Hormonal regulation of iron-stress response in sunflower roots: a morphological and cytological investigation

Summary Sunflowers are known to respond to Fe deficiency (-Fe) with a typical root tip swelling and the formation of root hairs and transfer cells in the rhizodermis. The possible regulation of this process was examined by a comparative study of root morphology and cytology of intact seedlings (Helianthus annuus L. cv. Giganteus) under -Fe and hormonal treatment in nutrient solution. Longitudinal sections of -Fe roots showed root tip swelling is due to cessation of cell elongation and isodiarnetric volume increase of the cortical cells. Enhanced cell division in the pericycle leads to the formation of lateral root primordia in the swollen zone. Xylem vessel differentiation is markedly accelerated and accompanied by early differentiation of the casparian band in the endodermis. Exogenous application of IAA (10^–8-10^–7 M) via the nutrient solution to Fe sufficient plants causes symptoms which closely mimick the characteristics of Fe deficiency including root hair development. Moreover, rhizodermal cells produce peripheral protuberances reminiscent of -Fe transfer cells. Ethylene-releasing ethephon (10^–4M) also causes subapical swelling and root hair formation. However, wall protuberance development is less pronounced. ABA (10^–5 M) leads to similar root thickening and root hair formation but without any comparable transfer cell differentiation. From the striking similarities between -Fe and IAA treatment it is concluded that this hormone (possibly in cooperation with ethylene) is involved in the Fe stress response of sunflower roots. The importance of a continuous polar IAA transport for this process is discussed.Abbreviations ABA abscisic acid - ACC 1-aminocyclopropane-1-carboxylic acid - Ethephone 2-chloro-ethylphosphonic acid - Fe(III)-EDTA ethylenediaminetetraacetic ferric-sodium salt - IAA indole-acetic acid - TIBA triiodobenzoic acid     

Nitric oxide as a critical factor for perception of UV-B irradiation by microtubules in Arabidopsis

Influence of ultraviolet-B (UV-B) as an abiotic stress factor on plant microtubules (MTs) and involvement of nitric oxide (NO) as a secondary messenger mediating plant cell response to environmental stimuli were investigated in this study. Taking into account that endogenous NO content in plant cells has been shown to be increased under a broad range of abiotic stress factors, the effects of UV-B irradiation and also the combined action of UV-B and NO donor sodium nitroprusside (SNP) or NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) on the MTs organization in different root cells of Arabidopsis thaliana were tested. Subsequently, realization of the MT-mediated processes such as root growth and development was studied under these conditions. Arabidopsis thaliana seedlings expressing the chimeric gene gfp-map4 were exposed to the enhanced UV-B with or without SNP or c-PTIO pretreatment. The UV-B irradiation alone led to a dose-dependent root growth inhibition and to morphological alterations of the primary root manifested in their swelling and excessive root hair formation. Moreover, dose-dependent randomization and depolymerization of MTs in both epidermal and cortical cells under the enhanced UV-B were found. However, SNP pretreatment of the UV-B irradiated A. thaliana seedlings recovered the UV-B inhibited root growth as compared to c-PTIO pretreatment. It has been shown that in 24 h after UV-B irradiation the organization of MTs in root epidermal cells of SNP-pretreated A. thaliana seedlings was partially recovered, whereas in c-PTIO-pretreated ones the organization of MTs has not been distinctly improved. Therefore, we suppose that the enhanced NO levels in plant cells can protect MTs organization as well as MT-related processes of root growth and development against disrupting effects of UV-B.     

