Root Hair Zone Research Articles

Salinity attenuates nickel-accumulating capacity of Atropa belladonna L. plants

Comparative analysis of growth and composition of Atropa belladonna L. plants was performed after separate and combined additions of NaCl and NiCl2 to the nutrient medium. Plants were grown in water culture on modified Johnson solution for 8 weeks until the formation of the fifth leaf pair. Thereafter, NiCl2 was introduced at final concentrations of 100 and 150 μM into the medium either separately or in combination with 100 mM NaCl. After completing the 7-day treatment with Ni ions, the plants' weight and the content of water and photosynthetic pigments were determined. The content of Ni, free polyamines (putrescine, spermidine, spermine), and atropine was determined in plant roots and leaves, whereas the content of Fe, proline, and malondialdehyde (MDA) was examined in leaves only. The distribution of Ni in various tissues was inspected using the dimethylglyoxime method. The presence of NiCl2 in growth media diminished the increments in fresh weight of shoots and roots; lowered the content of water, pigments, and iron in leaves; and initiated chlorosis. The leaves of Ni-treated plants accumulated larger amounts of atropine, putrescine, proline, and MDA with respect to the control levels of these compounds. In contrast to the action of Ni alone, the combined application of NaCl and NiCl2 was followed by the increased content of water and pigments in leaves. The presence of NaCl in the medium restricted the entry of Ni into roots and diminished the levels of MDA and proline in leaves. After growing the plants in the presence of 100 and 150 μM NiCl2, nickel was located in the root outer cortex and the rhizoderm. In plants treated with 150 μM NiCl2, nickel was also observed in tissues of the central cylinder, mostly in the pericycle, phloem, and xylem. In plants grown in the presence of 150 μM NiCl2 and 100 mM NaCl, the decreased accumulation of nickel was noted in the tissues of the central cylinder in the root hair zone. Thus, the combined action of Ni and moderate salinity reduced nickel accumulation in roots and aboveground organs of A. belladonna plants. The reduced Ni content in plants mitigated the toxic effect of Ni present in the medium. This was manifested in stabilization of leaf water status, an increase in the content of photosynthetic pigments, and alleviation of oxidative stress, which was assessed from the content of low-molecular organic compounds exhibiting stress-protective and antioxidant action (proline, MDA, free polyamines, and atropine).

Arabidopsis plasma membrane H+-ATPase genes AHA2 and AHA7 have distinct and overlapping roles in the modulation of root tip H+ efflux in response to low-phosphorus stress.

Phosphorus deficiency in soil is one of the major limiting factors for plant growth. Plasma membrane H+-ATPase (PM H+-ATPase) plays an important role in the plant response to low-phosphorus stress (LP). However, few details are known regarding the action of PM H+-ATPase in mediating root proton (H+) flux and root growth under LP. In this study, we investigated the involvement and function of different Arabidopsis PM H+-ATPase genes in root H+ flux in response to LP. First, we examined the expressions of all Arabidopsis PM H+-ATPase gene family members (AHA1-AHA11) under LP. Expression of AHA2 and AHA7 in roots was enhanced under this condition. When the two genes were deficient in their respective Arabidopsis mutant plants, root growth and responses of the mutants to LP were highly inhibited compared with the wild-type plant. AHA2-deficient plants exhibited reduced primary root elongation and lower H+ efflux in the root elongation zone. AHA7-deficient plants exhibited reduced root hair density and lower H+ efflux in the root hair zone. The modulation of H+ efflux by AHA2 or AHA7 was affected by the action of 14-3-3 proteins and/or auxin regulatory pathways in the context of root growth and response to LP. Our results suggest that under LP conditions, AHA2 acts mainly to modulate primary root elongation by mediating H+ efflux in the root elongation zone, whereas AHA7 plays an important role in root hair formation by mediating H+ efflux in the root hair zone.

