Lacking Root Hairs Research Articles

Interactions between root hairs and the soil microbial community affect the growth of maize seedlings.

Root hairs are considered important for rhizosphere formation, which affects root system functioning. Through interactions with soil microorganisms mediated by root exudation, root hairs may affect the phenotypes and growth of young plants. We tested this hypothesis by integrating results from two experiments: (1) a factorial greenhouse seedling experiment with Zea mays B73-wt and its root-hairless mutant, B73-rth3, grown in live and autoclaved soil, quantifying 15 phenotypic traits, seven growth rates, and soil microbiomes and (2) a semi-hydroponic system quantifying root exudation of maize genotypes. Possibly as compensation for lacking root hairs, B73-rth3 seedlings allocated more biomass to roots and grew slower than B73-wt seedlings in live soil, whereas B73-wt seedlings grew slowest in autoclaved soil, suggesting root hairs can be costly and their benefits were realized with more complete soil microbial assemblages. There were substantial differences in root exudation between genotypes and in rhizosphere versus non-rhizosphere microbiomes. The microbial taxa enriched in the presence of root hairs generally enhanced growth compared to taxa enriched in their absence. Our findings suggest the root hairs' adaptive valueextends to plant-microbe interactions mediated byroot exudates, affecting plant phenotypes, and ultimately, growth.

Do root hairs of barley and maize roots reinforce soil under shear stress?

Roots reinforce soil by acting as soil pins, dissipating shear stresses and anchoring the soil in place. By protruding into the soil and binding to soil particles, root hairs increase root-soil contact and aid root anchorage. However, it is not yet known whether this ability to anchor roots affects the root system’s ability to reinforce soil. Using a laboratory box shearing rig, this study explores whether root hairs affect soil shear resistance. The force required to shear soil columns permeated with roots lacking root hairs (barley brb and maize rth3 mutants) are compared to columns permeated with hairy roots (their respective wild types, WT) using unplanted soil columns as controls. Known root traits (e.g. root length density, root surface area density, average diameter, percentage of fine roots, and root tensile strength) were measured to ensure that differences in shear resistance could be attributed to the presence/absence of root hairs. All rooted columns required more force to shear than their respective unplanted columns but the thicker, stronger maize roots were more effective at soil reinforcement than the more numerous but weaker barley roots. After the maximum growth period, root hairs appeared to have a consistent and significant impact on peak shearing force. However, the WT root systems also produced greater root surface area density. As the rate at which peak shearing force increased with increasing root surface area density was similar for roots with and without root hairs, the increased peak shearing force of the WT columns cannot be attributed to resistance supplied by the presence of root hair but rather to a more prolific root system. Therefore, it was concluded that root diameter and root tensile strength most influenced root reinforcement of soil and as such, the relatively minute root hairs had negligible effects compared to their parent roots.

Expansin gene loss is a common occurrence during adaptation to an aquatic environment.

Expansins comprise a superfamily of plant cell wall loosening proteins that can be divided into four individual families (EXPA, EXPB, EXLA and EXLB). Aside from inferred roles in a variety of plant growth and developmental traits, little is known regarding the function of specific expansin clades, for which there are at least 16 in flowering plants (angiosperms); however, there is evidence to suggest that some expansins have cell-specific functions, in root hair and pollen tube development, for example. Recently, two duckweed genomes have been sequenced (Spirodela polyrhiza strains 7498 and 9509), revealing significantly reduced superfamily sizes. We hypothesized that there would be a correlation between expansin loss and morphological reductions seen among highly adapted aquatic species. In order to provide an answer to this question, we characterized the expansin superfamilies of the greater duckweed Spirodela, the marine eelgrass Zostera marina and the bladderwort Utricularia gibba. We discovered rampant expansin gene and clade loss among the three, including a complete absence of the EXLB family and EXPA-VII. The most convincing correlation between morphological reduction and expansin loss was seen for Utricularia and Spirodela, which both lack root hairs and the root hair expansin clade EXPA-X. Contrary to the pattern observed in other species, four Utricularia expansins failed to branch within any clade, suggesting that they may be the result of neofunctionalization. Last, an expansin clade previously discovered only in eudicots was identified in Spirodela, allowing us to conclude that the last common ancestor of monocots and eudicots contained a minimum of 17 expansins.

Arabidopsis ZINC FINGER PROTEIN1 Acts Downstream of GL2 to Repress Root Hair Initiation and Elongation by Directly Suppressing bHLH Genes.