Nitrogen source interacts with ROP signalling in root hair tip‐growth

Root hairs elongate in a highly polarized manner known as tip growth. Overexpression of constitutively active Rho of Plant (ROP)/RAC GTPases mutants induces swelling of root hairs. Here, we demonstrate that Atrop11^CA‐induced swelling of root hairs depends on the composition of the growth medium. Depletion of ammonium allowed normal root hair elongation in Atrop11^CA plants, induced the development of longer root hairs in wild‐type plants and suppressed the effect of Atrop11^CA expression on actin organization and reactive oxygen species distribution, whereas membrane localization of the protein was not affected. Ammonium at concentrations higher than 1 mM and the presence of nitrate were required for induction of swelling. Oscillations in wall and cytoplasmic pH are known to accompany tip growth in root hairs, and buffering of the growth medium decreased Atrop11^CA‐induced swelling. Fluorescence ratio imaging experiments revealed that in wild‐type root hairs, the addition of NH₄NO₃ to the growth medium induced an increase in the amplitude of extracellular and intracellular pH oscillations and an overall decrease in cytoplasmic pH at the cell apex. Based on these results, we suggest a model in which ROP GTPases and nitrogen‐dependent pH oscillations function in parallel pathways, creating a positive feedback loop during root hair growth.     

Organization of cortical microtubules and microfibril deposition in response to blue-light-induced apical swelling in a tip-growing Adiantum protonema cell

The arrangements of cortical microtubules (MTs) in a tip-growing protonemal cell of Adiantum capillus-veneris L. and of cellulose microfibrils (MFs) in its wall were examined during blue-light (BL)-induced apical swelling. In most protonemal cells which had been growing in the longitudinal direction under red light, apical swelling was induced within 2 h of the onset of BL irradiation, and swelling continued for at least 8 h. During the longitudinal growth under red light, the arrangement of MFs around the base of the apical hemisphere (the subapical region) was perpendicular to the cell axis, while a random arrangement of MFs was found at the very tip, and a roughly axial arrangement was observed in the cylindrical region of most cells. This orientation of MFs corresponds to that of the cortical MTs reported previously (Murata et al. 1987, Protoplasma 141, 135–138). In cells irradiated with BL, a random rather than transverse arrangement of both MTs and MFs was found in the subapical region. Time-course studies showed that this reorientation occurred within 1 h after the onset of the BL irradiation, i.e. it preceded the change in growth pattern. These results indicate that the orientation of cortical MTs and of cellulose MFs is involved in the regulation of cell diameter in a tip-growing Adiantum protonemal cell.Abbreviations BL blue light - MF(s) microfibril(s) - MT(s) microtubule(s)     

Responses of Tradescantia stamen hairs and pollen to UV-B irradiation

Clones 02 and 4430 of Tradescantia were tested in field, greenhouse and controlled environment chambers as monitors for the potentially hazardous UV-B irradiation increase that could result from stratospheric ozone decrease. In addition to about 16 hr of solar emissions at about 2100 micro-einsteins·m⁻²·s⁻¹ (400–700 nm) and 15 hr at about 1800 micro-einsteins·m⁻²·s⁻¹ in the field and greenhouse, respectively, plants were given 7 hr of supplemental UV-B irradiation per day for 27 days. After the first 7 days of UV-B irradiation exposure, cumulative data were recorded for 20 days. Cuttings of Tradescantia plants in controlled-environment, exposed to 16 hr of simulated solar emission of about 800 micro-einsteins·m⁻²·s⁻¹ (400–700 nm), were also exposed to 10 hr of supplemental UV-B irradiation per day for 1 or 2 days. All plants were checked for somatic aberrations (color changes in the flower petals and stamen hairs), number of hairs per stamen, and cells per hair. Pollen germination and pollen tube growth were noted after a 90-min UV-B irradiation period.Somatic aberrations occurred infrequently in the petals and were judged unreliable criteria for use in monitoring enhanced UV-B irradiation environments. The number of aberrant events within stamen hairs, however, was significantly increased by the UV-B irradiation treatments. while pollen germination and pollen tube growth were significantly reduced. These data indicate that color changes in stamen hairs and pollen viability are useful criteria for monitoring UV-B irradiation changes.     