Ethylene modulates root hair development in trifoliate orange through auxin-signaling pathway

Root hair, tubular outgrowths from root-specific epidermis, is strongly affected by plant growth regulators, such as ethylene and auxin, while the interaction between them is more attractive. A sand culture study was performed to analyze the mechanisms of exogenous ethylene (ETH, ethephon) and silver nitrate (Ag, ethylene effect inhibitor) interaction with auxin on root hair development of trifoliate orange (Poncirus trifoliata), a widely used rootstock for citrus production. The results showed that ETH up-regulated the genes of ethylene synthesis, receptor and signal transduction that leading to increased root hair density and length while Ag presented the opposite effects. Root hair initiation was attributed to epidermal cell development as well as the proportion of epidermal cell bulge to root hair when treated by ETH and Ag. Furthermore, ETH up-regulated the expression of auxin biosynthesis and transport genes, thus, increasing the endogenous auxin content in the root hair zone and subsequently enhancing root hair growth dramatically. However, application of Ag represented opposite effect, namely an inhibited effect on relevant genes expression. These results indicated that ethylene stimulated auxin biosynthesis and transport toward the root hair zone, where it achieved a local activation of the auxin signaling pathway leading to promotion of root hair development.

Exogenous application of abscisic acid (ABA) increases root and cell hydraulic conductivity and abundance of some aquaporin isoforms in the ABA-deficient barley mutant Az34.

Background and Aims Regulation of water channel aquaporins (AQPs) provides another mechanism by which abscisic acid (ABA) may influence water flow through plants. To the best of our knowledge, no studies have addressed the changes in ABA levels, the abundance of AQPs and root cell hydraulic conductivity (LpCell) in the same tissues. Thus, we followed the mechanisms by which ABA affects root hydraulics in an ABA-deficient barley mutant Az34 and its parental line 'Steptoe'. We compared the abundance of AQPs and ABA in cells to determine spatial correlations between AQP abundance and local ABA concentrations in different root tissues. In addition, abundance of AQPs and ABA in cortex cells was related to LpCell. Methods Root hydraulic conductivity (LpRoot) was measured by means of root exudation analyses and LpCell using a cell pressure probe. The abundance of ABA and AQPs in root tissues was assessed through immunohistochemical analyses. Isoform-specific antibodies raised against HvPIP2;1, HvPIP2;2 and HvPIP2;5 were used. Key Results Immunolocalization revealed lower ABA levels in root tissues of Az34 compared with 'Steptoe'. Root hydraulic conductivity (LpRoot) was lower in Az34, yet the abundance of HvPIPs in root tissues was similar in the two genotypes. Root hair formation occurred closer to the tip, while the length of the root hair zone was shorter in Az34 than in 'Steptoe'. Application of external ABA to the root medium of Az34 and 'Steptoe' increased the immunostaining of root cells for ABA and for HvPIP2;1 and HvPIP2;2 especially in root epidermal cells and the cortical cell layer located beneath, parallel to an increase in LpRoot and LpCell. Treatment of roots with Fenton reagent, which inhibits AQP activity, prevented the ABA-induced increase in root hydraulic conductivity. Conclusion Shortly after (<2 h) ABA application to the roots of ABA-deficient barley, increased tissue ABA concentrations and AQP abundance (especially the plasma-membrane localized isoforms HvPIP2;1 and HvPIP2;2) were spatially correlated in root epidermal cells and the cortical cell layer located beneath, in conjunction with increased LpCell of the cortical cells. In contrast, long-term ABA deficiency throughout seedling development affects root hydraulics through other mechanisms, in particular the developmental timing of the formation of root hairs closer to the root tip and the length of the root hair zone.

English

Roots in compacted soil experience both axial resistance and radial pressure. Axial resistance at root apex and radial pressure perpendicularly at root surface cause root deformation i.e., change in root shape. Although the effects of axial resistance are well described in literature, there is a lack of quantitative approach to examine the radial impact. In addition to morphological observation in root cross sections, this study quantified the radial impact of compacted soil on root surface and vascular cylinder by measuring root roundness (R) and flattening (F) of vetiver (Vetiveria zizanioides L.) seedlings. The plants grown in high bulk density soil (1.58 g cm-3) exhibited uneven root surface with invaginations, reduced R and increased F in root hair zone, compared with plants in low bulk density soil (1.20 g cm-3). However, soil compaction had little effect on vascular cylinder in terms of microscopic observation, the R and F values. It is concluded that the measurements of root roundness (R) and flattening (F) are consistent with microscopic observations and can be used to quantify root deformation under soil compaction.

GmEXPB2, a Cell Wall β-Expansin, Affects Soybean Nodulation through Modifying Root Architecture and Promoting Nodule Formation and Development.