Cys2His2-like fold group (C2H2)-type zinc finger proteins promote root hair growth and development by regulating their target genes. However, little is known about their potential negative roles in root hair initiation and elongation. Here, we show that the C2H2-type zinc finger protein named ZINC FINGER PROTEIN1 (AtZP1), which contains an ERF-associated amphiphilic repression (EAR) motif, negatively regulates Arabidopsis (Arabidopsis thaliana) root hair initiation and elongation. Our results demonstrate that AtZP1 is highly expressed in root hairs and that AtZP1 inhibits transcriptional activity during root hair development. Plants overexpressing AtZP1 lacked root hairs, while loss-of-function mutants had longer and more numerous root hairs than the wild type. Transcriptome analysis indicated that AtZP1 downregulates genes encoding basic helix-loop-helix (bHLH) transcription factors associated with root hair cell differentiation and elongation. Mutation or deletion of the EAR motif substantially reduced the inhibitory activity of AtZP1. Chromatin immunoprecipitation assays, AtZP1:glucocorticoid receptor (GR) induction experiments, electrophoretic mobility shift assays, and yeast one-hybrid assays showed that AtZP1 directly targets the promoters of bHLH transcription factor genes, including the key root hair initiation gene ROOT HAIR DEFECTIVE6 (RHD6) and root hair elongation genes ROOT HAIR DEFECTIVE 6-LIKE 2 (RSL2) and RSL4, and suppresses root hair development. Our findings suggest that AtZP1 functions downstream of GL2 and negatively regulates root hair initiation and elongation, by suppressing RHD6, RSL4, and RSL2 transcription via the GL2/ZP1/RSL pathway.

Interaction between root hairs and soil phosphorus on rhizosphere priming of soil organic matter

We hypothesized that the rhizosphere priming effect (RPE) of soil organic matter by mutant barley lacking root hairs is dependant on a large network of symbiotic arbuscular mycorrhizal fungi (AMF). We thus predicted that fertilizing with phosphate-P would reduce AMF abundance and, in turn, reduce RPE of mutant barley. We packed microcosms with a P-responsive soil in which we grew mutant barley lacking root hairs as well as wild type barley and narrowleaf plantain, each possessing root hairs. One set of microcosms was fertilized with phosphate-P while another set was not fertilized. The plants were grown in a labelling chamber with 13C-depleted CO2. Soil respiration and δ13C of headspace CO2 were measured after 3, 4 and 5 weeks and RPE was calculated using an isotope mass balance approach. Root hair length was measured and soils were analyzed for the 16:1ω5 neutral lipid fatty acid (i.e. AMF biomarker). AMF abundance was greater, whereas RPE was lower, in mutant barley soil under low-P than under high-P conditions. In the other two plant-types, P had no effect on AMF or on RPE. As our results contradict our prediction, we propose an alternative explanation based on plant N demand under high-P.

Forward Genetics Approach Reveals a Mutation in bHLH Transcription Factor-Encoding Gene as the Best Candidate for the Root Hairless Phenotype in Barley.

Root hairs are the part of root architecture contributing significantly to the root surface area. Their role is particularly substantial in maintaining plant growth under stress conditions, however, knowledge on mechanism of root hair differentiation is still limited for majority of crop species, including barley. Here, we report the results of a map-based identification of a candidate gene responsible for the lack of root epidermal cell differentiation, which results in the lack of root hairs in barley. The analysis was based on the root hairless barley mutant rhl1.b, obtained after chemical mutagenesis of spring cultivar ‘Karat’. The rhl1 gene was located in chromosome 7HS in our previous studies. Fine mapping allowed to narrow the interval encompassing rhl1 gene to 3.7 cM, which on physical barley map spans a region of 577 kb. Five high confidence genes are located within this region and their sequencing resulted in the identification of A>T mutation in one candidate, HORVU7Hr1G030250 (MLOC_38567), differing the mutant from its parent variety. The mutation, located in the 3′ splice-junction site, caused the retention of the last intron, 98 bp long, in mRNA of rhl1.b allele. This resulted in the frameshift, the synthesis of 71 abnormal amino acids and introduction of premature STOP codon in mRNA. The mutation was present in the recombinants from the mapping population (F2 rhl1.b × ‘Morex’) that lacked root hairs. The candidate gene encodes a bHLH transcription factor with LRL domain and may be involved in early stages of root hair cell development. We discuss the possible involvement of HORVU7Hr1G030250 in this process, as the best candidate responsible for early stages of rhizodermis differentiation in barley.