Oxidative stress and antioxidant content in Chlorella vulgaris after exposure to ultraviolet-B radiation

Growth of Chlorella vulgaris was measured in cultures irradiated with 0, 0.8, 2.0 and 4.4 kJ m² UV-B. Growth expressed as chlorophyll content, declined significantly with increased UV-B dose. Ultraviolet-B irradiated cultures in log phase of growth showed a 284% increase in oxygen radical generation and a 145% increase in lipid peroxidation compared with unirradiated cultures, whereas cultures in the stationary growth phase showed no significant changes in these parameters. The activities of superoxide dismutase and catalase increased by 40 and 500%, respectively, after exposure to a UV-B dose of 4.4 kJ m⁻². Contents of the lipophilic antioxidants α-tocopherol and β-carotene increased by 180 and 63 amol cell⁻¹ respectively, between log and stationary phases in unirradiated cultures; but in UV-B-irradiated cultures these increases were significantly depressed. Photoreducing capacities of chloroplasts were decreased following UV-B irradiation of both isolated chloroplasts and those isolated from irradiated algae. Cells exposed to UV-B exhibited increased size and starch accumulation. These results suggest that oxidative stress conditions related to UV-B exposure trigger an antioxidant response that includes an increase in the activity of the antioxidant enzymes (superoxide dismutase and catalase).     

Dynamics of growth,proliferation and differentiation of wheat root cells exposed to a high nickel concentration

The dynamics of root growth, proliferation of initial cells of the root cap, rhizodermis, and central metaxylem, as well as structural changes in the cells induced by a 72-h exposure to a high (0.1 mM) concentration of NiSO₄ were studied in 3-day-old wheat (Triticum aestivum L.) seedlings. In the roots of control plants, we observed a 12-h rhythm of changes in the length of the cells that completed elongating. Upon the treatment with nickel, this effect was negated, and a considerable reduction in the root length increment was observed in 12 h. In 24 h, root growth essentially ceased. Cell elongation was suppressed acropetally, and the cells, whose elongation was over, became shorter. In the meristem and apical part of the elongation zone, slow cell growth continued during the second and even third days. Autoradiography showed that the earliest effect of nickel on the processes of root morphogenesis observed in 6 h was a suppression of cell transition to DNA synthesis. The cells, where DNA synthesis has already started or which were in other stages of the cycle, continued to pass slowly through the cycle and completed it. Sister cells formed as a result of division subsequently left the cycle in the phase G1 and transited to dormancy. It was found that the main mechanism of cell proliferation cessation was the suppression of cell transition to DNA synthesis. In the cells elongating when exposed to nickel, tissue-specific changes in the nucleus structure were observed (chromatolysis in the rhizodermis and cortex, pycnosis in the endodermis, a disturbance of the nucleus structure in the central metaxylem). These disorders were only observed after cessation of elongation. Root incubation in 0.1 mM nickel solution did not affect the onset of cell differentiation in the xylem and metaphloem and shifted its beginning to the root tip. However, in 24 h the initiation and growth of root hairs were suppressed. It was concluded that tissue-specific nickel-induced changes in the nucleus structure in the elongating cells do not cause the cessation of root growth, although point to nickel toxic effect on the cells in the course of elongation.     

Causes of root growth retardation induced by ultraviolet-B irradiation of shoots in Barley seedlings

In barley seedlings (Hordeum vulgare L.) during two days after irradiation of shoots with UV-B (0.5 W/m², 6 h), the rate of elongation of primary roots decreased 2–3 times compared to that in control plants. The modulus of elasticity of roots (ε) increased at most twofold in 12 h after the onset of irradiation; the hydraulic conductivity (L _p) diminished by a factor of two in 12 h, and the root osmotic pressure gradually decreased by 0.08 MPa in 24 h. Changes in ε and L _p were shown to be related to oxidative stress in growing roots, which was evidenced from the increase in H₂O₂ level up to 15-fold increase in 6 h and in activity of guaiacol peroxidase (3.5-fold in 12 h). After 48 h, the characteristics of oxidative metabolism and root characteristics ε and L _p became identical in untreated and treated plants. On the third day, the rate of root growth in treated plants reached its initial value. It is concluded that the main causes of retardation of root growth under these conditions were as follows: the increase in cell wall rigidity related to formation of oxidative cross-links in the apoplast and the decrease in root osmotic pressure due to limited transport of assimilates from irradiated leaves. After the intensity of UV-B irradiation applied to shoots was enhanced (1.6 W/m², 4 h), another physiological status of roots was observed on the 2nd day characterized by twofold increase in L _p, tenfold decreased root elongation rate, and by a progressing increase of root diameter in growing roots. The comparison of root responses induced by irradiation of shoots with the root responses to sodium salicylate and ABA suggests that both agents might participate in the transmission of signals from irradiated leaves to roots.     

