Root Hair Length Research Articles

Barley root hair growth and morphology in soil, sand, and water solution media and relationship with nickel toxicity.

Barley, Hordeum vulgare (Doyce), was grown in the 3 media of soil, hydroponic sand solution (sand), and hydroponic water solution (water) culture at the same environmental conditions for 4 d. Barley roots were scanned, and root morphology was analyzed. Plants grown in the 3 media had different root morphology and nickel (Ni) toxicity response. Root elongations and total root lengths followed the sequence soil > sand > water. Plants grown in water culture were more sensitive to Ni toxicity and had greater root hair length than those from soil and sand cultures, which increased root surface area. The unit root surface area as root surface area per centimeter of length of root followed the sequence water > sand > soil and was found to be related with root elongation. Including the unit root surface area, the difference in root elongation and 50% effective concentration were diminished, and percentage of root elongations can be improved with a root mean square error approximately 10% for plants grown in different media. Because the unit root surface area of plants in sand culture is closer to that in soil culture, the sand culture method, not water culture, is recommended for toxicity parameter estimation. Environ Toxicol Chem 2016;35:2125-2133. © 2016 SETAC.

Root hair formation in rice (Oryza sativa L.) differs between root types and is altered in artificial growth conditions.

Root hairs are important sites for nutrient uptake, especially in P limiting conditions. Here we provide first insights into root hair development for the diverse root types of rice grown under different conditions, and show the first in situ images of rice root hairs in intact soil. Roots of plants grown in upland fields produced short root hairs that showed little responsiveness to P deficiency, and had a higher root hair density in the high P condition. These results were reproducible in rhizoboxes under greenhouse conditions. Synchrotron-based in situ analysis of root hairs in intact soil further confirmed this pattern of root hair formation. In contrast, plants grown in nutrient solution produced more and longer root hairs in low P conditions, but these were unequally distributed among the different root types. While nutrient solution-grown main roots had longer hairs compared to upland field-grown main roots, second order lateral roots did not form any root hairs in nutrient solution-grown plants. Furthermore, root hair formation for plants grown in flooded lowland fields revealed few similarities with those grown in nutrient solution, thus defining nutrient solution as a possible measure of maximal, but not natural root hair development. By combining root hair length and density as a measure for root hair impact on the whole soil-grown root system we show that lateral roots provided the majority of root hair surface.

Anisotropic Cell Expansion Is Affected through the Bidirectional Mobility of Cellulose Synthase Complexes and Phosphorylation at Two Critical Residues on CESA3.

Here we report that phosphorylation status of S211 and T212 of the CESA3 component of Arabidopsis (Arabidopsis thaliana) cellulose synthase impacts the regulation of anisotropic cell expansion as well as cellulose synthesis and deposition and microtubule-dependent bidirectional mobility of CESA complexes. Mutation of S211 to Ala caused a significant decrease in the length of etiolated hypocotyls and primary roots, while root hairs were not significantly affected. By contrast, the S211E mutation stunted the growth of root hairs, but primary roots were not significantly affected. Similarly, T212E caused a decrease in the length of root hairs but not root length. However, T212E stunted the growth of etiolated hypocotyls. Live-cell imaging of fluorescently labeled CESA showed that the rate of movement of CESA particles was directionally asymmetric in etiolated hypocotyls of S211A and T212E mutants, while similar bidirectional velocities were observed with the wild-type control and S211E and T212A mutant lines. Analysis of cell wall composition and the innermost layer of cell wall suggests a role for phosphorylation of CESA3 S211 and T212 in cellulose aggregation into fibrillar bundles. These results suggest that microtubule-guided bidirectional mobility of CESA complexes is fine-tuned by phosphorylation of CESA3 S211 and T212, which may, in turn, modulate cellulose synthesis and organization, resulting in or contributing to the observed defects of anisotropic cell expansion.

Salinity-induced reduction in root surface area and changes in major root and shoot traits at the phytomer level in wheat.

