Development of the root pole and cell patterning in Arabidopsis roots

Asymmetry and cell polarity in root development

Within living systems, striking juxtapositions in symmetry and asymmetry can be observed and the superficial appearance of symmetric organization often gives way to cellular asymmetries at higher resolution. It is frequently asymmetry and polarity that fascinate and challenge developmental biologists. In multicellular eukaryotes, cell polarity and asymmetry are essential for diverse cellular, tissue, and organismal level function and physiology and are particularly crucial for developmental processes. In plants, where cells are surrounded by rigid cell walls, asymmetric cell divisions are the foundation of pattern formation and differential cell fate specification. Thus, cellular asymmetry is a key feature of plant biology and in the plant root the consequences of these asymmetries are elegantly displayed. Yet despite the frequency of asymmetric (formative) cell divisions, cell/tissue polarity and the proposed roles for directional signaling in these processes, polarly localized proteins, beyond those involved in auxin or nutrient transport, are exceedingly rare. Indeed, although half of the asymmetric cell divisions in root patterning are oriented parallel to the axis of growth, laterally localized proteins directly involved in patterning are largely missing in action. Here, various asymmetric cell divisions and cellular and structural polarities observed in roots are highlighted and discussed in the context of the proposed roles for positional and/or directional signaling in these processes. The importance of directional signaling and the weight given to polarity in the root-shoot axis is contrasted with how little we currently understand about laterally oriented asymmetry and polarity in the root.

Positional signaling and expression of ENHANCER OF TRY AND CPC1 are tuned to increase root hair density in response to phosphate deficiency in Arabidopsis thaliana

Phosphate (Pi) deficiency induces a multitude of responses aimed at improving the acquisition of Pi, including an increased density of root hairs. To understand the mechanisms involved in Pi deficiency-induced alterations of the root hair phenotype in Arabidopsis (Arabidopsis thaliana), we analyzed the patterning and length of root epidermal cells under control and Pi-deficient conditions in wild-type plants and in four mutants defective in the expression of master regulators of cell fate, CAPRICE (CPC), ENHANCER OF TRY AND CPC 1 (ETC1), WEREWOLF (WER) and SCRAMBLED (SCM). From this analysis we deduced that the longitudinal cell length of root epidermal cells is dependent on the correct perception of a positional signal ('cortical bias') in both control and Pi-deficient plants; mutants defective in the receptor of the signal, SCM, produced short cells characteristic of root hair-forming cells (trichoblasts). Simulating the effect of cortical bias on the time-evolving probability of cell fate supports a scenario in which a compromised positional signal delays the time point at which non-hair cells opt out the default trichoblast pathway, resulting in short, trichoblast-like non-hair cells. Collectively, our data show that Pi-deficient plants increase root hair density by the formation of shorter cells, resulting in a higher frequency of hairs per unit root length, and additional trichoblast cell fate assignment via increased expression of ETC1.

Endodermal cell-cell contact is required for the spatial control of Casparian band development in Arabidopsis thaliana

BACKGROUND AND AIMS

Apoplasmic barriers in plants fulfil important roles such as the control of apoplasmic movement of substances and the protection against invasion of pathogens. The aim of this study was to describe the development of apoplasmic barriers (Casparian bands and suberin lamellae) in endodermal cells of Arabidopsis thaliana primary root and during lateral root initiation.

METHODS

Modifications of the endodermal cell walls in roots of wild-type Landsberg erecta (Ler) and mutants with defective endodermal development - scarecrow-3 (scr-3) and shortroot (shr) - of A. thaliana plants were characterized by light, fluorescent, confocal laser scanning, transmission and cryo-scanning electron microscopy.

