A root is a root is a root? Water uptake rates of Citrus root orders

Knowledge about the physiological function of root orders is scant. In this study, a system to monitor the water flux among root orders was developed using miniaturized chambers. Different root orders of 4‐year‐old Citrus volkameriana trees were analysed with respect to root morphology and water flux. The eight root orders showed a broad overlap in diameter, but differences in tissue densities and specific root area (SRA) were clearly distinguishable. Thirty per cent of the root branch biomass but 50% of the surface area (SA) was possessed by the first root order, while the fifth accounted for 5% of the SA (20% biomass). The root order was identified as a determinant of water flux. First‐order roots showed a significantly higher rate of water uptake than the second and third root orders, whereas the fourth and fifth root orders showed water excess. The water excess suggested the occurrence of hydraulic redistribution (HR) as a result of differences in osmotic potentials. We suggest that plants may utilize hydraulic redistribution to prevent coarse root desiccation and/or to increase nutrient acquisition. Our study showed that the novel ‘miniature depletion chamber’ method enabled direct measurement of water fluxes per root order and can be a major tool for future studies on root order traits.     

Can root electrical capacitance be used to predict root mass in soil?

Background

Electrical capacitance, measured between an electrode inserted at the base of a plant and an electrode in the rooting substrate, is often linearly correlated with root mass. Electrical capacitance has often been used as an assay for root mass, and is conventionally interpreted using an electrical model in which roots behave as cylindrical capacitors wired in parallel. Recent experiments in hydroponics show that this interpretation is incorrect and a new model has been proposed. Here, the new model is tested in solid substrates.

Methods

The capacitances of compost and soil were determined as a function of water content, and the capacitances of cereal plants growing in sand or potting compost in the glasshouse, or in the field, were measured under contrasting irrigation regimes.

Key Results

Capacitances of compost and soil increased with increasing water content. At water contents approaching field capacity, compost and soil had capacitances at least an order of magnitude greater than those of plant tissues. For plants growing in solid substrates, wetting the substrate locally around the stem base was both necessary and sufficient to record maximum capacitance, which was correlated with stem cross-sectional area: capacitance of excised stem tissue equalled that of the plant in wet soil. Capacitance measured between two electrodes could be modelled as an electrical circuit in which component capacitors (plant tissue or rooting substrate) are wired in series.

Conclusions

The results were consistent with the new physical interpretation of plant capacitance. Substrate capacitance and plant capacitance combine according to standard physical laws. For plants growing in wet substrate, the capacitance measured is largely determined by the tissue between the surface of the substrate and the electrode attached to the plant. Whilst the measured capacitance can, in some circumstances, be correlated with root mass, it is not a direct assay of root mass.     

Does the lack of root hairs alter root system architecture of Zea mays?

Plant and Soil - Root hairs are one root trait among many which enables plants to adapt to environmental conditions. How different traits are coordinated and whether some are mutually exclusive is...     

BMP4 signaling regulates formation of Hertwig’s epithelial root sheath during tooth root development

Although Hertwig’s epithelial root sheath (HERS) performs an important function in the formation of the tooth root, the developmental mechanisms that control HERS growth and differentiation remain to be thoroughly elucidated. Bone morphogenetic protein 4 (BMP4), which is secreted by mesenchymal cells, acts on the dental epithelium as a regulator of cell differentiation during crown formation. In an effort to determine whether BMP4 specifically regulates the development of HERS in the dental epithelium, we assessed the localizations of BMP4, BMP receptor-IB (BMPR-IB), and BMPR-II during molar root formation in the mouse. HERS cells were shown to express BMPR-IB and BMPR-II. BMP4-positive cells were detected densely in the dental papillae around HERS, thereby suggesting that BMP4 participated in HERS formation. Beads soaked in BMP4, NOGGIN, or phosphate-buffered saline (PBS) were implanted into the pulp cavity under culture conditions, and the length of HERS was evaluated with regard to the proliferating cells. After 12 h, both groups exhibited a similar HERS developmental pattern, with the length and shape of HERS bearing a close resemblance to one another. However, after 48 h, the observed HERS elongation was significantly shorter in the BMP4-treated group. In addition, proliferative cell nuclear antigens were detectable only in the NOGGIN- and PBS-treated groups. These findings demonstrate that mesenchymally expressed BMP4 regulates HERS development by preventing elongation and maintaining cell proliferation. BMP4 may, therefore, prove useful as a root-formation regulatory agent in a variety of tissue-engineering applications. Akihiro Hosoya and Ji-Youn Kim contributed equally to this work. This work was supported by the Seoul R&BD Program (NT070139).     

Experimentally reduced root–microbe interactions reveal limited plasticity in functional root traits in Acer and Quercus

Background and Aims

Interactions between roots and soil microbes are critical components of below-ground ecology. It is essential to quantify the magnitude of root trait variation both among and within species, including variation due to plasticity. In addition to contextualizing the magnitude of plasticity relative to differences between species, studies of plasticity can ascertain if plasticity is predictable and whether an environmental factor elicits changes in traits that are functionally advantageous.