The aim of this study was to investigate the effect of salinity stress on root growth at the phytomer level in wheat to provide novel site-specific understanding of salinity damage in roots. Seedlings of 13 wheat varieties were grown hydroponically. Plants were exposed to three concentrations of NaCl, 0 (control), 50 and 100mM, from 47 days after sowing. In a destructive harvest 12 days later we determined the number of live leaves, adventitious roots, seminal roots and newly formed roots at the youngest phytomer; length and diameter of main axes; and length and diameter of root hairs and their number per millimetre of root axis. Elongation rate of main axes and root hair density were then derived. Root surface area at each root-bearing phytomer (Pr) was mechanistically modelled. New root formation was increased by salt exposure, while number of live leaves per plant decreased. The greatest salinity effect on root axis elongation was observed at the youngest roots at Pr1 and Pr2. Both the 50mM and the 100mM levels of salinity reduced root hair length by approximately 25% and root hair density by 40% compared with the control whereas root hairs alone contributed around 93% of the estimated total root surface area of an individual tiller. Decrease in main axis length of new roots, root hair density and root hair length combined to reduce estimated root surface area by 36-66% at the higher NaCl concentration. The varietal response towards the three salinity levels was found to be trait-specific. The data highlight reduction in root surface area as a major but previously largely unrecognized component of salinity damage. Salinity resistance is trait-specific. Selection for retention of root surface area at a specific phytomer position following salt exposure might be useful in development of salinity-tolerant crop varieties.

Fine-tuning by strigolactones of root response to low phosphate.

Strigolactones are plant hormones that regulate the development of different plant parts. In the shoot, they regulate axillary bud outgrowth and in the root, root architecture and root-hair length and density. Strigolactones are also involved with communication in the rhizosphere, including enhancement of hyphal branching of arbuscular mycorrhizal fungi. Here we present the role and activity of strigolactones under conditions of phosphate deprivation. Under these conditions, their levels of biosynthesis and exudation increase, leading to changes in shoot and root development. At least for the latter, these changes are likely to be associated with alterations in auxin transport and sensitivity. On the other hand, strigolactones may positively affect plant-mycorrhiza interactions and thereby promote phosphate acquisition by the plant. Strigolactones may be a way for plants to fine-tune their growth pattern under phosphate deprivation.

Cell wall-associated ROOT HAIR SPECIFIC 10, a proline-rich receptor-like kinase, is a negative modulator of Arabidopsis root hair growth.

Plant cell growth is restricted by the cell wall, and cell wall dynamics act as signals for the cytoplasmic and nuclear events of cell growth. Among various receptor kinases, ROOT HAIR SPECIFIC 10 (RHS10) belongs to a poorly known receptor kinase subfamily with a proline-rich extracellular domain. Here, we report that RHS10 defines the root hair length of Arabidopsis thaliana by negatively regulating hair growth. RHS10 modulates the duration of root hair growth rather than the growth rate. As poplar and rice RHS10 orthologs also showed a root hair-inhibitory function, this receptor kinase-mediated function appears to be conserved in angiosperms. RHS10 showed a strong association with the cell wall, most probably through its extracellular proline-rich domain (ECD). Deletion analysis of the ECD demonstrated that a minimal extracellular part, which includes a few proline residues, is required for RHS10-mediated root hair inhibition. RHS10 suppressed the accumulation of reactive oxygen species (ROS) in the root, which are necessary for root hair growth. A yeast two-hybrid screening identified an RNase (RNS2) as a putative downstream target of RHS10. Accordingly, RHS10 overexpression decreased and RHS10 loss increased RNA levels in the hair-growing root region. Our results suggest that RHS10 mediates cell wall-associated signals to maintain proper root hair length, at least in part by regulating RNA catabolism and ROS accumulation.

Rhizosheaths on wheat grown in acid soils: phosphorus acquisition efficiency and genetic control.

