Pattern in the Root Epidermis: An Interplay of Diffusible Signals and Cellular Geometry

The epidermis of roots is composed of hair and non-hair cells. Patterning of this epidermis results from spatially regulated differentiation of these cell types. Root epidermal development in vascular plants may be divided into three broad groups based on the mode of hair development; Type 1: any cell in the epidermis can form a root hair; Type 2: the smaller product of an asymmetric cell division forms a root hair; Type 3: the epidermis is organized into discrete files of hair and non-hair cells. TheArabidopsisroot epidermis is composed of discrete files of hair and non-hair cells (Type 3). Genetic and physiological evidence indicates that ethylene is a positive regulator of hair cell development. Genes with opposite roles in the development of hair cells in the shoot (trichomes) and hair cells in the root have been identified. Plants with presumptive loss of function alleles in theTRANSPARENT TESTA GLABRA (TTG)orGLABRA2(GL2) genes are devoid of trichomes indicating that these genes are positive regulators of trichome development. The development of supernumerary root hair cells in these mutant backgrounds illustrates that these genes are also negative regulators of root hair cell development. A model that explains the spatial pattern of epidermal cell differentiation implicates ethylene or its precursor 1-amino-1-cyclopropane carboxylate as a diffusible signal. Possible roles for theTTGandGL2genes in relation to the ethylene signal are discussed.     

Chemosystematics of Brassicales

The order Brassicales, sensu APG III, belongs to Eurosids, and comprises 17 families and 398 genera. The present work discusses the chemical features of Brassicales through the micromolecular chemical data of its taxa and selected taxonomic markers to assess pertinent affinities between its families by correlating their chemosystematic parameters. Although the chemical data of all families were obtained, the data of Brassicaceae, Capparaceae, and Cleomaceae were the most studied. The chemistry of the Brassicales species is diverse, but it reveals the chemical affinity of its families due to occurrence of flavonoids (35%) and glucosinolates (25%), which were characterized as good chemical markers. The flavonoids consist primarily of flavones and flavonols, presenting a low flavone/flavonol ratio. These micromolecules commonly contain unprotected hydroxyls, which are mainly protected by glucosilation, revealing the basal features of its taxa. In Brassicales, the predominantly allyl glucosinolates are usually found in Brassicaceae, Capparaceae, and Cleomaceae families. In the present study, the results of the chemosystematic analysis confirmed the affinity among the Brassicaceae, Capparaceae, and Cleomaceae families, and supported the concept of their monophyly in the Brassicales order. However, more chemical data of the other families is required to improve the chemosystematic conclusions.     

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.     

Embryology of Koeberlinia (Koeberliniaceae): Evidence for core-Brassicalean affinities

Koeberlinia, comprising a single xerophytic species K. spinosa, had previously been placed in various families, mainly Capparaceae. Current molecular evidence now places it in its own family Koeberliniaceae, thought to be related to the Bataceae/Salvadoraceae among the 17 other families of the Brassicales. We investigated 55 embryological characters of the genus, most of which are not understood yet, and thereby assessed its systematic relationships. Koeberlinia has many embryological features in common with the Capparaceae and seven other core-Brassicalean families (i.e., Brassicaceae, Cleomaceae, Emblingiaceae, Gyrostemonaceae, Pentadiplandraceae, Resedaceae, and Tovariaceae), specifically by possessing a campylotropous ovule with a nonmultiplicative (two-cell-layered) outer integument, reniform seeds with a curved embryo, and a fibrous exotegmen in the mature seed coat. However, Koeberlinia is clearly distinguished from them by a tenuinucellate rather than crassinucellate ovule as previously reported, markedly enlarged apical nucellar epidermal cells, and an "exotestal" seed coat. Embryologically, Koeberlinia resembles neither the Bataceae nor the Salvadoraceae, although only limited embryological data are available for these two families. Embryological evidence thus favors its joining the core Brassicales, but additional molecular analyses and embryological studies on the missing data of the Bataceae and Salvadoraceae are needed for final confirmation of its phylogenetic position.     

Distinct and overlapping roles of single-repeat MYB genes in root epidermal patterning

Cell specification in the root epidermis of Arabidopsis generates a position-dependent pattern of root-hair cells and non-hair cells. Here we conduct a comprehensive analysis of the five members of a single-repeat R3 MYB gene family, including CAPRICE (CPC), TRIPTYCHON (TRY), ENHANCER of TRY and CPC 1, 2, and 3 (ETC1, ETC2, and ETC3), and study their role and functional relationship in root epidermal cell specification. Based on genetic and expression analyses, CPC, TRY and ETC1, but not ETC2 or ETC3, promote the hair cell fate by inhibiting non-hair specification. Further, we find that single-repeat MYB activity is required for epidermal patterning throughout root development, beginning during embryogenesis. We also identify a novel regulatory interaction whereby GLABRA2 (GL2) promotes TRY (but not CPC or ETC1) expression in the root epidermis, which generates a second lateral inhibition feedback loop. Gene fusion experiments combining CPC regulatory elements with protein-coding regions of each single-repeat MYB gene suggest that all five proteins are functionally similar, although TRY and ETC2 exhibit distinctions from CPC/ETC1/ETC3. These results provide new insight into the function of these single-repeat MYBs and suggest that divergence of their regulatory sequences is largely responsible for their distinct roles in epidermal cell patterning.     