KEY RESULTS

In wild-type plant roots Casparian bands initiate at approx. 1600 µm from the root cap junction and suberin lamellae first appear on the inner primary cell walls at approx. 7000-8000 µm from the root apex in the region of developing lateral root primordia. When a single cell replaces a pair of endodermal and cortical cells in the scr-3 mutant, Casparian band-like material is deposited ectopically at the junction between this 'cortical' cell and adjacent pericycle cells. Shr mutant roots with an undeveloped endodermis deposit Casparian band-like material in patches in the middle lamellae of cells of the vascular cylinder. Endodermal cells in the vicinity of developing lateral root primordia develop suberin lamellae earlier, and these are thicker, compared wih the neighbouring endodermal cells. Protruding primordia are protected by an endodermal pocket covered by suberin lamellae.

CONCLUSIONS

The data suggest that endodermal cell-cell contact is required for the spatial control of Casparian band development. Additionally, the endodermal cells form a collet (collar) of short cells covered by a thick suberin layer at the base of lateral root, which may serve as a barrier constituting a 'safety zone' protecting the vascular cylinder against uncontrolled movement of water, solutes or various pathogens.

Temperature compensation of auxin dependent developmental patterning

The establishment of localized auxin gradients plays a central role in developmental patterning in plants. Auxin levels and responses have been shown to increase with temperature although developmental patterning is not affected. This suggests the existence of a homeostatic mechanism that ensures that patterning occurs normally over a range of temperatures. We recently described the cloning and characterization of BOBBER1 (BOB1), an Arabidopsis gene which encodes a small heat shock protein. BOB1 is required for the establishment of auxin gradients and for normal developmental patterning. BOB1 is also required for organismal thermotolerance and localizes to heat shock granules at elevated temperatures. Since BOB1 functions in both temperature responses and developmental patterning we propose that BOB1 may encode a component of a developmental temperature compensation mechanism.

Plants and arbuscular mycorrhizal fungi: an evolutionary-developmental perspective

Arbuscular mycorrhizas (AMs) are widespread symbiotic associations that are commonly described as the result of co-evolution events between fungi and plants where both partners benefit from the reciprocal nutrient exchange. Here, we review data from fossil records, characterizations of AM fungi in basal plants and live cell imaging of angiosperm colonization processes from an evolutionary-developmental perspective. The uniformity of plant cell responses to AM colonization in haploid gametophytes and diploid sporophytes, in non-root organs, and throughout many seed plant clades highlights the ancient origin of the interaction and suggests the existence of common molecular and cellular processes. The possibility that pre-existing mechanisms involved in plant cell division were recruited by plants to accommodate AM fungi is discussed.

AAP1 transports uncharged amino acids into roots of Arabidopsis

Amino acids are available to plants in some soils in significant amounts, and plants frequently make use of these nitrogen sources. The goal of this study was to identify transporters involved in the uptake of amino acids into root cells. Based on the fact that high concentrations of amino acids inhibit plant growth, we hypothesized that mutants tolerating toxic levels of amino acids might be deficient in the uptake of amino acids from the environment. To test this hypothesis, we employed a forward genetic screen for Arabidopsis thaliana mutants tolerating toxic concentrations of amino acids in the media. We identified an Arabidopsis mutant that is deficient in the amino acid permease 1 (AAP1, At1g58360) and resistant to 10 mm phenylalanine and a range of other amino acids. The transporter was localized to the plasma membrane of root epidermal cells, root hairs, and throughout the root tip of Arabidopsis. Feeding experiments with [(14)C]-labeled neutral, acidic and basic amino acids showed significantly reduced uptake of amino acids in the mutant, underscoring that increased tolerance of aap1 to high levels of amino acids is coupled with reduced uptake by the root. The growth and uptake studies identified glutamate, histidine and neutral amino acids, including phenylalanine, as physiological substrates for AAP1, whereas aspartate, lysine and arginine are not. We also demonstrate that AAP1 imports amino acids into root cells when these are supplied at ecologically relevant concentrations. Together, our data indicate an important role of AAP1 for efficient use of nitrogen sources present in the rhizosphere.

