moreover, this phenotype was observed only when plants were grown on low-nitrate supply. In addition, NPF3.1 gene expression was upregulated by low exogenous nitrate concentrations and the npf3.1 mutants exhibited a not yet described GA-related phenotype under these conditions. All together, these results indicated that NPF3.1 is indeed involved in GAs transport in planta under low-nitrate conditions.

Effect and localization of phenanthrene in maize roots

Polycyclic aromatic hydrocarbons (PAHs) have a toxic effect on plants, which limits the efficiency of phytomanagement of contaminated soils. The mechanisms underlying their toxicity are not fully understood. A cultivation experiment was carried out with maize, used as model plant, exposed to sand spiked with phenanthrene (50 or 150 mg kg(-1) dw). Epi-fluorescence microscopic observation of root sections was used to assess suberization of exodermis and endodermis and phenanthrene localization along the primary root length. For 10 days of cultivation, exodermis and endodermis suberization of exposed maize was more extensive. However, after 20 days of exposure, exodermis and endodermis of non-exposed roots were totally suberized, whilst PHE-exposed roots where less suberized. Early extensive suberization may act as barrier against PHE penetration, however longer exposure inhibits root maturation. Phenanthrene patches were located only near suberized exodermis and endodermis, which may therefore act as retention zones, where the hydrophobic phenanthrene accumulates during its radial transport.

Permeability of Plant Young Root Endodermis to Cu Ions and Cu-Citrate Complexes in Corn and Soybean

The non-selective apoplastic passage of Cu and Cu-citrate complexes into the root stele of monocotyledonous corn and dicotyledonous soybean was investigated using an inorganic-salt-precipitation technique. Either Cu ions or Cu-citrate complexes were drawn into root through the apoplast from the root growth medium, and K4[Fe(CN)6] was subsequently perfused through xylem vessels or the entire root cross section. Based on microscopic identification of the reddish-brown precipitates of copper ferrocyanide in the cell walls of the xylem of corn and soybean roots, Cu(2+) passed through the endodermal barrier into the xylem of both species. When the solution containing 200 μM CuSO4 and 400 μM sodium citrate (containing 199.98 μM Cu-citrate, 0.02 μM Cu(2+)) was drawn via differential pressure gradients into the root xylem while being perfused with K4[Fe(CN)6] through the entire root cross-section, reddish-brown precipitates were observed in the walls of the stele of soybean, but not corn root. However, when a CuSO4 solution containing 0.02 or 0.2 μM free Cu(2+) was used, no reddish-brown precipitates were detected in the stele of either of the two plants. Results indicated that endodermis was permeable to Cu-citrate complexes in primary roots of soybean, but not corn. The permeability of the endodermal barrier to the Cu-citrate complex may vary between dicotyledonous and monocotyledonous plants, which has considerable implications for chelant-enhanced phytoextraction.

Fate map of Medicago truncatula root nodules

Legume root nodules are induced by N-fixing rhizobium bacteria that are hosted in an intracellular manner. These nodules are formed by reprogramming differentiated root cells. The model legume Medicago truncatula forms indeterminate nodules with a meristem at their apex. This organ grows by the activity of the meristem that adds cells to the different nodule tissues. In Medicago sativa it has been shown that the nodule meristem is derived from the root middle cortex. During nodule initiation, inner cortical cells and pericycle cells are also mitotically activated. However, whether and how these cells contribute to the mature nodule has not been studied. Here, we produce a nodule fate map that precisely describes the origin of the different nodule tissues based on sequential longitudinal sections and on the use of marker genes that allow the distinction of cells originating from different root tissues. We show that nodule meristem originates from the third cortical layer, while several cell layers of the base of the nodule are directly formed from cells of the inner cortical layers, root endodermis and pericycle. The latter two differentiate into the uninfected tissues that are located at the base of the mature nodule, whereas the cells derived from the inner cortical cell layers form about eight cell layers of infected cells. This nodule fate map has then been used to re-analyse several mutant nodule phenotypes. This showed, among other things, that intracellular release of rhizobia in primordium cells and meristem daughter cells are regulated in a different manner.

A gateway with a guard: how the endodermis regulates growth through hormone signaling

The endodermis is a defining feature of plant roots and is most widely studied as a differentially permeable barrier limiting solute uptake from the soil into the vascular stream. Recent work has revealed that this inner cell layer is also an important signaling center for hormone-mediated control of growth. Auxin, gibberellic acid, abscisic acid and strigalactones all appear to depend on the endodermis to regulate root biology and point to this cell type as having important inter-cell layer regulatory activity, as well. In this review I discuss recent work detailing the importance of the endodermis in growth control and how this function is affected during responses to the environment.

Regulation of plant vascular stem cells by endodermis-derived EPFL-family peptide hormones and phloem-expressed ERECTA-family receptor kinases

Plant vasculatures are complex tissues consisting of (pro)cambium, phloem, and xylem. The (pro)cambium serves as vascular stem cells that produce all vascular cells. The Arabidopsis ERECTA (ER) receptor kinase is known to regulate the architecture of inflorescence stems. It was recently reported that the er mutation enhances a vascular phenotype induced by a mutation of TDR/PXY, which plays a significant role in procambial proliferation, suggesting that ER participates in vascular development. However, detailed molecular mechanisms of the ER-dependent vascular regulation are largely unknown. Here, this work found that ER and its paralogue, ER-LIKE1, were redundantly involved in procambial development of inflorescence stems. Interestingly, their activity in the phloem was sufficient for vascular regulation. Furthermore, two endodermis-derived peptide hormones, EPFL4 and EPFL6, were redundantly involved in such regulation. It has been previously reported that EPFL4 and EPFL6 act as ligands of phloem-expressed ER for stem elongation. Therefore, these findings indicate that cell-cell communication between the endodermis and the phloem plays an important role in procambial development as well as stem elongation. Interestingly, similar EPFL-ER modules control two distinct developmental events by slightly changing their components: the EPFL4/6-ER module for stem elongation and the EPFL4/6-ER/ERL1 module for vascular development.