Rhizosheaths comprise soil bound to roots, and in wheat (Triticum aestivum L.) rhizosheath size correlates with root hair length. The aims of this study were to determine the effect that a large rhizosheath has on the phosphorus (P) acquisition by wheat and to investigate the genetic control of rhizosheath size in wheat grown on acid soil.Near-isogenic wheat lines differing in rhizosheath size were evaluated on two acid soils. The soils were fertilized with mineral nutrients and included treatments with either low or high P. The same soils were treated with CaCO3 to raise the pH and detoxify Al(3+) Genotypic differences in rhizosheath size were apparent only when soil pH was low and Al(3+) was present. On acid soils, a large rhizosheath increased shoot biomass compared with a small rhizosheath regardless of P supply. At low P supply, increased shoot biomass could be attributed to a greater uptake of soil P, but at high P supply the increased biomass was due to some other factor. Generation means analysis indicated that rhizosheath size on acid soil was controlled by multiple, additive loci. Subsequently, a quantitative trait loci (QTL) analysis of an F6 population of recombinant inbred lines identified five major loci contributing to the phenotype together accounting for over 60% of the total genetic variance. One locus on chromosome 1D accounted for 34% of the genotypic variation. Genetic control of rhizosheath size appears to be relatively simple and markers based on the QTL provide valuable tools for marker assisted breeding.

Plant biostimulants: physiological responses induced by protein hydrolyzed-based products and humic substances in plant metabolism

In recent years, the use of biostimulants in sustainable agriculture has been growing. Biostimulants can be obtained from different organic materials and include humic substances (HS), complex organic materials, beneficial chemical elements, peptides and amino acids, inorganic salts, seaweed extracts, chitin and chitosan derivatives, antitranspirants, amino acids and other N-containing substances. The application of biostimulants to plants leads to higher content of nutrients in their tissue and positive metabolic changes. For these reasons, the development of new biostimulants has become a focus of scientific interest. Among their different functions, biostimulants influence plant growth and nitrogen metabolism, especially because of their content in hormones and other signalling molecules. A significant increase in root hair length and density is often observed in plants treated with biostimulants, suggesting that these substances induce a “nutrient acquisition response” that favors nutrient uptake in plants via an increase in the absorptive surface area. Furthermore, biostimulants positively influence the activity and gene expression of enzymes functioning in the primary and secondary plant metabolism. This article reviews the current literature on two main classes of biostimulants: humic substances and protein-based biostimulants. The characteristic of these biostimulants and their effects on plants are thoroughly described.

Root Responses to Boron Deficiency Mediated by Ethylene

Low boron (B) supply alters the architecture of the root system in Arabidopsis thaliana seedlings, leading to a reduction in the primary root growth and an increase in the length and number of root hairs. At short-term (hours), B deficiency causes a decrease in the cell elongation of the primary root, resulting in a lower growth. Experimental approaches using ethylene insensitive Arabidopsis mutants, inhibitors of ethylene response, and GUS reporter lines suggest that ethylene is involved in these responses of the primary root to B deficiency. Furthermore, it has been shown that auxin participates in the inhibition of cell elongation under short-term B deprivation. These results support that an interaction between ethylene and auxin plays an important role in controlling the primary root elongation, in which a number of genes related to the synthesis, transport, and signaling of both phytohormones could modulate this effect. Evidence for a root cross-talk among both hormones and other possible intermediates (abscisic acid, calcium sensors, and reactive oxygen species) in response to B deficiency is provided and discussed.

Comparative Morphophysiological Analyses and Molecular Profiling Reveal Pi-Efficient Strategies of a Traditional Rice Genotype.

Phosphate (Pi) deficiency severely affects crop yield. Modern high yielding rice genotypes are sensitive to Pi deficiency whereas traditional rice genotypes are naturally compatible with low Pi ecosystems. However, the underlying molecular mechanisms for low Pi tolerance in traditional genotypes remain largely elusive. To delineate the molecular mechanisms for low Pi tolerance, two contrasting rice genotypes, Dular (low Pi tolerant), and PB1 (low Pi sensitive), have been selected. Comparative morphophysiological, global transcriptome and lipidome analyses of root and shoot tissues of both genotypes grown under Pi deficient and sufficient conditions revealed potential low Pi tolerance mechanisms of the traditional genotype. Most of the genes associated with enhanced internal Pi utilization (phospholipid remobilization) and modulation of root system architecture (RSA) were highly induced in the traditional rice genotype, Dular. Higher reserves of phospholipids and greater accumulation of galactolipids under low Pi in Dular indicated it has more efficient Pi utilization. Furthermore, Dular also maintained greater root growth than PB1 under low Pi, resulting in larger root surface area due to increased lateral root density and root hair length. Genes involved in enhanced low Pi tolerance of the traditional genotype can be exploited to improve the low Pi tolerance of modern high yielding rice cultivars.