Positional information and mobile transcriptional regulators determine cell pattern in the Arabidopsis root epidermis

The root epidermis is a model system for deciphering the mechanism underpinning the formation of cellular pattern in planar groups of cells. The epidermis comprises rows of hair cells (H) and non-hair-bearing epidermal cells (N). Laser ablation and clonal analysis have shown that the fates of epidermal cells are flexible through development and that positional information which may be located in the cell wall or extracellular matrix determines cell fate. A leucine-rich repeat protein called SCRAMBLED is required for the development of cell pattern which may be involved in the perception of positional information. It is proposed that positional signals then initiate the cell-specific expression of a number of transcription factors that complete the patterning process, resulting in the expression of hair-promoting genes in hair cells (H) and their repression in the hairless cells (N).

Stepwise understanding of root development

Recent studies using Arabidopsis propose a framework of root development and pattern formation that can be divided to three processes. First, a positional signal that is delivered from neighboring cells controls the fate of undifferentiated cells. Then, cell fate is fixed through a protein-network that includes various transcription factors. Finally, the expression of a particular gene-set leads to fate-dependent cell differentiation, resulting in oriented cell division, cell specification and cell elongation. In addition, these processes could be modified by chromatin stabilization and protein degradation. We focus on three fundamental patterns of root development, circumferential pattern, radial pattern and proximo-distal pattern, and on novel approaches to identify genes that are responsible for the spatiotemporal regulation of root development.

Differential Expression of AtXTH17, AtXTH18, AtXTH19 and AtXTH20 Genes in Arabidopsis Roots. Physiological Roles in Specification in Cell Wall Construction

Xyloglucan endotransglucosylase/hydrolases (XTHs) are a class of enzymes that are capable of splitting and reconnecting xyloglucan molecules, and are implicated in the construction and restructuring of the cellulose/xyloglucan framework. Thirty-three members of the XTH gene family are found in the genome of Arabidopsis thaliana, but their roles remain unclear. Here, we describe the tissue-specific and growth stage-dependent expression profiles of promoter::GUS fusion constructs for four Arabidopsis XTH genes, AtXTH17, AtXTH18, AtXTH19 and AtXTH20, which are phylogenetically closely related to one another. AtXTH17 and AtXTH18 were expressed in all cell types in the elongating and differentiating region of the root, while AtXTH19 was expressed in the apical dividing and elongating regions, as well as in the differentiation zone, and was up-regulated by auxin. In contrast, AtXTH20 was expressed specifically in vascular tissues in the basal mature region of the root. This expression analysis also disclosed cis-regulatory sequences that are conserved among the four genes, and are responsible for the root-specific expression profile. These results indicate that the four XTH genes, which were generated by gene duplication, have diversified their expression profile within the root in such a way as to take responsibility for particular physiological roles in the cell wall dynamics.

An auxin-responsive SCARECROW-like transcriptional activator interacts with histone deacetylase

Members of the plant-specific GRAS family of putative transcription factors are involved in various aspects of plant development. SCARECROW (SCR) is a member of this protein family and plays a significant role in the radial patterning of both roots and shoots. However, little is known about the regulation of SCR expression and its mode of action in plants. Here, we report on the isolation and characterization of a Brassica napus SCARECROW-like protein, BnSCL1, isolated by selecting for proteins that interact with the Arabidopsis histone deacetylase AtHDA19 in a yeast two-hybrid screen. BnSCL1 contains domains conserved in the GRAS family of proteins, interacts with AtHDA19 through a VHIID domain, and exerts transcription activation of reporter genes . BnSCL1 is expressed predominantly in the roots, where its expression is regulated by auxin, as it also is in shoots and mature leaves. These results indicate that BnSCL1 is a member of the GRAS family, and suggest that its mode of action in plant auxin response may involve interaction with HDA19.

