Lectin-enhanced accumulation of manganese-limited Rhizobium leguminosarum cells on pea root hair tips.

The ability of Rhizobium leguminosarum 248 to attach to developing Pisum sativum root hairs was investigated during various phases of bacterial growth in yeast extract-mannitol medium. Direct cell counting revealed that growth of the rhizobia transiently stopped three successive times during batch culture in yeast extract-mannitol medium. These interruptions of growth, as well as the simultaneous autoagglutination of the bacteria, appeared to be caused by manganese limitation. Rhizobia harvested during the transient phases of growth inhibition appeared to have a better attachment ability than did exponentially growing rhizobia. The attachment characteristics of these manganese-limited rhizobia were compared with those of carbon-limited rhizobia (G. Smit, J. W. Kijne, and B. J. J. Lugtenberg, J. Bacteriol. 168:821-827, 1986, and J. Bacteriol. 169:4294-4301, 1987). In contrast to the attachment of carbon-limited cells, accumulation of manganese-limited rhizobia (cap formation) was already in full progress after 10 min of incubation; significantly delayed by 3-O-methyl-D-glucose, a pea lectin haptenic monosaccharide; partially resistant to sodium chloride; and partially resistant to pretreatment of the bacteria with cellulase. Binding of single bacteria to the root hair tips was not inhibited by 3-O-methyl-D-glucose. Whereas attachment of single R. leguminosarum cells to the surface of pea root hair tips seemed to be similar for both carbon- and manganese-limited cells, the subsequent accumulation of manganese-limited rhizobia at the root hair tips is apparently accelerated by pea lectin molecules. Moreover, spot inoculation tests with rhizobia grown under various culture conditions indicated that differences in attachment between manganese- and carbon-limited R. leguminosarum cells are correlated with a significant difference in infectivity in that manganese-limited rhizobia, in contrast to carbon-limited rhizobia, are infective. This growth-medium-dependent behavior offers and explanation for the seemingly conflicting data on the involvement of host plant lectins in attachment of rhizobia to root hairs of leguminous plants. Sym plasmid-borne genes do not play a role in manganese-limitation-induced attachment of R. leguminosarum.     

Involvement of Rhizobium leguminosarum nodulation genes in gene expression in pea root hairs

The mRNA population in pea root hairs was characterized by means of in vitro translation of total root hair RNA followed by 2-dimensional gel electrophoresis of the translation products. Root hairs contain several mRNAs not detectable in total RNA preparations from roots. Most of these root hair-specific mRNAs occur in elongating root hairs at higher levels than in mature root hairs. The expression of some genes in pea root hairs is typically affected by inoculation with Rhizobium leguminosarum. One gene, encoding RH-42, is specifically induced while the expression of another gene, encoding RH-44, is markedly enhanced. Using R. leguminosarum mutants it was shown that the nodC gene is required for the induction and enhancement of expression of the RH-42 and RH-44 genes, respectively, while the Rhizobium chromosomal gene pss1, involved in exopolysaccharide synthesis, is not essential. After induction of the nod genes with apigenin the bacteria excrete into the culture medium a factor that causes root hair deformation. This deformation factor stimulates the expression of the RH-44 gene but does not induce the expression of the gene encoding RH-42.     

Environmentally induced plasticity of root hair development in Arabidopsis

Postembryonic development of plants is dependent on both intrinsic genetic programs and environmental factors. The plasticity of root hair patterning in response to environmental signals was investigated in the Columbia-0 wild type and 19 Arabidopsis mutants carrying lesions in various parts of the root hair developmental pathway by withholding phosphate or iron (Fe) from the nutrient medium. In the aging primary root and in laterals of the wild type, the number of root hairs increased in response to phosphate and Fe deficiency in a manner typical of each growth type. Although an increase in root hair density in -phosphorus plants was mainly achieved by the formation of extra hairs over both tangential and radial wall of underlying cortical cells, roots of -Fe plants were characterized by a high percentage of extra hairs with two tips. Root hair patterning and hair length was differentially affected by the presence or absence of phosphate and Fe among the genotypes under investigation, pointing to separate cascades of gene activation under all three growth conditions. Divergence in root hair patterning was most pronounced among mutants with defects in genes that affect the first stages of differentiation, suggesting that nutritional signals are perceived at an early stage of epidermal cell development. During elongation of the root hairs, no differences in the requirement of gene products between the growth types were obvious. The role of genes involved in root hair development in the aging primary root of Arabidopsis under the various growth conditions is discussed.     