Nodulation is an essential process for biological nitrogen (N2) fixation in legumes, but its regulation remains poorly understood. Here, a β-expansin gene, GmEXPB2, was found to be critical for soybean (Glycine max) nodulation. GmEXPB2 was preferentially expressed at the early stage of nodule development. β-Glucuronidase staining further showed that GmEXPB2 was mainly localized to the nodule vascular trace and nodule vascular bundles, as well as nodule cortical and parenchyma cells, suggesting that GmEXPB2 might be involved in cell wall modification and extension during nodule formation and development. Overexpression of GmEXPB2 dramatically modified soybean root architecture, increasing the size and number of cortical cells in the root meristematic and elongation zones and expanding root hair density and size of the root hair zone. Confocal microscopy with green fluorescent protein-labeled rhizobium USDA110 cells showed that the infection events were significantly enhanced in the GmEXPB2-overexpressing lines. Moreover, nodule primordium development was earlier in overexpressing lines compared with wild-type plants. Thereby, overexpression of GmEXPB2 in either transgenic soybean hairy roots or whole plants resulted in increased nodule number, nodule mass, and nitrogenase activity and thus elevated plant N and phosphorus content as well as biomass. In contrast, suppression of GmEXPB2 in soybean transgenic composite plants led to smaller infected cells and thus reduced number of big nodules, nodule mass, and nitrogenase activity, thereby inhibiting soybean growth. Taken together, we conclude that GmEXPB2 critically affects soybean nodulation through modifying root architecture and promoting nodule formation and development and subsequently impacts biological N2 fixation and growth of soybean.

Morphological effects at radicle level by direct contact of invasive Acacia dealbata Link

Seeds of Lactuca sativa L. (model plant) and Quillaja saponaria Mol. (native tree) were subjected to germination in direct contact with leaves, pods and seeds from invasive Acacia dealbata Link. (Fabaceae) to reveal whether morphological changes occur at radicle level. Bioassays were carried out with plant material deposited underneath A. dealbatás canopy under natural conditions at Mediterranean ecosystem in South America. Segments of radicle tip, cell elongation zone and root hair zone were analyzed under scanning electron microscopy. The three plant parts from A. dealbata inhibited the formation of root hairs and altered the rhizodermis (deformation and tissue destruction) of the two recipient species. In the case of Q. saponaria, an increase in thickness in the cell elongation zone was observed and internal tissues up to the vascular cylinder in the root hair zone were intensely damaged. The root cap and meristematic region suffered structural changes and damage resulting from the contact with leaves and pods. All morphological effects have been attributed to the impact caused at root level by potential allelochemicals released by different tested plant materials of A. dealbata. These results suggest that the morphological changes and damage produced by A. dealbata at the radicle level may pose a threat to the growth and survival of the recipient species.

CHANGES IN THE MORPHOLOGY OF THE ROOT SYSTEM OF WHEAT INOCULATED WITH Azospirillum brasilense Sp7 AND BACTERIOPHAGE FAb-Sp7

The Azospirillum is one of the most common objects to study associative interactions between bacteria and plants. In the rhizosphere of cereals the Azospirillum cells form effective association and demonstrate a stimulating effect on plant growth and development, in particular in wheat. However, the details of these positive effects are still unknown. For example, there is no evidence of whether the Azospirillum cells are infected with bacteriophages, and little is known about the role that the bacteriohages of soil microorganisms can play in the development of associative relationship between microorganisms and plants. Nevertheless, the bacteriophages are well known to control the bacteria number in the population and be involved in genetic transduction. Note that the studied bacteriophages are mostly isolated from environment, in particular, from soil, while the bacteriphages of associative soil bacteria still remain poorly studied. To evaluated changes in the morphology of the wheat (Triticum aestivum L.) root system in Saratovskaya 29 variety after inoculation with Azospirillum brasilense Sp7 bacterial cells and bacteriophage FAb-Sp7, isolated from these cells, we first inoculated the 3-day old wheat seedlings with A. brasilense Sp7 at 108, 106, 104 cells per 1 ml. It resulted in a decrease in the length of the root, and the number of lateral roots grew considerably at 102 cells per 1 ml. After treatment wiht bacteriophage FAb-Sp7 an average number of roots remained unchanged. It was shown that wheat seedling inoculation with Azospirillum suspension resulted in a 3-fold decrease in root elongation zone, and the suction zone reduced 1.5 times. Treatment of wheat seedlings with a suspension containing Azospirillum and phages resulted in a 3.3-fold decrease in the tensile zone, but the root hair zone remained unchanged. Using confocal microscopy we demonstrated that incubation of wheat seedlings with Azospirillum at 102 cells per 1 ml significantly increased the number and length of root hairs (on average by 40 %) compared with the control. It was shown that a simultaneous inoculation with Azospirillum Sp7 cells and bacteriophage FAb-Sp7 also increased the number and length of root hairs (60 % on average) compared with control. It is suggested that bacteriophages of Azospirillum are involved into complex interaction of bacteria with wheat root system, and can influence on and, ultimately, determine the effectiveness of associative relationships between the plant and microorganisms.