Root morphological traits that determine phosphorus-acquisition efficiency and critical external phosphorus requirement in pasture species

Annual pasture legume species can vary more than 3-fold in their critical external phosphorus (P) requirement (i.e. P required for 90% of maximum yield). In this work we investigated the link between root morphology, P acquisition and critical external P requirement among pasture species. The root morphology acclimation of five annual pasture legumes and one grass species to low soil P availability was assessed in a controlled-environment study. The critical external P requirement of the species was low (Dactylis glomerata L., Ornithopus compressus L., Ornithopus sativus Brot.), intermediate (Biserrula pelecinus L., Trifolium hirtum All.) or high (Trifolium subterraneum L.). Root hair cylinder volumes (a function of root length, root hair length and average root diameter) were estimated in order to assess soil exploration and its impact on P uptake. Most species increased soil exploration in response to rates of P supply near or below their critical external P requirement. The legumes differed in how they achieved their maximum root hair cylinder volume. The main variables were high root length density, long root hairs and/or high specific root length. However, total P uptake per unit surface area of the root hair cylinder was similar for all species at rates of P supply below critical P. Species that maximised soil exploration by root morphology acclimation were able to prolong access to P in moderately P-deficient soil. However, among the species studied, it was those with an intrinsic capacity for a high root-hair-cylinder surface area (i.e. long roots and long root hairs) that achieved the lowest critical P requirement.

P uptake, root properties and biomass of six cover-crops

Phosphorus (P) is an essential and limited nutrient. To avoid shortages in the coming decades, P recycling and efficiency of P use will have to be enhanced. Cover crops may help to transform less available P forms into bioavailable P forms. The aim of this investigation was the comparison of P uptake with root traits of six cover crops. Root and shoot biomass was analysed for P content. Root hair length and root length was measured on the basis of 6400 dpi root scans using ImageJ and WinRhizo. In addition, the root hair cylinder was calculated. P uptake was found to be strongly correlated with the root hair cylinder and the total crop biomass. Keywords: cover crops, phosphorus uptake, root hair cylinder, root length, root hairs

Rhizosphere carboxylates and morphological root traits in pasture legumes and grasses

The legumes most often used in temperate pastures such as Trifolium subterraneum have relatively high external P requirements for maximum growth. We investigated root traits associated with P acquisition in current and novel pasture legumes, as well as temperate grasses which have lower P requirements. Thirteen legume species, two pasture grasses, and three high carboxylate-exuding crop species (Lupinus albus, L. angustifolius, Cicer arietinum) were grown in a glasshouse for six weeks. Rhizosphere carboxylates and root morphological traits were measured. Ornithopus spp. had rhizosphere carboxylates in similar quantities to the Lupinus spp. (> 40 μmol g−1 root dry mass). Trifolium subterraneum lines had relatively large average root diameter, reduced specific root length and very short average root hair length resulting in specific root hair cylinder volumes (RHCVs) only 14–20 % of the grasses. However, O. sativus, O. compressus and Biserrula pelecinus had specific RHCVs more comparable to the grasses. Novel pasture legume species with root morphology more comparable to that of grasses than T. subterraneum were identified. Of these, Ornithopus spp. were notable as they also had high rhizosphere carboxylates relative to root dry mass.

High variation in the percentage of root length colonised by arbuscular mycorrhizal fungi among 139 lines representing the species subterranean clover (Trifolium subterraneum)