Ontogenetic transition from specialized root hairs to specific root-fungus symbiosis in the dominant Mediterranean seagrass Posidonia oceanica

Terrestrial plants typically take up nutrients through roots or mycorrhizae while freshwater plants additionally utilize leaves. Their nutrient uptake may be enhanced by root hairs whose occurrence is often negatively correlated with mycorrhizal colonization. Seagrasses utilize both leaves and roots and often form root hairs, but seem to be devoid of mycorrhizae. The Mediterranean seagrass Posidonia oceanica is an exception: its adults commonly lack root hairs and regularly form a specific association with a single pleosporalean fungus. Here we show that at two sites in the southern Adriatic, all its seedlings possessed abundant root hairs with peculiar morphology (swollen terminal parts) and anatomy (spirally formed cell walls) as apparent adaptations for better attachment to the substrate and increase of breaking strain. Later on, their roots became colonized by dark septate mycelium while root hairs were reduced. In adults, most of terminal fine roots possessed the specific fungal association while root hairs were absent. These observations indicate for the first time that processes regulating transition from root hairs to root fungal colonization exist also in some seagrasses. This ontogenetic shift in root traits may suggests an involvement of the specific root symbiosis in the nutrient uptake by the dominant Mediterranean seagrass.

Análisis del desarrollo de plantas de mortiño (Vaccinium meridionale Swart.) bajo dos sistemas de propagación: clonal y sexual

La investigación realizada en el CI La Selva de Corpoica, Rionegro-Antioquia, a 2.120 msnm, formación ecológica bosque húmedo montano bajo, buscó caracterizar el crecimiento y desarrollo de plantas de mortiño, Vaccinium meridionale Sw., provenientes de poblaciones espontáneas, obtenidas a partir de semilla y multiplicación vegetativa. Estas se trasplantaron a rizotrones, con vidrios laterales en dos de sus costados, para el registro del crecimiento de las raíces. Las plantas tomaron 1.663 y 1.367 días, desde la siembra de la semilla y el enraizamiento de estacas a floración, respectivamente. Las raíces de ambos tipos de material no presentaron pelos radicales. En las dos clases de plantas no se apreciaron períodos de alternancia en el crecimiento de las órganos aéreos y los subterráneos, con relaciones alométricas entre la parte aérea y las raíces del orden de 6,3 para los individuos obtenidos por semilla y 7,0 para los de multiplicación asexual. A pesar de que se detectaron flujos de floración, se apreció formación continua de flores en ambos materiales. Los registros de desarrollo, en las plantas propagadas sexualmente, exhibieron una mayor dispersión que aquellos obtenidos con las multiplicadas vegetativamente.

Characteristics of a root hair-less line of Arabidopsis thaliana under physiological stresses

The plasma membrane-associated Ca(2+)-binding protein-2 of Arabidopsis thaliana is involved in the growth of root hair tips. Several transgenic lines that overexpress the 23 residue N-terminal domain of this protein under the control of the root hair-specific EXPANSIN A7 promoter lack root hairs completely. The role of root hairs under normal and stress conditions was examined in one of these root hair-less lines (NR23). Compared with the wild type, NR23 showed a 47% reduction in water absorption, decreased drought tolerance, and a lower ability to adapt to heat. Growth of NR23 was suppressed in media deficient in phosphorus, iron, calcium, zinc, copper, or potassium. Also, the content of an individual mineral in NR23 grown in normal medium, or in medium lacking a specific mineral, was relatively low. In wild-type plants, the primary and lateral roots produce numerous root hairs that become elongated under phosphate-deficient conditions; NR23 did not produce root hairs. Although several isoforms of the plasma membrane phosphate transporters including PHT1;1-PHT1;6 were markedly induced after growth in phosphate-deficient medium, the levels induced in NR23 were less than half those observed in the wild type. In phosphate-deficient medium, the amounts of acid phosphatase, malate, and citrate secreted from NR23 roots were 38, 9, and 16% of the levels secreted from wild-type roots. The present results suggest that root hairs play significant roles in the absorption of water and several minerals, secretion of acid phosphatase(s) and organic acids, and in penetration of the primary roots into gels.

A galacturonic acid-containing xyloglucan is involved in Arabidopsis root hair tip growth.