Microtubule reorganization as a response to implementation of NO signals in plant cells

The effects of an exogeneous NO donor, sodium nitroprusside, on the orientation and organization of cortical microtubules in Arabidopsis thaliana root cells expressing GFP-MAP4 were studied in vivo. It was found that sodium nitroprusside treatment (10–500 μM, 24 h) caused the acceleration of primary root growth and enhanced initiation of root hairs in the differentiation zone. The influence of sodium nitroprusside revealed in changes in the orientation and organization of cortical microtubules in different types of cells of A. thaliana root. The most sensitive to sodium nitroprusside exposure were microtubules in epidermal cells of the elongation zone, where native transverse orientation of cortical microtubules turned into random, oblique, or longitudinal relative to the primary root axis. We suggest that NO, as one of the intracellular secondary messengers, triggers cell differentiation by reorientation of cortical microtubules, possibly via tubulin nitrotyrosination.     

Arabidopsis PCaP2 modulates the phosphatidylinositol 4,5‐bisphosphate signal on the plasma membrane and attenuates root hair elongation

Phosphatidylinositol 4,5‐bisphosphate [PtdIns(4,5)P₂] serves as a subcellular signal on the plasma membrane, mediating various cell‐polarized phenomena including polar cell growth. Here, we investigated the involvement of Arabidopsis thaliana PCaP2, a plant‐unique plasma membrane protein with phosphoinositide‐binding activity, in PtdIns(4,5)P₂ signaling for root hair tip growth. The long‐root‐hair phenotype of the pcap2 knockdown mutant was found to stem from its higher average root hair elongation rate compared with the wild type and to counteract the low average rate caused by a defect in the PtdIns(4,5)P₂‐producing enzyme gene PIP5K3. On the plasma membrane of elongating root hairs, the PCaP2 promoter‐driven PCaP2–green fluorescent protein (GFP), which complemented the pcap2 mutant phenotype, overlapped with the PtdIns(4,5)P₂ marker 2xCHERRY‐2xPH^PLC in the subapical region, but not at the apex, suggesting that PCaP2 attenuates root hair elongation via PtdIns(4,5)P₂ signaling on the subapical plasma membrane. Consistent with this, a GFP fusion with the PCaP2 phosphoinositide‐binding domain PCaP2^N23, root hair‐specific overexpression of which caused a low average root hair elongation rate, localized more intense to the subapical plasma membrane than to the apical plasma membrane similar to PCaP2–GFP. Inducibly overexpressed PCaP2–GFP, but not its derivative lacking the PCaP2^N23 domain, replaced 2xCHERRY‐2xPH^PLC on the plasma membrane in root meristematic epidermal cells, and suppressed FM4‐64 internalization in elongating root hairs. Moreover, inducibly overexpressed PCaP2 arrested an endocytic process of PIN2–GFP recycling. Based on these results, we conclude that PCaP2 functions as a negative modulator of PtdIns(4,5)P₂ signaling on the subapical plasma membrane probably through competitive binding to PtdIns(4,5)P₂ and attenuates root hair elongation.     