Observations on root hair production by lucerne,maize and perennial ryegrass grown in a sandy loam

Summary Root hair production by young plants of lucerne, maize and perennial ryegrass grown in a sandy loam was assessed by examining roots growing at a soil-glass interface. Results are given for the percentage frequency distribution of root hair lengths and the numbers of root hairs produced per mm root. The mean lengths of root hairs observed on lucerne, maize and perennial ryegrass roots were 0.35, 0.90 and 1.12 mm respectively. Lucerne produced an average of 105 root hairs per mm of root, whereas maize produced 161 and perennial ryegrass produced 88. The total length of root hairs per mm length of root was estimated to be 37, 146 and 99 mm for lucerne, maize and perennial ryegrass resp. Letcombe Laboratory     

Role of manganese in low-pH-induced root hair formation in<Emphasis Type="Italic"> Lactuca sativa</Emphasis> cv. Grand Rapids seedlings

Root hair formation is induced by low pH in lettuce (Lactuca sativa L. cv. Grand Rapids) seedlings cultured in mineral medium. The role of mineral concentrations in this phenomenon was investigated, especially for manganese. When lettuce seedlings were cultured in media that were deficient in calcium (Ca), manganese (Mn), boron (B) or molybdenum (Mo), morphological changes were induced in roots. Deficiency of other nutrients had little effect on root hair formation. Ca or B deficiency inhibited the growth of the main root and the formation of root hairs, regardless of pH. Mn or Mo deficiency increased root hair formation at pH 6 and suppressed main root growth slightly. In contrast, increasing the Mn concentration suppressed low-pH-induced root hair formation. The Mn content of roots grown at pH 4 was only about 15% of that at pH 6. In contrast, the Mo content of roots grown at low pH was about six times that of roots grown at neutral pH. These results suggest that root hair formation induced by low pH is at least partly mediated by decreased Mn uptake in root cells.     

Heterogeneous nutrient supply promotes maize growth and phosphorus acquisition: additive and compensatory effects of lateral roots and root hairs

Background and AimsRoot proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition.MethodsNear-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg⁻¹ soil.ResultsBoth WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients.ConclusionsIn addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.     

A method for the isolation of root hairs from the model legume Medicago truncatula

A new method for the isolation of root hairs from the model legume, Medicago truncatula, was developed. The procedure involves the propagation of detached roots on agar plates and the collection of root hairs by immersion in liquid nitrogen. Yields of up to 40 micro g of root hair protein were obtained from 50-100 root tips grown for 3 weeks on a single plate. The high purity of the root hair fraction was monitored by western blot analysis using an antibody to the pea epidermis specific protein PsRH2. Sequence analyses revealed that the protein homologous to PsRH2 in M. truncatula, MtRH2, is identical to the root protein MtPR10-1. The MtRH2 protein proved to be a useful endogenous marker to monitor root hair isolation since it is also specifically expressed in the root epidermis.     

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.     

A study of the role of root morphological traits in growth of barley in zinc-deficient soil

Zinc (Zn) deficiency reduces crop yields globally. This study investigated the importance of root morphological traits, especially root hairs, in plant growth and Zn uptake. Wild-type barley (Hordeum vulgare) Pallas and its root-hairless mutant brb were grown in soil and solution culture at different levels of Zn supply for 16 d. Root morphological traits (root length, diameter, and surface area) were measured using the WinRHIZOPro Image Analysis system. In soil culture, Pallas had greater shoot dry matter, shoot Zn concentration, shoot Zn content, and Zn uptake per cm(2) root surface area than brb, primarily under zinc deficiency. Both Pallas and brb developed longer roots under Zn deficiency. Development of root hairs was not affected by plant Zn status. In solution culture, there were no significant genotypic differences in any of the parameters measured, indicating that mutation in brb does not affect growth and Zn uptake. However, both Pallas and brb developed longer and thinner roots, and root hair growth was less than in soil culture, and was not affected by plant Zn status. The better growth and greater Zn uptake of Pallas compared with brb in Zn-deficient soil can be attributed primarily to greater root surface area due to root hairs in Pallas rather than other root morphological differences.     

Membrane transport and cytokinin action in root hairs of Medicago sativa

In an effort to develop a cellular model for studying cytokinin action in higher plants, we investigated the effect of cytokinins on growth and membrane transport in root hairs of alfalfa (Medicago sativa␣L.) seedlings. Alfalfa seedlings grown for 24 h in the presence of cytokinins showed increased root hair length and formation of root hairs close to the root cap. Increased growth of root hairs was observed within 10 min of cytokinin application and was accompanied by a hyperpolarization of the plasma membrane. The ion-transport inhibitor diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) blocked both cytokinin-induced root hair growth and hyperpolarization but did not, by itself, alter growth or membrane potential. Finally, hyperpolarization was induced by extracellular cytokinin but not by injection of cytokinin into the cytoplasm. These findings show that root hairs undergo several rapid responses to cytokinin, including changes in membrane transport and growth. We conclude that multiple cytokinin response in root hairs may be mediated by events involving perception and ion transport at the plasma membrane. Received: 18 July 1996 / Accepted: 1 October 1996     