Involvement of 14-3-3 protein GRF9 in root growth and response under polyethylene glycol-induced water stress.

Plant 14-3-3 proteins are phosphoserine-binding proteins that regulate a wide array of targets via direct protein-protein interactions. In this study, the role of a 14-3-3 protein, GRF9, in plant response to water stress was investigated. Arabidopsis wild-type, GRF9-deficient mutant (grf9), and GRF9-overexpressing (OE) plants were treated with polyethylene glycol (PEG) to induce mild water stress. OE plant showed better whole-plant growth and root growth than the wild type under normal or water stress conditions while the grf9 mutant showed worse growth. In OE plants, GRF9 favours the allocation of shoot carbon to roots. In addition, GRF9 enhanced proton extrusion, mainly in the root elongation zone and root hair zone, and maintained root growth under mild water stress. Grafting among the wild type, OE, and grf9 plants showed that when OE plants were used as the scion and GRF9 was overexpressed in the shoot, it enhanced sucrose transport into the root, and when OE plants were used as rootstock and GRF9 was overexpressed in the root, it caused more release of protons into the root surface under water stress. Taken together, the results suggest that under PEG-induced water stress, GRF9 is involved in allocating more carbon from the shoot to the root and enhancing proton secretion in the root growing zone, and this process is important for root response to mild water stress.

The Arabidopsis nitrate transporter NRT2.5 plays a role in nitrate acquisition and remobilization in nitrogen-starved plants.

Nitrogen is a key mineral nutrient playing a crucial role in plant growth and development. Understanding the mechanisms of nitrate uptake from the soil and distribution through the plant in response to nitrogen starvation is an important step on the way to improve nitrogen uptake and utilization efficiency for better growth and productivity of plants, and to prevent negative effects of nitrogen fertilizers on the environment and human health. In this study, we show that Arabidopsis NITRATE TRANSPORTER 2.5 (NRT2.5) is a plasma membrane-localized high-affinity nitrate transporter playing an essential role in adult plants under severe nitrogen starvation. NRT2.5 expression is induced under nitrogen starvation and NRT2.5 becomes the most abundant transcript amongst the seven NRT2 family members in shoots and roots of adult plants after long-term starvation. GUS reporter analyses showed that NRT2.5 is expressed in the epidermis and the cortex of roots at the root hair zone and in minor veins of mature leaves. Reduction of NRT2.5 expression resulted in a decrease in high-affinity nitrate uptake without impacting low-affinity uptake. In the background of the high-affinity nitrate transporter mutant nrt2.4, an nrt2.5 mutation reduced nitrate levels in the phloem of N-starved plants further than in the single nrt2.4 mutants. Growth analyses of multiple mutants between NRT2.1, NRT2.2, NRT2.4, and NRT2.5 revealed that NRT2.5 is required to support growth of nitrogen-starved adult plants by ensuring the efficient uptake of nitrate collectively with NRT2.1, NRT2.2 and NRT2.4 and by taking part in nitrate loading into the phloem during nitrate remobilization.

SPX1 is an important component in the phosphorus signalling network of common bean regulating root growth and phosphorus homeostasis.

Proteins containing the SPX domain are believed to play vital roles in the phosphorus (P) signalling network in plants. However, the functions of SPX proteins in legumes remain largely unknown. In this study, three SPX members, PvSPX1-PvSPX3 were cloned from common bean (Phaseolus vulgaris L.). It was found that the transcripts of all three PvSPX members were significantly enhanced in both bean leaves and roots by phosphate (Pi) starvation. Among them, the expression of nuclear localized PvSPX1 showed more sensitive and rapid responses to Pi starvation. Consistently, only overexpression of PvSPX1 resulted in increased root P concentration and modified morphology of transgenic bean hairy roots, such as inhibited root growth and an enlarged root hair zone. It was further demonstrated that PvSPX1 transcripts were up-regulated by overexpressing PvPHR1, and overexpressing PvSPX1 led to increased transcripts of 10 Pi starvation-responsive genes in transgenic bean hairy roots. Taken together, it is suggested that PvSPX1 is a positive regulator in the P signalling network of common bean, and is downstream of PvPHR1.