Root hairs provide a model system to study plant cell growth, yet little is known about the polysaccharide compositions of their walls or the role of these polysaccharides in wall expansion. We report that Arabidopsis thaliana root hair walls contain a previously unidentified xyloglucan that is composed of both neutral and galacturonic acid-containing subunits, the latter containing the β-D-galactosyluronic acid-(1→2)-α-D-xylosyl-(1→ and/or α-L-fucosyl-(1→2)-β-D-galactosyluronic acid-(1→2)-α-D-xylosyl-(1→) side chains. Arabidopsis mutants lacking root hairs have no acidic xyloglucan. A loss-of-function mutation in At1g63450, a root hair-specific gene encoding a family GT47 glycosyltransferase, results in the synthesis of xyloglucan that lacks galacturonic acid. The root hairs of this mutant are shorter than those of the wild type. This mutant phenotype and the absence of galacturonic acid in the root xyloglucan are complemented by At1g63450. The leaf and stem cell walls of wild-type Arabidopsis contain no acidic xyloglucan. However, overexpression of At1g63450 led to the synthesis of galacturonic acid-containing xyloglucan in these tissues. We propose that At1g63450 encodes XYLOGLUCAN-SPECIFIC GALACTURONOSYLTRANSFERASE1, which catalyzes the formation of the galactosyluronic acid-(1→2)-α-D-xylopyranosyl linkage and that the acidic xyloglucan is present only in root hair cell walls. The role of the acidic xyloglucan in root hair tip growth is discussed.

INOCULATION OF RHODODENDRON CULTIVARS IN VITRO WITH DIFFERENT STRAINS OF ERICOID ENDOMYCORRHIZAE

Rhododendron, as most of the Ericaceae, lacking root hairs, is generally mycorrhized by typical ericoid endomycorrhizae. In vitro grown plantlets are aseptic and thus can be artificially infected by specific strains in order to evaluate the best one for the establishment of the symbiosis. The objective of this research was the evaluation of the best mycorrhizal inoculum, within 11 fungal strains: (Hymeuoscyphus ericae, 6 different strains of Oidiodeudrou maius and 4 sterile non-classified mycelia isolated on Vaccinium or Calluna), for 3 commercial rhododendron cultivars ('Nova Zembla', 'Roseum Elegans' and 'Madame Masson'). Rooting and growth of mycorrhized plantlets were compared with those of control untreated cuttings of the same varieties and with cuttings treated with IBA (indole-butyric acid). After rooting, all of the plants were observed and measured for one year. Contrary to what was observed in previous experiments on Vaccinium and Azalea, the different fungal strains did not significantly influence the rooting time or percentage, which in rhododendron was only affected by the genotype, but had significant influence on the subsequent growth of plants. The effect of different mycorrhizal inocula on the growth of the tested cultivars is presented and discussed.

Evolution of mycoheterotrophy in plants: a phylogenetic perspective

Evolution of mycoheterotrophy in plants: a phylogenetic perspective

Multi-instrumental Analysis of Tissues of Sunflower Plants Treated with Silver(I) Ions - Plants as Bioindicators of Environmental Pollution.

The aim of this work is to investigate sunflower plants response on stressinduced by silver(I) ions. The sunflower plants were exposed to silver(I) ions (0, 0.1, 0.5,and 1 mM) for 96 h. Primarily we aimed our attention to observation of basic physiologicalparameters. We found that the treated plants embodied growth depression, coloured changes and lack root hairs. Using of autofluorescence of anatomical structures, such aslignified cell walls, it was possible to determine the changes of important shoot and rootstructures, mainly vascular bungles and development of secondary thickening. Thedifferences in vascular bundles organisation, parenchymatic pith development in the rootcentre and the reduction of phloem part of vascular bundles were well observable.Moreover with increasing silver(I) ions concentration the vitality of rhizodermal cellsdeclined; rhizodermal cells early necrosed and were replaced by the cells of exodermis.Further we employed laser induced breakdown spectroscopy for determination of spatialdistribution of silver(I) ions in tissues of the treated plants. The Ag is accumulated mainlyin near-root part of the sample. Moreover basic biochemical indicators of environmentalstress were investigated. The total content of proteins expressively decreased withincreasing silver(I) ions dose and the time of the treatment. As we compare the resultsobtained by protein analysis - the total protein contents in shoot as well as root parts - wecan assume on the transport of the proteins from the roots to shoots. This phenomenon canbe related with the cascade of processes connecting with photosynthesis. The secondbiochemical parameter, which we investigated, was urease activity. If we compared theactivity in treated plants with control, we found out that presence of silver(I) ions markedlyenhanced the activity of urease at all applied doses of this toxic metal. Finally we studiedthe effect of silver(I) ions on activity of urease in in vitro conditions.