Morphological and molecular confirmation of Albugo resedae (Albuginales; Oomycota) as a distinct species from A. candida

The genus Albugo s.str. causes white blister rust on four families of the Brassicales, Brassicaceae, Capparaceae, Cleomaceae, and Resedaceae. Recent phylogenetic studies have revealed that several host specific lineages are present within Albugo on Brassicales, while it was also confirmed that Albugo candida has an exceptionally wide host range which extends from Brassicaceae to Cleomaceae and Capparaceae. The Albugo species infecting the Resedaceae was attributed in monographic studies as well as local floras to either A. resedae or, applying a broader species concept, to A. candida. In the present study, A. resedae specimens were morphologically and molecularly compared to the five Albugo species so far confirmed from Brassicales, A. candida, A. koreana, A. laibachii, A. lepidii, and A. voglmayrii. Both morphological differences of oospore ornamentation and phylogenetic analysis of cox2 mtDNA sequences provided evidence that A. resedae is distinct from A. candida and from the additional four species so far described from Brassicaceae. It thus seems possible that so far unknown factors restrict Albugo candida to Brassicaceae and its sister families, Cleomaceae and Capparaceae.     

Different pathways are involved in phosphate and iron stress-induced alterations of root epidermal cell development

Low bioavailability of phosphorus (P) and iron (Fe) induces morphogenetic changes in roots that lead to a higher surface-to-volume ratio. In Arabidopsis, an enlargement in the absorptive surface area is achieved by an increase in the length and frequency of hairs in roots of Fe- and P-deficient plants. The extra root hairs are often located in positions that are occupied with non-hair cells under normal conditions, i.e. over a tangential wall of underlying cortical cells. An involvement of auxin and ethylene in root epidermis cell development of Fe- and P-deficient plants was inferred from phenotypical analysis of hormone-related Arabidopsis mutants and from the application of substances that interfere with either synthesis, transport, or perception of the hormones. Application of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid or the auxin analog 2,4-D caused a marked increase in root hair density in plants of all growth types and confers a phenotype characteristic of ethylene-overproducing mutants. Hormone insensitivity and application of hormone antagonists inhibited the initiation of extranumerary root hairs induced by Fe deficiency, but did not counteract the formation of extra hairs in response to P deprivation. A model is presented summarizing putative pathways for alterations in root epidermal cell patterning induced by environmental stress.     

Scanning Electron Microscopy of Plant Roots

A glycol methacrylate infiltration and polymerization techniquewas used to prepare clover roots inoculated with Rhizobium forscanning reflection electron microscopy. Root hairs and epidermalcells were coated with many bacteria; some bacteria seemed tobe embedded in the wall surface. Root hair tips were often smoothbut some older root hair surfaces showed a fibrillar meshworkpattern. Small granules c. 0.18 µm diameter were presenton the root hair and epidermal cell walls. The root cap, someroot hairs, and some epidermal cells were covered by an amorphousfilm thought to be the mucigel.     

Brassicales: an update on chromosomal evolution and ancient polyploidy

Brassicales comprise 17 families, c. 400 genera and more than 4600 species. Despite the mustard family (crucifers, Brassicaceae) continuing to be the subject of intensive research, the remaining 16 families are largely under studied. Here I summarize the available data on chromosome number and genome size variation across Brassicales in the context of a robust phylogenetic framework. This analysis has revealed extensive knowledge gaps in karyological data for non-crucifer and species-rich families in particular (i.e., Capparaceae, Cleomaceae, Resedaceae and Tropaeolaceae). A parsimonious interpretation of the combined chromosomal and phylogenetic data set suggests that the ancestral pre-Brassicales genome had 9 or 14 chromosome pairs, later multiplied by the At-β (beta) whole-genome duplication (WGD) to n?=?18 or 28. This WGD was followed by post-polyploid diploidization marked by diversification to 12 or 13 families and independent decreases in chromosome numbers. Family-specific WGDs are proposed to precede the diversification of Capparaceae, Resedaceae and Tropaeolaceae.     