Scanning electron microscopic aspects of short tuberized roots, with special reference to cell rhizodermis evolution under drought and rehydration

Abstract. Different morphological aspects of short tuberized roots initiated during drought stress in mesophytic species, such as Sinapis alba L., were studied with the scanning electron microscopy technique of cryofixation. Specially adapted for direct and immediate observation of fresh living roots, this method has given precise information about rhizodermal organization and evolution during drought and rehydration.
The main difference from a normal lateral root grown in a well-watered soil appears in the basal enlarged zone of the short root where all the cells show the same round and turgid aspect. In the medium zone of the short root, rhizodermis differentiation into alternating rows of short (trichoblasts) and long (atrichoblasts) cells, which characterizes the typical Sinapis root, remains clearly discernible, though not so regular as in normal lateral roots.
The turgid state of rhizodermal cells all over the short tuberized root grown in a drying soil suggests an effective regulatory mechanism for water deficit avoidance.
During the first hours of rehydration, immediate absorption of water is noticeable through the rapid swelling of some long cells which appear to protrude considerably among other rhizodermal cells. However, these protrusions will not give rise to hairs, as further observations of short roots after growth has resumed show very distinctly that progressive hair formation occurs in the medium zone of the root, and that their emergence originates from trichoblasts only.
These observations may indicate that atrichoblasts, on account of their highly vacuolated condition, are the first cells to absorb water and that they may even be stimulated, in some environmental conditions, to initiate hair formation, although they are not so well adapted to do so as the short cells in this species.     

Endoplasmic microtubules configure the subapical cytoplasm and are required for fast growth of Medicago truncatula root hairs

To investigate the configuration and function of microtubules (MTs) in tip-growing Medicago truncatula root hairs, we used immunocytochemistry or in vivo decoration by a GFP linked to a MT-binding domain. The two approaches gave similar results and allowed the study of MTs during hair development. Cortical MTs (CMTs) are present in all developmental stages. During the transition from bulge to a tip-growing root hair, endoplasmic MTs (EMTs) appear at the tip of the young hair and remain there until growth arrest. EMTs are a specific feature of tip-growing hairs, forming a three-dimensional array throughout the subapical cytoplasmic dense region. During growth arrest, EMTs, together with the subapical cytoplasmic dense region, progressively disappear, whereas CMTs extend further toward the tip. In full-grown root hairs, CMTs, the only remaining population of MTs, converge at the tip and their density decreases over time. Upon treatment of growing hairs with 1 microM oryzalin, EMTs disappear, but CMTs remain present. The subapical cytoplasmic dense region becomes very short, the distance nucleus tip increases, growth slows down, and the nucleus still follows the advancing tip, though at a much larger distance. Taxol has no effect on the cytoarchitecture of growing hairs; the subapical cytoplasmic dense region remains intact, the nucleus keeps its distance from the tip, but growth rate drops to the same extent as in hairs treated with 1 microM oryzalin. The role of EMTs in growing root hairs is discussed.     

Ultrastructure,differentiation and cell wall texture of trichoblasts and root hairs of ceratopteris thalictroides (L.) brongn. (Parkeriaceae)

Trichoblasts and root hairs of Ceratopteris thalictroides (L.) Brongn. were studied by different techniques to survey their morphological features. Trichoblasts could be identified at an early stage by an intra-vacuolar precipitate appearing during fixation. Special attention was paid to root-hair initiation. No structures or changes were observed that play a role in the initiation of papilla formation. When the papilla is formed, vesicles and periplasmic membranes can be observed which may play a role in the weakening of the cell wall during the papilla outgrowth.During root-hair growth, the nucleus of the trichoblast moves from the trichoblast to a subapical position in the root hair. The nuclei of all root cells contain 2 types of nuclear inclusions, one of which is proteinaceous.The cell wall of the Ceratopteris root hair has a helicoidal texture and because the cortical microtubules run longitudinally in the root hair, no correlation can be made between the directions of microtubules and microfibrils in these root hairs.     