A rooting procedure for lentil (Lens culinaris Medik.) and other hypogeous legumes (pea, chickpea and Lathyrus) based on explant polarity

The present study assessed the rooting response of lentil nodal segments in relation to explant polarity, hormone, salt and carbohydrate concentrations of the medium. Nodal segments of lentil with an axillary bud cultured in an inverted orientation (apical end in medium) showed higher rooting frequencies than explants cultured in a normal orientation (basal end in medium). The highest rooting percentage (95.35%) and average number of shoots regenerated per explant (2.4) were obtained from explants placed in an inverted orientation on Murashige and Skoog (MS) medium salts with 3% sucrose, supplemented with 5 microM indole acetic acid (IAA) and 1 microM kinetin (KN). Reducing or increasing phytohormone concentration did not alter significantly root regeneration of inverted explants. Sucrose at 3% allowed higher root regeneration frequencies compared to 1.5% sucrose. MS full concentration permitted regeneration of longer shoots with more nodes per regenerated shoot, compared to MS half-strength, which regenerated more shoots of shorter length and with less nodes. Inverted nodal segments of other hypogeous legumes (pea, chickpea and Lathyrus) also exhibited higher rooting frequencies than explants cultured in a normal orientation on MS medium with 3% sucrose and supplemented with 5 microM IAA and 1 microM KN. The most novel application of this study is the culture of nodal segments of hypogeous legumes in an inverted orientation. This procedure is a considerable improvement over other published procedures concerning in vitro rooting of lentil, pea, chickpea and Lathyrus.

How and where to build a root hair

The root hair of Arabidopsis has become a model system for investigations of the patterning and morphogenesis of cells in plants. A cascade of transcriptional regulators controls the pattern of cellular differentiation. Recently, one of the genes that plays a specific role in cellular differentiation in roots, WEREWOLF, has been shown to be functionally equivalent to GLABRA1, which functions only in the shoot. The cloning of genes defined by mutants with defective root-hair growth has provided insights into the roles of the cell wall, ion transport and the cytoskeleton during hair growth. Genetic analyses continue to identify mutants that will be instructive in furthering our understanding of the growth and development of root-hair cells.

Maize VP1 complements Arabidopsis abi3 and confers a novel ABA/auxin interaction in roots

The maize Vp1 gene and abi3 gene of Arabidopsis are believed to be orthologs based on similarities of the mutant phenotypes and amino acid sequence conservation. Here we show that expression of VP1 driven by the 35S promoter can partially complement abi3-6, a deletion mutant allele of abi3. The visible phenotype of seed produced from VP1 expression in the abi3 mutant background is nearly indistinguishable from wild type. VP1 fully restores abscisic acid (ABA) sensitivity of abi3 during seed germination and suppresses the early flowering phenotype of abi3. The temporal regulation of C1-beta-glucuronidase (GUS) and chlorophyll a/b binding protein (cab3)-GUS reporter genes in developing seeds of 35S-VP1 lines were similar to wild type. On the other hand, two qualitative differences are observed between the 35S-VP1 line and wild type. The levels of CRC and C1-GUS expression are markedly lower in the seeds of 35S-VP1 lines than in wild type suggesting incomplete complementation of gene activation functions. Similar to ectopic expression of ABI3 (Parcy et al., 1994), ectopic expression of VP1 in vegetative tissue enhances ABA inhibition of root growth. In addition, 35S-VP1 confers strong ABA inducible expression of the normally seed-specific cruciferin C (CRC) gene in leaves. In contrast, ectopic ABA induction of C1-GUS is restricted to a localized region of the root elongation zone. The ABA-dependent C1-GUS expression expanded to a broader area in the root tissues treated with exogenous application of auxin. Interestingly, auxin-induced lateral root formation is completely suppressed by ABA in 35S-VP1 plants but not in wild type. These results indicate VP1 mediates a novel interaction between ABA and auxin signaling that results in developmental arrest and altered patterns of gene expression.

Xylogenesis: the birth of a corpse

Xylogenesis is a complex developmental process culminating in programmed cell death as a truly terminal differentiation event. In Arabidopsis, the availability of vascular-patterning mutants, and the identification of genes and their products that are involved in cell specification, secondary-wall deposition and lignification, are providing clues to the functions of some of the sequences in the large expressed sequence tag databases derived from the xylem-rich tissues of trees. An in vitro system, the Zinnia mesophyll cell system, provides an alternative system for those cell-biological experiments that are difficult to tackle in intact plants. In particular, a combination of molecular-genetic and cell-biological approaches has made possible the elucidation of some of the features of plant programmed cell death.