Role of root hairs in phosphorus depletion from a macrostructured soil

One rape (Brassica napus cv. Wesroona) plant and four cotton (Gossypium hirsutum cv. Sicot 3) plants were grown in plastic cells containing soil labelled with 407 kBq of³³P g⁻¹ soil. After 5–8 days of growth, the³³P depletion zones of all plants were autoradiographed and³³P uptake by plants was measured. The autoradiographs were scanned with a microdensitometer and the optical densities at several places within the³³P depletion zones of roots were obtained. The volume of soil explored by root hairs was estimated from measurements of root diameters and lengths of roots and root hairs. About half of the total³³P depleted by cotion roots came from outside the root hair cylinder whereas most of³³P taken up by rape was from within the root hair cylinder. Plants grown in a macrostructured soil may have roots growing in voids, within aggregates or on the surfaces of aggregates. The results of this study demonstrate that root hairs have a strong influence on the accessibility of phosphorus to roots in such a soil, and thus on the phosphorus nutrition of plants.     

Microtubule dynamics in living root hairs: transient slowing by lipochitin oligosaccharide nodulation signals

The incorporation of a fusion of green fluorescent protein and tubulin-alpha 6 from Arabidopsis thaliana in root hairs of Lotus japonicus has allowed us to visualize and quantify the dynamic parameters of the cortical microtubules in living root hairs. Analysis of individual microtubule turnover in real time showed that only plus polymer ends contributed to overall microtubule dynamicity, exhibiting dynamic instability as the main type of microtubule behavior in Lotus root hairs. Comparison of the four standard parameters of in vivo dynamic instability--the growth rate, the disassembly rate, and the frequency of transitions from disassembly to growth (rescue) and from growth to disassembly (catastrophe)--revealed that microtubules in young root hairs were more dynamic than those in mature root hairs. Either inoculation with Mesorhizobium loti or purified M. loti lipochitin oligosaccharide signal molecules (Nod factors) significantly affected the growth rate and transition frequencies in emerging and growing root hairs, making microtubules less dynamic at a specific window after symbiotic inoculation. This response of root hair cells to rhizobial Nod factors is discussed in terms of the possible biological significance of microtubule dynamics in the early signaling events leading to the establishment and progression of the globally important Rhizobium/legume symbiosis.     

The Involvement of Endogenous Phenolic Compounds in the Response of Pea Seedling Roots to Inoculation with Rhizobium leguminosarum at Various Temperatures

Endogenous phenolic compounds (PC) affecting Rhizobium leguminosarum bv. viceae propagation were isolated from the roots of etiolated pea (Pisum sativum L.) seedlings before and within one or two day after inoculation. It was established that, during the first day after inoculation, PC-induced stimulation of bacterial growth in roots was replaced by its inhibition, which was somewhat more pronounced at 8°C. The ratio between PC fractions was also changed during the first day after inoculation, especially strongly at low temperature; and this was evidently the cause for Rhizobium growth inhibition in root cells.     

Variation in phosphorus uptake efficiency by genotypes of cowpea (Vigna unguiculata) due to differences in root and root hair length and induced rhizosphere processes

The lengths of roots and root hairs and the extent of root-induced processes affect phosphorus (P) uptake efficiency by plants. To assess the influence of variation in the lengths of roots and root hairs and rhizosphere processes on the efficiency of soil phosphorus (P) uptake, a pot experiment with a low-P soil and eight selected genotypes of cowpea (Vigna unguiculata (L) WALP) was conducted. Root length, root diameter and root hair length were measured to estimate the soil volume exploited by roots and root hairs. The total soil P was considered as a pool of Olsen-P, extractable with 0.5 M NaHCO₃ at pH 8.5, and a pool of non-Olsen-P. Model calculations were made to estimate P uptake originated from Olsen-P in the root hair zone and the Olsen-P moving by diffusion into the root hair cylinder and non-Olsen-P uptake. The mean uptake rate of P and the mean rate of non-Olsen-P depletion were also estimated. The genotypes differed significantly in lengths of roots and root hairs, and in P uptake, P uptake rates and growth. From 6 to 85% of total P uptake in the soil volume exploited by roots and root hairs was absorbed from the pool of non-Olsen-P. This indicates a considerable activity of root-induced rhizosphere processes. Hence the large differences show that traits for more P uptake-efficient plants exist in the tested cowpea genotypes. This opens the possibility to breed for more P uptake-efficient varieties as a way to bring more sparingly soluble soil P into cycling in crop production and obtain capitalisation of soil P reserves.     

Effect of growth temperature on the composition of phenols in pea roots

It was shown that, in pea (Pisum sativum L.) roots, flavans are a dominating component of the soluble phenol fraction. In plants grown at low temperature (8°C), flavan content during the early growth phase was lower than in plants grown at 22°C, but later it increased and was by 40% higher than in plants grown at 22°C. Total phenol content in the two treatments differed insignificantly. Low growth temperature decreased the content of some phenolic compounds in pea seedling roots.     