Heat Shock Increases the Tolerance of Plants to UV-B Radiation: 1. Growth,Development, and Water Supply to Tissues

The effects of heat shock (HS); UV-B irradiation; and the consecutive action of these factors on the growth, development, and water supply of seven-day-old melon (Melo sativusSager., cv. Torpeda) seedlings were investigated. Depending on the HS severity, we observed growth stimulation (after treatment at 45°C for 1 h), growth retardation (after treatment at 45°C for 2 h or at 48°C for 1 h), or complete growth inhibition and cell death (after treatment at 45°C for 3 h or at 55°C for 1 h). UV-B irradiation (18.3 kJ/(m²h)), depending on its duration, stimulated (5–10 min), retarded (60 min), or resulted in complete growth inhibition and plant death (90 min). HS treatment (at 45°C for 1 h) prior to UV-B irradiation (for 1 h) favorably affected both the growth and water balance of seedlings. Apparently, the HS pretreatment increases the tolerance of seedlings to high doses of UV-B radiation.     

Restoration of cell growth and proliferation in the wheat roots after their inhibition by nickel sulfate

The dynamics of cell growth and proliferation restoration in different tissues and quiescent center (QC) in the wheat (Triticum aestivum L.) seedling roots and also the differentiation of rhizodermal cells and lateral root initiation after 48-h treatment with 100 μM NiSO₄ were studied. Within 24 h after nickel removal from medium, root growth was resumed due to the increase in the rate of cell growth in the meristem and the region where cell elongation started in control roots. Stimulation of cell proliferation was restored in the main part of the meristem and later in the initial cells of the files and QC. Cell proliferation was not observed in the QC. The time of cell proliferation resumption in the roots and in tested tissues depended on the degree of their injury by nickel treatment. In most tested roots, DNA synthesis and cell division were restored in 32 h. In the cells leaving the meristem due to the resumption of their growth and proliferation, growth of root hairs started. In 48 h, the number of roots with perished cells in the rhizodermis in the meristem was sharply increased and the regeneration of the damaged region by the cells of outer cortex was observed. Only after the appearance of root hairs, the cells coming from the meristem started to elongate. In most roots, the formation of the new elongation zone occurred in 56 h. During its formation, the initiation of lateral root primordia was shifted in the basipetal direction. It was concluded that the cessation of cell growth and proliferation under the influence of high concentration of heavy metal (HM) ions is not lethal for the root. At the action of toxic HM concentrations, the plant strategy is the maintenance of meristematic cell capacity for cell growth and proliferation resumption. The cellular mechanism of this capacity maintenance is the transition of meristematic cells from G₁ phase to dormancy due to growth inhibition and the inhibition of the transition to DNA synthesis.     

Biophysical parameters as informative tools for elucidating the causes of root growth retardation under stressful conditions

The root growth rate in barley (Hordeum vulgare L.) seedlings was measured in parallel with temporal changes in longitudinal (δl) and transverse (δD/D) cell-wall extensibilities and membrane hydraulic conductivity (L _p) in the root extension zone. The root growth rate and biophysical parameters examined were sensitive to UV-B irradiation of shoots or roots and to excessive content of ammonium, glutamate, or nickel in the nutrient medium. The root responses to the above treatments were compared with the effects of abscisic acid, salicylate, hydrogen peroxide, diethylstilbestrol, α-naphthyl acetate, oryzalin, and ionomycin. The progressive reduction of root growth under the action of various stressors was accompanied by typical temporal patterns of the growth zone parameters: the δl extensibility declined monotonically, while δD/D and L _p changed nonmonotonically, exhibiting the reversion from the initial decrease to the eventual increase above the control values. The decline of δl indicated that the root growth suppression was mainly due to changes in cell-wall mechanical properties caused probably by disorganization of cortical microtubules. It was found that the decline in δD/D and L _p was caused primarily by the appearance of oxidative stress, disorders in cytoplasmic H⁺ homeostasis in root cells, and the consequent transient activation of the plasmalemmal H⁺-pump. Conversely, the increase in δD/D and L _p upon the abrupt retardation of root growth was presumably caused by the increase in cytoplasmic Ca²⁺ content, disassembling of cortical microtubules, and by partial inhibition of the plasmalemmal H⁺-pump. The reversion of δD/D and L _p changes upon progressive reduction of root growth can be used as an indicator to distinguish moderate and severe stress conditions in the root growth zone. Furthermore, this reversion indicates the increasing disbalance in the homeostasis of reactive oxygen species, cytosolic Ca²⁺, and cytosolic H⁺ upon severe stress.     