Inhibition of germination and early growth of rape seed (Brassica napus L.) by MCPA in anionic and ester form

MCPA (4-chloro-2-methylphenoxy) acetic acid is a common synthetic auxin used as a herbicide. The purpose of this study was to determine the effects of four new forms of MCPA being the herbicidal ionic liquids (HILs) with MCPA as an anion and two previously known formulations (potassium–sodium salt and 2-ethylhexyl ester) on seed germination and seedling development of winter oilseed rape (Brassica napus). Rape plants are susceptible to MCPA and volunteers can be a big problem in crop rotation. Seedling fresh weight and root length were quantified, mitotic activity, as well as lipid, starch, hydrogen peroxide and polyphenol contents were assessed by light and fluorescence microscopy and the computer-aided cytophotometer. In primary roots mitotic activity was almost completely inhibited under the influence of herbicides, cell elongation zones and root hair zones were significantly reduced, and a characteristic bolded root segment formed just above a meristem. In contrast to the traditional salt formulation the new HILs were weak inducers of hydrogen peroxide synthesis, but were potent stimulators of the synthesis of phenolic compounds and storage as well as emergency substances such as lipids and starch. All tested forms of MCPA caused strong phytotoxic effect on winter rape seedlings, but the tested HILs were more effective.

Colonization of Fusarium Wilt‐Resistant and Susceptible Watermelon Roots by a Green‐Fluorescent‐Protein‐tagged Isolate of Fusarium oxysporum f.sp. niveum

AbstractInteractions between watermelon and a green fluorescent protein (GFP)‐tagged isolate of Fusarium oxysporum f.sp. niveum race 1 (Fon‐1) were studied to determine the differences in infection and colonization of watermelon roots in cultivars resistant to and susceptible to Fusarium wilt. The roots of watermelon seedlings were inoculated with a conidial suspension of the GFP‐tagged isolate, and confocal laser scanning microscopy was used to visualize colonization, infection and disease development. The initial infection stages were similar in both the resistant and susceptible cultivars, but the resistant cultivar responded differentially after the pathogen had penetrated the root. The pathogen penetrated and colonized resistant watermelon roots, but further fungal advance appeared to be halted, and the fungus did not enter the taproot, suggesting that resistance is initiated postpenetration. However, the tertiary and secondary lateral roots of resistant watermelon also were colonized, although not as extensively as susceptible roots, and the hyphae had penetrated into the central cylinder of lateral roots forming a dense hyphal mat, which was followed by a subsequent collapse of the lateral roots. The initial infection zone for both the wilt‐susceptible and wilt‐resistant watermelon roots appeared to be the epidermal cells within the root hair zone, which the fungus penetrated directly after forming appressoria. Areas where secondary roots emerged and wounded root tissue also were penetrated preferentially.

Quantitative imaging of rhizosphere pH and CO2 dynamics with planar optodes.

Live imaging methods have become extremely important for the exploration of biological processes. In particular, non-invasive measurement techniques are key to unravelling organism-environment interactions in close-to-natural set-ups, e.g. in the highly heterogeneous and difficult-to-probe environment of plant roots: the rhizosphere. pH and CO2 concentration are the main drivers of rhizosphere processes. Being able to monitor these parameters at high spatio-temporal resolution is of utmost importance for relevant interpretation of the underlying processes, especially in the complex environment of non-sterile plant-soil systems. This study introduces the application of easy-to-use planar optode systems in different set-ups to quantify plant root-soil interactions. pH- and recently developed CO2-sensors were applied to rhizobox systems to investigate roots with different functional traits, highlighting the potential of these tools. Continuous and highly resolved real-time measurements were made of the pH dynamics around Triticum turgidum durum (durum wheat) roots, Cicer arietinum (chickpea) roots and nodules, and CO2 dynamics in the rhizosphere of Viminaria juncea. Wheat root tips acidified slightly, while their root hair zone alkalized their rhizosphere by more than 1 pH unit and the effect of irrigation on soil pH could be visualized as well. Chickpea roots and nodules acidified the surrounding soil during N2 fixation and showed diurnal changes in acidification activity. A growing root of V. juncea exhibited a large zone of influence (mm) on soil CO2 content and therefore on its biogeochemical surrounding, all contributing to the extreme complexity of the root-soil interactions. This technique provides a unique tool for future root research applications and overcomes limitations of previous systems by creating quantitative maps without, for example, interpolation and time delays between single data points.