The efficiency of Arabidopsis thaliana (Brassicaceae) root hairs in phosphorus acquisition

Arabidopsis thaliana root hairs grow longer and denser in response to low-phosphorus availability. In addition, plants with the root hair response acquire more phosphorus than mutants that have root hairs that do not respond to phosphorus limiting conditions. The purpose of this experiment was to determine the efficiency of root hairs in phosphorus acquisition at high- and low-phosphorus availability. Root hair growth, root growth, root respiration, plant phosphorus uptake, and plant phosphorus content of 3-wk-old wild-type Arabidopsis (WS) were compared to two root hair mutants (rhd6 and rhd2) under high (54 mmol/m) and low (0.4 mmol/m) phosphorus availability. A cost-benefit analysis was constructed from the measurements to determine root hair efficiency. Under high-phosphorus availability, root hairs did not have an effect on any of the parameters measured. Under low-phosphorus availability, wild-type Arabidopsis had greater total root surface area, shoot biomass, phosphorus per root length, and specific phosphorus uptake. The cost-benefit analysis shows that under low phosphorus, wild-type roots acquire more phosphorus for every unit of carbon respired or unit of phosphorus invested into the roots than the mutants. We conclude that the response of root hairs to low-phosphorus availability is an efficient strategy for phosphorus acquisition.     

Root hair deformation in the white clover/Rhizobium trifolii symbiosis

Rhizobium trifolii most frequently infects its host white clover (Trifolium repens L.) by means of infection threads formed in markedly curled root hairs. Rhizobium infections are classified as either lateral or apical based on whether they originate in the branches or at the apex of the root hairs. A quantitative estimate of lateral and apical infection in the region of the host root (Trifolium repens L. cv. Regal Ladino) that possessed mature and immature root hairs at the time of inoculation with Rhizobium trifolii TAI (CSIRO, Canberra City, Australia) indicated that lateral infection occurred more frequently in the mature root hair region of the root. Apical infections were more common in the immature root hair region. Cell free filtrates collected from R. trifolii cultured in association with the host roots induced branching in white clover root hairs. A partially purified preparation of the branching factor was obtained from freeze-dried filtrates by ethanol extraction and ion exchange chromatography. Preliminary studies on the characteristics of these substances suggest that some are dialyzable and heat stable white others are non-dialyzable and heat labile. The dialyzable, heat-stable compounds contain neutral sugars and range between 1200 to 10000 daltons in size. In roots that were exposed to low concentrations (6–25 μg-ml^?1) of these partially purified deformation factors before inoculation, the developmentally mature root hairs were deformed at the time of inoculation. Nodules appeared in the mature and immature root hair region of these plants at the same time. In plants exposed to water, nodules were observed in the immature root hair region and mature root hair regions 3 and 5 days after inoculation, respectively. Based on these results, we conclude that the nodule development was hastened in the plants exposed to the root hair-deforming substances because the mature root hairs of these plants were made infectible at the time of inoculation by this exposure.     

Chlorogenic acid participates in the regulation of shoot,root and root hair development in Hypericum perforatum

Chlorogenic acid (CGA), a product of the phenylpropanoid pathway, is one of the most widespread soluble phenolic compounds in the plant kingdom. Although CGA is known to have important roles in plant function, its relevance in plant de novo organogenesis is not yet understood. With a series of experiments, here we show that CGA has a potential role in shoot, root and root hair development. In the first phase of our investigation, we developed an efficient and novel thin cell layer (TCL) regeneration protocol for Hypericum perforatum which could bridge all the in vitro morphogenetic stages between single cell and complete plant. Tissues at different morphogenetic states were analysed for their phenolic profile which revealed that shoot differentiation from callus tissues of H. perforatum is accompanied by the onset of CGA production. Further, the relevance of CGA in de novo organogenesis was deciphered by culturing highly organogenic root explants on media augmented with various concentrations of CGA. Results of this experiment showed that CGA concentrations lower than 10.0 mg l^?1 did not affect shoot organogenesis, whereas, higher concentrations significantly reduced this process in a concentration-dependent manner. In spite of the differential concentration-dependent effects of CGA on shoot regeneration, supplementation of CGA did not have any effect on the production of lateral roots and root hairs. Interestingly, CGA showed a concentration-dependent positive correlation with lateral roots and root hairs production in the presence of α-naphthaleneacetic acid (NAA). When the culture medium was augmented with 2-aminoindane-2-phosphonic acid (AIP), an inhibitor of phenylalanine ammonia lyase (PAL), induction of shoots, lateral roots and root hairs from the explants was significantly affected. Addition of an optimum concentration of CGA in these cultures partially restored all these organogenic processes.     

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.