Root, root hair, and symbiotic mutants of the model legume Lotus japonicus.

To gain an overview of plant factors controlling nodule number and organogenesis, an extensive screening using model legume Lotus japonicus was carried out. This screening involved 40,000 M2 seeds, and 32 stable mutant lines were isolated. From these, 16 mutant lines maintaining the phenotypic variation were selected and genetically analyzed. With respect to nodule number, four loci were identified, Ljsym77, Ljsym78, slippery root (slp), and radial organization1 (rdo1). The former two mutants have an increased number of nodules, while the latter two have a decreased number. Ljsym78-1 and Ljsym78-2 are hypernodulating mutants with a branched root system and were found to be allelic to Ljsym16. The phenotype of the Ljsym77 mutant was highly pleiotropic, being deficient in light and gravity responses. The slp mutant was isolated as a low-nodulating mutant lacking root hairs. Concerning nodule organogenesis, nine symbiotic loci were identified, including the two loci alb1 and fen1. Mutants affecting the developmental process of nodule organogenesis were placed in three phenotypic categories: Nod- (Ljsym70 to Ljsym73), Hist- (alb1-1, alb1-2, and Ljsym79), and Fix- (fen1, Ljsym75, and Ljsym81).

Genotypic differences in the presence of hairs on roots and gynophores of peanuts ( Arachis hypogaea L.) and their significance for phosphorus uptake

Root hairs substantially increase the surface area of plant roots with positive effects for phosphorus (P) uptake, but the ability of peanuts to form root hairs has been questioned. The aim was to examine hair development on roots and gynophores of a variety of peanut genotypes and to relate genotypic differences in hair formation to differences in P uptake. Five out of eighteen genotypes completely lacked hairs on both organs whereas others consistently developed hairs on roots and gynophores, although with considerable variation in hair density. The ability to form root hairs as well as root hair density concurred with the presence and density of hairs on gynophores, suggesting a possible connection between both developmental processes. The contribution of root hairs to P uptake was studied in three genotypes differing in hair density. The final amount of P taken up by roots did not differ between genotypes but two distinct P uptake strategies could be identified. The genotype lacking root hairs maintained P uptake due to the development of a large root system whereas densely covered roots of genotype 'Wasedairyu' were three times as efficient in extracting P from a P-deficient soil. Furthermore P uptake through gynophores contributed about 20% to the total P uptake of Wasedairyu but only insignificant amounts to other genotypes. The ability to form hairs on roots and gynophores can therefore be seen as an adaptation to low P availability and if combined with a large root system, could substantially increase the tolerance of peanuts to P deficiency.

Auxin and ethylene promote root hair elongation in Arabidopsis.

Genetic and physiological studies implicate the phytohormones auxin and ethylene in root hair development. To learn more about the role of these compounds, we have examined the root hair phenotype of a number of auxin- and ethylene-related mutants. In a previous study, Masucci and Schiefelbein (1996) showed that neither the auxin response mutations aux1 and axr1 nor the ethylene response mutations etr1 and ein2 have a significant effect on root hair initiation. In this study, we found that mutants deficient in either auxin or ethylene response have a pronounced effect on root hair length. Treatment of wild-type, axr1 and etr1 seedlings with the synthetic auxin, 2,4-D, or the ethylene precursor ACC, led to the development of longer root hairs than untreated seedlings. Furthermore, axr1 seedlings grown in the presence of ACC produce ectopic root hairs and an unusual pattern of long root hairs followed by regions that completely lack root hairs. These studies indicate that both auxin and ethylene are required for normal root hair elongation.

Occurrence of Vine Decline Diseases of Melons in Honduras.