Visualization of grapevine root colonization by the Saharan soil isolate Saccharothrix algeriensis NRRL B-24137 using DOPE-FISH microscopy

There is currently a gap of knowledge regarding whether some beneficial bacteria isolated from desert soils can colonize epi- and endophytically plants of temperate regions. In this study, the early steps of the colonization process of one of these bacteria, Saccharothrix algeriensis NRRL B-24137, was studied on grapevine roots to determine if this beneficial strain can colonize a non-natural host plant. An improved method of fluorescence in situ hybridization (FISH), the double labeling of oligonucleotide probes (DOPE)-FISH technique was used to visualize the colonization behavior of such bacteria as well as to determine if the method could be used to track microbes on and inside plants. A probe specific to Saccharothrix spp. was firstly designed. Visualization of the colonization behavior of S. algeriensis NRRL B-24137 on and inside roots of grapevine plants was then carried out with DOPE-FISH microscopy. The results showed that 10 days after inoculation, the strain could colonize the root hair zone, root elongation zone, as well as root emergence sites by establishing different forms of bacterial structures as revealed by the DOPE-FISH technique. Further observations showed that the strain could be also endophytic inside the endorhiza of grapevine plants. Taking into account the natural niches of this beneficial strain, this study exemplifies that, in spite of its isolation from desert soil, the strain can establish populations as well as subpopulations on and inside grapevine plants and that the DOPE-FISH tool can allow to detect it.

Aluminium toxic effects on seedling root survival affect plant composition along soil reaction gradients – a case study in dry sandy grasslands

AbstractQuestionAluminium (Al) toxicity is one of the most important factors restricting plant establishment on acidic soils, but its ecological significance for the occurrence of species along natural pH gradients is still under investigation. Are species occurring on acidic sandy soils less susceptible to Al toxic effects on germination and seedling root growth rate than species from calcareous sandy soils? How strong is the explanatory power of species' Al susceptibility for their occurrence along a pH gradient, as represented by their Ellenberg indicator value (EIV) for soil reaction (R)? Can Al tolerance of species be used as an independent trait to support Ellenberg's empirically‐derived reaction indicator values?LocationDry sandy grasslands in Southern Germany, with soil reactions ranging from acidic to calcareous.MethodsWe tested early seedling responses to different Al concentrations in 15 species from dry sandy grasslands. A filter paper‐based system was used to germinate seeds under Al concentrations of up to 10 mM. Germination, absolute root growth and the length of the root hair zone were recorded 7 and 14 d after first germination. Al concentrations that reduced root growth by 50 or 95% (ED50 and ED95, respectively) were correlated to Ellenberg indicator values (EIV) for soil reaction.ResultsEIV explained 66% of the variance in species' Al sensitivity. Tolerated Al concentrations resemble those concentrations the individual species are exposed to in their natural habitats.ConclusionsAmong all soil factors varying with soil pH, Al is one of the strongest restrictions to species' occurrence in acidic soils. Al acts as an environmental filter by allowing only Al‐tolerant seedlings to grow roots and establish. Al sensitivity is a measurable objective trait that could form a crucial physiological component in defining R indicator values.

A conceptual model of root hair ideotypes for future agricultural environments: what combination of traits should be targeted to cope with limited P availability?