Methods

To compare functional traits and trait plasticities in fine root tissues with natural and reduced levels of colonization by microbial symbionts, trimmed and surface-sterilized root segments of 2-year-old Acer rubrum and Quercus rubra seedlings were manipulated. Segments were then replanted into satellite pots filled with control or heat-treated soil, both originally derived from a natural forest. Mycorrhizal colonization was near zero in roots grown in heat-treated soil; roots grown in control soil matched the higher colonization levels observed in unmanipulated root samples collected from field locations.

Key Results

Between-treatment comparisons revealed negligible plasticity for root diameter, branching intensity and nitrogen concentration across both species. Roots from treated soils had decreased tissue density (approx. 10–20 %) and increased specific root length (approx. 10–30 %). In contrast, species differences were significant and greater than treatment effects in traits other than tissue density. Interspecific trait differences were also significant in field samples, which generally resembled greenhouse samples.

Conclusions

The combination of experimental and field approaches was useful for contextualizing trait plasticity in comparison with inter- and intra-specific trait variation. Findings that root traits are largely species dependent, with the exception of root tissue density, are discussed in the context of current literature on root trait variation, interactions with symbionts and recent progress in standardization of methods for quantifying root traits.     

Involvement of Arabidopsis thaliana phospholipase Dζ2 in root hydrotropism through the suppression of root gravitropism

Root hydrotropism is the phenomenon of directional root growth toward moisture under water-deficient conditions. Although physiological and genetic studies have revealed the involvement of the root cap in the sensing of moisture gradients, and those of auxin and abscisic acid (ABA) in the signal transduction for asymmetric root elongation, the overall mechanism of root hydrotropism is still unclear. We found that the promoter activity of the Arabidopsis phospholipase Dζ2 gene (PLDζ2) was localized to epidermal cells in the distal root elongation zone and lateral root cap cells adjacent to them, and that exogenous ABA enhanced the activity and extended its area to the entire root cap. Although pldζ2 mutant root caps did not exhibit a morphological phenotype in either the absence or presence of exogenous ABA, the inhibitory effect of ABA on gravitropism, which was significant in wild-type roots, was not observed in pldζ2 mutant roots. In root hydrotropism experiments, pldζ2 mutations significantly retarded or disturbed root hydrotropic responses. A drought condition similar to that used in a hydrotropism experiment enhanced the PLDζ2 promoter activity in the root cap, as did exogenous ABA. These results suggest that PLDζ2 responds to drought through ABA signaling in the root cap and accelerates root hydrotropism through the suppression of root gravitropism.     

Cell division patterns of the protoderm and root cap in the “closed” root apical meristem ofArabidopsis thaliana

Summary The peripheral root cap and protoderm inArabidopsis thaliana are organized into modular packets of cells derived from formative T-divisions of the root cap/protoderm (RCP) initials and subsequent proliferative divisions of their daughter cells. Each module consists of protoderm and peripheral root cap packets derived from the same periclinal T-division event of an RCP initial. Anatomical analyses are used to interpret the history of extensively coordinated cell divisions producing this modular construction. Within a given layer of root cap, the columella and RCP initials divided in a centrifugal sequence from the innermost columella initials toward the RCP initials. All RCP initials in the lineages around the circumference of the root divided nearly simultaneously in waves to form one module prior to the next wave of initial divisions forming a younger module. The peripheral root cap and protoderm packets within each module completed four rounds of proliferative divisions in the axial plane to produce, on average, 16 cells per packet in the basalmost modules in axial view. Peripheral root cap and protoderm cells predominantly in the T-type (trichoblast) lineages also underwent radial divisions as they were displaced basipetally. The regularity in the cellular pattern within the modules suggests a timing mechanism controlling highly coordinated cell division in the initials and their daughter cells.Abbreviations RAM root apical meristem - RCP root cap protoderm - prc peripheral root cap     

Significance of root hairs at the field scale – modelling root water and phosphorus uptake under different field conditions

Plant and Soil - Our aim was to determine whether there is a synergistic interaction between the arbuscular mycorrhizal (AM) fungus Rhizoglomus intraradices and the bacterium Brevibacterium...     

Immunoelectron microscopic localization of calmodulin in corn root cells

Methods for the localization of plant calmodulin by immuno-gold and immuno-peroxidase electron microscopy have been developed. In both corn root-cap cells and meristematic cells, calmodulin was found to be localized in the nucleus, cytoplasm, mitochondria as well as in the cell wall, In the meristematic cells, calmodulin was distinctly localized on the plasma membrane, cytoplasmic face of rough endoplasmic rcticulum and polyribosomes. Characteristically, calmodulin was present in the amyloplasts of root-cap cells. The widespread distribution of calmodulin may reflect its plciotropic functions in plant cellular activities.     

Root development: new meanings for root canals?