The role of actin in root hair morphogenesis: studies with lipochito-oligosaccharide as a growth stimulator and cytochalasin as an actin perturbing drug

Root hairs develop from bulges on root epidermal cells and elongate by tip growth, in which Golgi vesicles are targeted, released and inserted into the plasma membrane on one side of the cell. We studied the role of actin in vesicle delivery and retention by comparing the actin filament configuration during bulge formation, root hair initiation, sustained tip growth, growth termination, and in full-grown hairs. Lipochito-oligosaccharides (LCOs) were used to interfere with growth ( De Ruijter et al . 1998 , Plant J. 13, 341–350), and cytochalasin D (CD) was used to interfere with actin function. Actin filament bundles lie net-axially in cytoplasmic strands in the root hair tube. In the subapex of growing hairs, these bundles flare out into fine bundles. The apex is devoid of actin filament bundles. This subapical actin filament configuration is not present in full-grown hairs; instead, actin filament bundles loop through the tip. After LCO application, the tips of hairs that are terminating growth swell, and a new outgrowth appears from a site in the swelling. At the start of this outgrowth, net-axial fine bundles of actin filaments reappear, and the tip region of the outgrowth is devoid of actin filament bundles. CD at 1.0 μ m , which does not affect cytoplasmic streaming, does not inhibit bulge formation and LCO-induced swelling, but inhibits initiation of polar growth from bulges, elongation of root hairs and LCO-induced outgrowth from swellings. We conclude that elongating net-axial fine bundles of actin filaments, which we call FB-actin, function in polar growth by targeting and releasing Golgi vesicles to the vesicle-rich region, while actin filament bundles looping through the tip impede vesicle retention.     

Regulation of root hair density by phosphorus availability in Arabidopsis thaliana

We characterized the response of root hair density to phosphorus (P) availability in Arabidopsis thaliana. Arabidopsis plants were grown aseptically in growth media with varied phosphorus concentrations, ranging from 1 mmol m⁻³ to 2000 mmol m⁻³ phosphorus. Root hair density (number of root hairs per mm of root length) was analysed starting at 7 d of growth. Root hair density was highly regulated by phosphorus availability, increasing significantly in roots exposed to low-phosphorus availability. The initial root hairs produced by the radicle were not sensitive to phosphorus availability, but began to respond after 9 d of growth. Root hair density was about five times greater in low phosphorus (1 mmol m⁻³) than in high phosphorus (1000 mmol m⁻³) media. Root hair density decreased logarithmically in response to increasing phosphorus concentrations within that range. Root hair density also increased in response to deficiencies of several other nutrients, but not as strongly as to low phosphorus. Indoleacetic acid (IAA), the auxin transport inhibitor 2-(p-chlorophenoxy)-2-methylpropionic acid (CMPA), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), and the ethylene synthesis inhibitor amino-oxyacetic acid (AOA) all increased root hair density under high phosphorus but had very little effect under low phosphorus. Low phosphorus significantly changed root anatomy, causing a 9% increase in root diameter, a 31% decrease in the cross-sectional area of individual trichoblasts, a 40% decrease in the cross-sectional area of individual atrichoblasts, and 45% more cortical cells in cross-section. The larger number of cortical cells and smaller epidermal cell size in low phosphorus roots increased the number of trichoblast files from eight to 12. Two-thirds of increased root hair density in low phosphorus roots was caused by increased likelihood of trichoblasts to form hairs, and 33% of the increase was accounted for by changes in low phosphorus root anatomy resulting in an increased number of trichoblast files. These results show that phosphorus availability can fundamentally alter root anatomy, leading to changes in root hair density, which are presumably important for phosphorus acquisition.