Phosphorus (P) often limits crop production and is frequently applied as fertilizer; however, supplies of quality rock phosphate for fertilizer production are diminishing. Plants have evolved many mechanisms to increase their P acquisition, and an understanding of these traits could result in improved long-term sustainability of agriculture. This Viewpoint focuses on the potential benefits of root hairs to sustainable production. First the various root-related traits that could be deployed to improve agricultural sustainability are catalogued, and their potential costs and benefits to the plant are discussed. A novel mathematical model describing the effects of length, density and longevity of root hairs on P acquisition is developed, and the relative benefits of these three root-hair traits to plant P nutrition are calculated. Insights from this model are combined with experimental data to assess the relative benefits of a range of root hair ideotypes for sustainability of agriculture. A cost-benefit analysis of root traits suggests that root hairs have the greatest potential for P acquisition relative to their cost of production. The novel modelling of root hair development indicates that the greatest gains in P-uptake efficiency are likely to be made through increased length and longevity of root hairs rather than by increasing their density. Synthesizing this information with that from published experiments we formulate six potential ideotypes to improve crop P acquisition. These combine appropriate root hair phenotypes with architectural, anatomical and biochemical traits, such that more root-hair zones are produced in surface soils, where P resources are found, on roots which are metabolically cheap to construct and maintain, and that release more P-mobilizing exudates. These ideotypes could be used to inform breeding programmes to enhance agricultural sustainability.

The infection process of Fusarium oxysporum f.sp. vasinfectum in Australian cotton

The infection process of two commercial Australian cotton varieties differing in susceptibility to Fusarium oxysporum f. sp. vasinfectum (Fov) was investigated. There were no differences in the early stages of infection between the host cultivars. Conidia germinated and developed an extensive hyphal net around the root. Penetration of epidermal cells was most common in the root hair zone, causing colonisation of the root hairs and epidermal cells before hyphae invaded the cortex by both intra- and intercellular growth. Colonisation of the stele of the moderately resistant variety Sicot 189 was arrested within 3 days, while growth in the highly susceptible Siokra 1–4 continued for 5 days. The restriction of Fov in the xylem of Sicot 189 indicates a differential activation of defence responses in this cultivar that corresponds with field resistance.

Live imaging of intra- and extracellular pH in plants using pHusion, a novel genetically encoded biosensor

Changes in pH are now widely accepted as a signalling mechanism in cells. In plants, proton pumps in the plasma membrane and tonoplast play a key role in regulation of intracellular pH homeostasis and maintenance of transmembrane proton gradients. Proton transport in response to external stimuli can be expected to be finely regulated spatially and temporally. With the ambition to follow such changes live, a new genetically encoded sensor, pHusion, has been developed. pHusion is especially designed for apoplastic pH measurements. It was constitutively expressed in Arabidopsis and targeted for expression in either the cytosol or the apoplast including intracellular compartments. pHusion consists of the tandem concatenation of enhanced green fluorescent protein (EGFP) and monomeric red fluorescent protein (mRFP1), and works as a ratiometric pH sensor. Live microscopy at high spatial and temporal resolution is highly dependent on appropriate immobilization of the specimen for microscopy. Medical adhesive often used in such experiments destroys cell viability in roots. Here a novel system for immobilizing Arabidopsis seedling roots for perfusion experiments is presented which does not impair cell viability. With appropriate immobilization, it was possible to follow changes of the apoplastic and cytosolic pH in mesophyll and root tissue. Rapid pH homeostasis upon external pH changes was reflected by negligible cytosolic pH fluctuations, while the apoplastic pH changed drastically. The great potential for analysing pH regulation in a whole-tissue, physiological context is demonstrated by the immediate alkalinization of the subepidermal apoplast upon external indole-3-acetic acid administration. This change is highly significant in the elongation zone compared with the root hair zone and control roots.

Alleviation of Al-toxicity in wheat by boron 1. Anatomical and ultrastructure effects of aluminium on root of aluminium-tolerant cultivar ofTriticum aestivum

The effect of aluminium was investigated on the seedlings pre-treated by two concentrations of boron 4 μM or 32 μM grown in water culture using a concentration of 500 μM Al for 3 days. Semithin and ultrathin sections of the apical region of the roots and transmission electron microscopy micrographs were analysed of fourteen-day-old Al-tolerant (‘Sakha 93’) cultivar of Triticum aestivum. Results showed that the amelioration effect of boron treatment was pronounced at 32 μM B level. Rigidity of cell wall and plasma membrane in the wheat root apex cells (Zone of root hairs) could result from the formation of bonds of ions of the toxicant with components within their structures and appears as dark precipitants in cross sections. Cross sections of the apical region of the control plant roots showed well-developed normal anatomical structure and cell ultrastructure typical for those of root regions. Slight alterations under the influence of aluminium or boron alone or both of them and the role exhibited by boro...