During Arabidopsis root development, a radial pattern of tissues is extended by the meristem. These tissues form continuous layers and recent data suggest that tissue continuity is instrumental for constraining the direction of signaling in a process termed channeling. In the ground tissue, fate-determining signals originate from contiguous cells of the same layer, possibly due to specific symplastic connections. Mutant analysis supports the hypothesis that vascular tissue continuity may facilitate and depend on the directional transport of a vascular fate-determining signal, possibly the phytohormone auxin.     

Is the shoot a root with a view?

Recently, it has been shown that the same sets of genes act in both root and shoot to regulate cell fate and patterning. One gene cassette regulates epidermal cell fate, another cassette regulates ground tissue derived cell fate and organization. Ectopic expression and laser ablation have been used to probe the mechanisms by which these genes perform their tissue and organ-specific functions.     

Stem cells: The root of prostate cancer?

The cancer stem cell (CSC) model states that tumors contain a reservoir of self-renewing cells that maintain the heterogeneous cell population of the tumor. These cells appear to be resistant to therapy and can therefore survive to repopulate the tumor during progression to therapy resistant disease. The biology of CSCs is still not definitive since it is difficult to isolate them from solid tumors and analyze their characteristics in vitro. Another challenge is to correlate these characteristics with tumor development and progression in vivo. Using the prostate CSC as a model, this review presents the CSC hypothesis, reviews the origin, identification and functions of prostate CSCs, and discusses the clinical implications and therapeutic challenges CSCs have for cancer therapy.     

Modular construction of the protoderm and peripheral root cap in the “open” root apical meristem ofTrifolium repens cv. Ladino

Summary Roots with open apical organization are defined by not having specific tiers of initial cells in the root apical meristem; those with closed apical organization have specific initial tiers to which all cell files can be traced. An example of the clear organization of closed roots is the development protocol of the root cap and protoderm. The key event in differentiating these tissues is the T-division, a periclinal division of the root cap/protoderm (RCP) initial that establishes a module. Each module comprises two packets, the protoderm and peripheral root cap. Consecutive T-divisions of the same RCP initial produce up to five modules on average in a lineage of cells in white clover (Trifolium repens cv. Ladino), with all lineages around the circumference of the root dividing in waves to form one module prior to the next. On average, clover has approximately 32 axial protoderm and peripheral root cap cells in each module, and 32 RCP lineages. The occurrence of RCP T-divisions in white clover, a root with open apical organization, and the subsequent modular construction of the root cap and protoderm, provides a link between open and closed roots and suggests a common developmental feature that most roots of seed plants may share independent of their root meristem organization type. The open apical organization of the white clover root varies from roots with closed apical organization in that the RCP initials occur in staggered positions instead of connected to discrete tiers, and the peripheral root cap and columella daughter cells form additional layers of cells. White clover also forms root hairs on all protoderm cells irrespective of their position relative to the underlying cortical cells.Abbreviations RAM root apical meristem - RCP root cap protoderm - prc peripheral root cap     

Hairy roots — a short cut to transgenic root nodules

To facilitate molecular studies of symbiotic nitrogen fixation a procedure for rapid production of transgenic root nodules was established on the legumeLotus corniculatus (Bird'sfoot trefoil). Regeneration of transgenic plants is not required as transgenic nodules are formed onAgrobacterium rhizogenes incited roots inoculated withRhizobium. Easy identification of transformed roots is possible using a set ofA. rhizogenes acceptor strains carrying assayable marker genes such as chloramphenicol acetyltransferase (CAT), -glucuronidase (GUS), or luciferase (LUC) under control of the cauliflower mosaic virus (CaMV) 35S promoter. Counterselection ofA. rhizogenes after infection of plants was improved using an auxotrophy marker.Abbreviations CAT chloramphenicol acetyltransferase - GUS -glucuronidase - LUC luciferase - Ri root inducing - TL left T-DNA - CaMV Cauliflower Mosaic Virus     

Establishment of crown–root domain borders in mouse incisor

Teeth are composed of two domains, the enamel-covered crown and the enamel-free root. The understanding of the initiation and regulation of crown and root domain formation is important for the development of bioengineered teeth. In most teeth the crown develops before the root, and erupts to the oral cavity whereas the root anchors the tooth to the jawbone. However, in the continuously growing mouse incisor the crown and root domains form simultaneously, the crown domain forming the labial and the root domain the lingual part of the tooth. While the crown–root border on the incisor distal side supports the distal enamel extent, reflecting an evolutionary diet adaptation, on the incisor mesial side the root-like surface is necessary for the attachment of the interdental ligament between the two incisors. Therefore, the mouse incisor exhibits a functional distal–mesial asymmetry. Here, we used the mouse incisor as a model to understand the mechanisms involved in the crown–root border formation. We analyzed the cellular origins and gene expression patterns leading to the development of the mesial and distal crown–root borders. We discovered that Barx2, En1, Wnt11, and Runx3 were exclusively expressed on the mesial crown–root border. In addition, the distal border of the crown–root domain might be established by cells from a different origin and by an early Follistatin expression, factor known to be involved in the root domain formation. The use of different mechanisms to establish domain borders gives indications of the incisor functional asymmetry.