A Possible Role for the Thick-walled Epidermal Cells in the Mycorrhizal Hair Roots of Lysinema ciliatum R. Br. and other Epacridaceae

Hair roots ofLysinema ciliatum R. Br. and some other Epacridaceaehave thick-walled cells in the epidermis. These are preferentiallycolonized with mycorrhizal fungi. Individual epidermal cellscontaining hyphal coils separate at the middle lamella and arereleased into the soil. Other colonized cells remain attachedto the roots, usually in groups, surrounded by bare exodermis,where epidermal cells have either collapsed or been sloughedoff. It is suggested that these colonized thick walled cellscan serve to prolong the mycorrhizal association and to infectnew hair roots as these emerge. The thick wall has a very specializedstructure and composition and could have a number of roles,either acting as a substrate or protective coat or in controllingwater status and uptake. Young hair-roots are surrounded bya mucilage sheath that is similar in appearance to that in Ericaceaeand apparently produced by root cap cells, not the epidermis. Lysinema ciliatum R. Br.; ericoid mycorrhiza; hair root; root cap; cortex; epidermis; exodermis     

Two ways to skin a plant: The analysis of root and shoot epidermal development in Arabidopsis

The post-embryonic architecture of higher plants is derived from the activity of two meristems that are formed in the embryo: the shoot meristem and the root meristem. The epidermis of the shoot is derived from the outermost layer of cells covering the shoot meristem through repeated anticlinal divisions. By contrast, the epidermis of the root is derived from an internal ring of cells, located at the centre of the root meristem, by a precise series of both periclinal and anticlinal divisions. Each epidermis has an independent origin. In Arabidopsis the mature shoot epidermis is composed of a small number of cell types: hair cells (trichomes), stomatal guard cells and other epidermal cells. In shoots, hairs take the form of branched trichomes that are surrounded at their base by a ring of accessory cells in a sheet of epidermal cells. The root epidermis is composed of two cell types: trichoblasts that form root hair cells and atrichoblasts that form non-hair cells. Mutations affecting both the patterning and the morphogenesis of cells in both shoot and root epidermis have recently been described. Most of these mutations affect development in a single epidermis, but at least one, ttg, is involved in development in both epidermal systems.     

The root epidermis of<Emphasis Type="Italic"> Echium plantagineum</Emphasis> L.: a novel type of pattern based on the distribution of short and long root hairs

The great majority of angiosperm species form a group in which either every cell in the root epidermis produces a root hair, or the cells that produce these hairs are randomly distributed. We describe, for the first time, pattern in the root epidermal cells of a species within this group. The seedling root of Echium plantagineum L. (Boraginaceae) has an epidermis in which almost every cell produces a root hair, but these are of two types, short hairs (up to 200 micro m) and long hairs (>200 micro m), which are in separate cell files, with the cells bearing long hairs usually separated by one or two files of cells bearing short hairs; the epidermal cells with the long root hairs are longer than the epidermal cells with the short root hairs. The long root hairs are initiated and develop earlier than the short root hairs. Transverse sections of the region of the root which contains only developing long root hairs show that the hair cells are located above anticlinal walls between underlying cortical cells. We regard the distribution of root epidermal cells in E. plantagineum as a sub-type of this group. We discuss the possible evolution, from this sub-type, of another group that is characterised by hair cells and non-hair cells occurring in separate files.     

Drought-induced Short Roots in Arabidopsis thaliana: Structural Characteristics

In Arabidopsis thaliana, as in other Brassicaceae species, a progressive drought stress induced changes in root morphogenesis: from a threshold plant water deficit, the new emerging roots remain short, hairless and often take a tuberized shape at their base while drought persists. The organization of these drought-induced roots was examined in light microscopy in Arabidopsis thaliana, Columbia wild-type ecotype, and compared to the normal, well-watered lateral roots. The main structural traits were the absence of elongation zone, the arrest of cell cap expansion, the lack of root hairs (despite epidermal differentiation in trichoblasts and atrichoblasts) and the radial enlargement of epidermal and cortical cells. The early differentiation, close to the short root apex, of large and highly lignified metaxylem elements, the absence of starch accumulation in hypertrophied cortical cells appeared to be characteristic of the species Arabidopsis, as compared to other Brassicaceae. These structural alterations are discussed in terms of drought-induced changes in gene expression with regard to similar modifications described in root morphogenesis and root hair-defective Arabidopsis mutants.     

Epidermal Patterning Genes Impose Non-cell Autonomous Cell Size Determination and have Additional Roles in Root Meristem Size Control

The regulation of cellular growth is of vital importance for embryonic and postembryonic patterning. Growth regulation in the epidermis has importance for organ growth rates in roots and shoots, proposing epidermal cells as an interesting model for cellular growth regulation. Here we assessed whether the root epidermis is a suitable model system to address cell size determination. In Arabidopsis thaliana L., root epidermal cells are regularly spaced in neighbouring tricho- (root hair) and atrichoblast (non-hair) cells, showing already distinct cell size regulation in the root meristem. We determined cell sizes in the root meristem and at the onset of cellular elongation, revealing that not only division rates but also cellular shape is distinct in tricho- and atrichoblasts. Intriguingly, epidermal-patterning mutants, failing to define differential vacuolization in neighbouring epidermal cell files, also display non-differential growth. Using these epidermal-patterning mutants, we show that polarized growth behaviour of epidermal tricho- and atrichoblast is interdependent, suggesting non-cell autonomous signals to integrate tissue expansion. Besides the interweaved cell-type-dependent growth mechanism, we reveal an additional role for epidermal patterning genes in root meristem size and organ growth regulation. We conclude that epidermal cells represent a suitable model system to study cell size determination and interdependent tissue growth.