Development of more phosphorus (P)-efficient pasture legumes could improve the P balance of pasture systems. Trifolium subterraneum (subterranean clover) is the most widely-grown annual pasture legume in southern Australia and is host to arbuscular mycorrhizal fungi (AMF). To examine the potential to alter colonisation level through breeding, 97 lines from the core collection of subterranean clover (core lines) and 42 cultivars (cultivar lines) were grown in a glasshouse in a low-P sandy acid field soil with indigenous AMF (Experiment 1). To examine the robustness of rankings, two cultivars (Riverina and Woogenellup) were also grown in 11 field soils (Experiment 2). To examine whether the presence of AMF changed other root traits important for P uptake, cultivars Riverina and Woogenellup were grown in two low-P field soils (sand, sandy loam), either pasteurised to remove indigenous AMF or unpasteurised, and with and without addition of inoculum of AMF (Experiment 3). The range of colonisation for core lines and cultivar lines was similar (∼12–68% of root length), however, the core lines had higher mean colonisation (37% vs 33%) and a greater proportion of lines with >40% of root length colonised (41% vs 24% of lines). Woogenellup was more heavily colonised than Riverina in eight soils and had a similar level of colonisation in three soils. In the sand, pasteurisation resulted in shoot DM increasing by up to 100% (possibly due to reduction in root diseases). Inoculation, in pasteurised and unpasteurised soil, resulted in an increase in shoot DM, shoot P concentration and shoot P content of ∼100%, increased average root diameter by up to 13%, decreased specific root length by up 30% and decreased root mass ratio; effects on average root hair length were variable. In the unpasteurised sandy loam, shoot P concentrations were higher than in the sand, and inoculation slightly increased shoot P concentration, but not shoot DM; the effects of inoculation on root traits were mostly small. We conclude that potential may exist to select subterranean clover for higher or lower colonisation by AMF using cultivar lines. Also, indigenous AMF may be less effective at enhancing plant P uptake and growth than inoculants. Finally, key root traits for enhancing plant P-uptake may change in the presence of AMF, but not sufficiently so to warrant deliberate inclusion or exclusion of AMF when screening germplasm for such traits.

SCARN a Novel Class of SCAR Protein That Is Required for Root-Hair Infection during Legume Nodulation.

Rhizobial infection of legume root hairs requires a rearrangement of the actin cytoskeleton to enable the establishment of plant-made infection structures called infection threads. In the SCAR/WAVE (Suppressor of cAMP receptor defect/WASP family verpolin homologous protein) actin regulatory complex, the conserved N-terminal domains of SCAR proteins interact with other components of the SCAR/WAVE complex. The conserved C-terminal domains of SCAR proteins bind to and activate the actin-related protein 2/3 (ARP2/3) complex, which can bind to actin filaments catalyzing new actin filament formation by nucleating actin branching. We have identified, SCARN (SCAR-Nodulation), a gene required for root hair infection of Lotus japonicus by Mesorhizobium loti. Although the SCARN protein is related to Arabidopsis thaliana SCAR2 and SCAR4, it belongs to a distinct legume-sub clade. We identified other SCARN-like proteins in legumes and phylogeny analyses suggested that SCARN may have arisen from a gene duplication and acquired specialized functions in root nodule symbiosis. Mutation of SCARN reduced formation of infection-threads and their extension into the root cortex and slightly reduced root-hair length. Surprisingly two of the scarn mutants showed constitutive branching of root hairs in uninoculated plants. However we observed no effect of scarn mutations on trichome development or on the early actin cytoskeletal accumulation that is normally seen in root hair tips shortly after M. loti inoculation, distinguishing them from other symbiosis mutations affecting actin nucleation. The C-terminal domain of SCARN binds to ARPC3 and ectopic expression of the N-terminal SCAR-homology domain (but not the full length protein) inhibited nodulation. In addition, we found that SCARN expression is enhanced by M. loti in epidermal cells and that this is directly regulated by the NODULE INCEPTION (NIN) transcription factor.

Polyethylene Glycol (PEG)-Treated Hydroponic Culture Reduces Length and Diameter of Root Hairs of Wheat Varieties

Wheat is an important cereal crop worldwide that often suffers from moisture deficits at the reproductive stage. Polyethylene glycol (PEG)-treated hydroponic conditions create negative osmotic potential which is compared with moisture deficit stress. An experiment was conducted in a growth chamber to study the effects of PEG on root hair morphology and associated traits of wheat varieties. Plants of 13 wheat varieties were grown hydroponically and three different doses of PEG 6000 (w/v): 0% (control), 0.3% and 0.6% (less than −1 bar) were imposed on 60 days after sowing for 20 days’ duration. A low PEG concentration was imposed to observe how initial low moisture stress might affect root hair development. PEG-treated hydroponic culture significantly decreased root hair diameter and length. Estimated surface area reduction of root hairs at the main axes of wheat plants was around nine times at the 0.6% PEG level compared to the control plants. Decrease in root hair diameter and length under PEG-induced culture decreased “potential” root surface area per unit length of main root axis. A negative association between panicle traits, length and dry weight and the main axis length of young roots indicated competition for carbon during their development. Data provides insight into how a low PEG level might alter root hair development.