A survey of vine decline diseases of melons (Cucumis melo L.) was conducted on three geographically separate farms (12 fields) in the area of San Lorenzo, Honduras, during the spring of 1996. Symptoms were typical of the vine declines in that the leaves began yellowing and collapsing in the crown just prior to harvest and the decline radiated outward. Crown lesions, typical of many of the vine declines, were observed infrequently. Most plants exhibited no crown lesion. Roots exhibited brown lesions and root corking, and were generally lacking root hairs, although the roots were not macerated. Fruit were small sized, low in sugars, and sunburned. Isolations were made from the crown area, primary root, and secondary roots of affected plants by excising 3- to 5-mm pieces and surface sterilizing for 30 to 60 s with 0.5% sodium hypochlorite. Four tissue pieces from each plant part were placed on 2.0% water agar plus 0.1 g of streptomycin sulfate per liter and their identity maintained. Plates were examined for fungal growth daily for 5 days and hyphal tips from all colonies were transferred to potato dextrose agar and Synthetischer nährstoffärmer agar (2) to allow for sporulation. Rhizoctonia solani Kühn was the predominant fungus isolated from affected plants in several fields, with an incidence as high as 75%. A Pythium sp. was isolated from primary and secondary roots of 50% of the plants on the Santa Rosa farm. Vines exhibiting tan to light brown crown lesions either on one side of the vine or encompassing the crown, but without gumming, always produced colonies of Fusarium semitectum Berk. & Ravenel. Fusarium solani (Mart.) Appel & Wollenweb. emend. W. C. Snyder & H. N. Hans. was isolated from roots of more than 30% of the plants, although this is not unusually high. Crown lesions typical of charcoal rot were observed in some fields, with Macrophomina phaseolina (Tassi) Goidanich being isolated from 65% of the plants in one field. Charcoal rot was a minor problem in all fields but one. Didymella bryoniae (Auersw.) Rehm was not isolated from any of the plants. A Stagonospora-like fungus, which has been demonstrated recently as a potential contributor to vine decline (1), was isolated on 17% of the plants from five fields on the Embarcadero Farm. Monosporascus cannonballus Pollack & Uecker was isolated infrequently in one field on the Santa Rosa Farm. This is the first report of M. cannonballus and a Stagonospora-like fungus in Honduras.

Fossil ectomycorrhizae from the Middle Eocene

Fossil ectomycorrhizae were found recently among permineralized plant remains in the middle Eocene Princeton chert of British Columbia. The ectomycorrhizae are associated with roots of Pinus and have a Hartig net that extends to the endodermis, a pseudoparenchymatous mantle, and contiguous extramatrical hyphae that are simple-septate. The mycorrhizal rootlets lack root hairs and dichotomize repeatedly to form large, coralloid clusters. Reproductive structures are absent. Based on the morphological characteristics, and the identity of the host, the closely related basidiomycete genera Rhizopogon and Suillus are suggested as comparable extant mycorrhizal fungi. These exquisitely preserved specimens represent the first unequivocal occurrence of fossil ectomycorrhizae and demonstrate that such associations were well-established at least 50 million years ago.

Boletinellus MerulioidesAlters Root Morphology ofPinus DensifloraWithout Mycorrhizal Formation

A stimulatory effect of Boletinellus merulioides on the roots of Pinus densiflora was evaluated using pure culture synthesis. Seedlings of P. densiflora were inoculated with cultures of B. merulioides at high and low glucose levels. After 180 days, shoot weight was not enhanced over controls by inoculation with the fungus, but root weight was significantly increased and roots were highly branched. Short roots were dichotomously branched, lacked root hairs, and had an internal morphology similar to that of ectomycorrhizal rootlets. A loose weft of fungal hyphae surrounded the short roots, but did not penetrate the epidermis. Higher glucose levels further increased root weight and branching. Filtrates from liquidgrown cultures of B. merulioides and Pisolithus tinctorius were compared using high performance liquid chromatography. Both fungi produced measurable quantities of indole-3-acetic acid, the in-vitro production of which was enhanced by higher glucose levels.

Microscopic evidence on how iron deficiency limits nodule initiation in Lupinus angustifolius L.

Lupins (Lupinus angustifolius L. ev. Yandee), grown in solution culture, have been used to study the sites and process of infection by Bradyrhizobium sp. (Lupinus) and the impairment of nodulation by iron deficiency. Infection leading to nodulation occurred in an area of epidermal cells either lacking root hairs or with very young root hairs at the time of inoculation. Cells aged 13 h or over appeared not to be infected. Infection was initiated in the outer cortex. Rare, short infection threads were evident on day 4 after inoculation, 2 d after the initial division of cortical cells resulting from the bradyrhizobial inoculation. Bacteria had been released into the cytoplasm of cortical cells within 5 d after inoculation. Bacteroids multiplied in the cytoplasm, segregated passively and spread in the infection zones by repeated division of the invaded cells. Under iron deficiency, initial cell division occurred in the outer cortex of host roots, as in iron-sufficient plants after inoculation. Iron deficiency then limited further division of cortical cells. Only a few surviving infection sites developed nodules with normal structure but development was much slower than in iron-sufficient plants.