Mycorrhiza alters the profile of root hairs in trifoliate orange

Root hairs and arbuscular mycorrhiza (AM) coexist in root systems for nutrient and water absorption, but the relation between AM and root hairs is poorly known. A pot study was performed to evaluate the effects of four different AM fungi (AMF), namely, Claroideoglomus etunicatum, Diversispora versiformis, Funneliformis mosseae, and Rhizophagus intraradices on root hair development in trifoliate orange (Poncirus trifoliata) seedlings grown in sand. Mycorrhizal seedlings showed significantly higher root hair density than non-mycorrhizal seedlings, irrespective of AMF species. AMF inoculation generally significantly decreased root hair length in the first- and second-order lateral roots but increased it in the third- and fourth-order lateral roots. AMF colonization induced diverse responses in root hair diameter of different order lateral roots. Considerably greater concentrations of phosphorus (P), nitric oxide (NO), glucose, sucrose, indole-3-acetic acid (IAA), and methyl jasmonate (MeJA) were found in roots of AM seedlings than in non-AM seedlings. Levels of P, NO, carbohydrates, IAA, and MeJA in roots were correlated with AM formation and root hair development. These results suggest that AMF could alter the profile of root hairs in trifoliate orange through modulation of physiological activities. F. mosseae, which had the greatest positive effects, could represent an efficient AM fungus for increasing fruit yields or decreasing fertilizer inputs in citrus production.

Root development: Pulse control

The length of root hairs, a critical determinant of a plant's absorption capacity, is dictated by a translational pulse of the transcription factor RSL4. The intensity and duration of this pulse depends on the integration of endogenous and environmental information that tunes the phenotypic readout to the environment.

Variation in root traits associated with nutrient foraging among temperate pasture legumes and grasses

AbstractTemperate pasture legumes (e.g. Trifolium and Medicago spp.) often have a higher phosphorus (P) requirement for maximum productivity than pasture grasses. This is partly attributed to differences between legumes and grasses in their ability to acquire P from soil. We are the first to report differences in root morphology traits important for soil P acquisition in a range of novel pasture legumes being developed for use in temperate pastures of southern Australia. Up to a 3·6‐fold range in specific root length (SRL) (79–281 m root g−1 root) and 6·1‐fold range in root hair length (RHL) (0·12–0·75 mm) was found between the pasture species. The commonly used Trifolium subterraneum and Medicago sativa had relatively low SRLs and short root hairs, while Ornithopus compressus, O. sativus and Biserrula pelecinus had RHLs and SRLs more similar to those of two grass species that were also assessed. Specific root length was highly correlated with average root diameter, and root traits were relatively stable at different plant ages. We surmise that large differences among pasture legume species in the effective volume of soil explored could translate into significant differences in their critical P requirements (i.e. soil P concentration to achieve 90% of maximum shoot yield).

Altered expression of TaRSL4 gene by genome interplay shapes root hair length in allopolyploid wheat.

Polyploidy is a major driving force in plant evolution and speciation. Phenotypic changes often arise with the formation, natural selection and domestication of polyploid plants. However, little is known about the consequence of hybridization and polyploidization on root hair development. Here, we report that root hair length of synthetic and natural allopolyploid wheats is significantly longer than those of their diploid progenitors, whereas no difference is observed between allohexaploid and allotetraploid wheats. The expression of wheat gene TaRSL4, an orthologue of AtRSL4 controlling the root hair development in Arabidopsis, was positively correlated with the root hair length in diploid and allotetraploid wheats. Moreover, transcript abundance of TaRSL4 homoeologue from A genome (TaRSL4-A) was much higher than those of other genomes in natural allopolyploid wheat. Notably, increased root hair length by overexpression of the TaRSL4-A in wheat led to enhanced shoot fresh biomass under nutrient-poor conditions. Our observations indicate that increased root hair length in allohexaploid wheat originated in the allotetraploid progenitors and altered expression of TaRSL4 gene by genome interplay shapes root hair length in allopolyploid wheat.