Root Cortical Cells Research Articles

Growth and functional responses of different cultivars of Lotus corniculatus to aluminum and low pH stress

Aluminum toxicity is an important stress factor in acid soils. Growth, respiration and permeability properties of root cells were studied in five cultivars of Lotus corniculatus subjected to aluminum (Al) or low pH stress. The cultivars showed significant differences in root elongation under stress conditions, which correlated with changes in membrane potential (E(M)) of root cortical cells. A pH drop from 5.5 to 4.0 resulted in significant membrane depolarization and root growth inhibition. The strongest inhibition was observed in cv. São Gabriel (33.6%) and least in cv. UFRGS (25.8%). Application of an extremely high Al concentration (2mM) stopped the root growth in cv. INIA Draco, while inhibition in cv. UFRGS reached only 75%. The E(M) values of cortical cells of Lotus roots varied between -115 and -144mV. Treatment with 250microM of AlCl(3) (pH 4) resulted in rapid membrane depolarization. The extent of the membrane depolarization ranged between 51mV (cv. UFGRS) and 16mV (cv. INIA Draco). The membrane depolarization was followed by a loss of K(+) from Al-treated roots (2mM Al) and resulted in a decrease of the diffusion potential (E(D)). The total amount of K(+) in Al-treated roots dropped from 31.4 to 16.8micromolg(-1) FW in sensitive cv. INIA Draco, or from 26.1 to 22.7micromolg(-1) FW in tolerant cv. UFGRS. The rate of root respiration under control conditions as well as under Al treatment was higher in cv. INIA Draco than in cv. UFRGS. Al-induced inhibition of root respiration was 21-34% of the control.

Symbiotic use of pathogenic strategies: rhizobial protein secretion systems

Rhizobia - a diverse group of soil bacteria - induce the formation of nitrogen-fixing nodules on the roots of legumes. Nodulation begins when the roots initiate a molecular dialogue with compatible rhizobia in the soil. Most rhizobia reply by secreting lipochitooligosaccharidic nodulation factors that enable entry into the legume. A molecular exchange continues, which, in compatible interactions, permits rhizobia to invade root cortical cells, differentiate into bacteroids and fix nitrogen. Rhizobia also use additional molecular signals, such as secreted proteins or surface polysaccharides. One group of proteins secreted by rhizobia have homologues in bacterial pathogens and may have been co-opted by rhizobia for symbiotic purposes.

Regulação do desenvolvimento de micorrizas arbusculares

As micorrizas arbusculares (MAs) são associações simbióticas mutualistas entre fungos do filo Glomeromycota e a maioria das plantas terrestres. A formação e o funcionamento das MAs depende de um complexo processo de troca de sinais, que resulta em mudanças no metabolismo dos simbiontes e na diferenciação de uma interface simbiótica no interior das células das raízes. Os mecanismos que regulam a formação das MAs são pouco conhecidos, mas sabe-se que a concentração de fosfato (P) na planta é um fator determinante para o desenvolvimento da simbiose. A disponibilidade de P na planta pode afetar o balanço de açúcares e de fitormônios (FHs), além da expressão de genes de defesa vegetal. Com o advento da genômica e proteômica, vários genes essenciais para o desenvolvimento das MAs já foram identificados e seus mecanismos de regulação estão sendo estudados. Até o presente, sabe-se que as plantas secretam substâncias que estimulam a germinação de esporos e o crescimento de fungos micorrízicos arbusculares (FMAs). Há evidências também de que os FMAs sintetizam moléculas sinalizadoras, que são reconhecidas pelas plantas hospedeiras. Pelo menos três genes são essenciais para o reconhecimento dessa molécula e a transdução do sinal molecular. Discutem-se os papéis desses genes e os possíveis mecanismos que regulam sua expressão, bem como os papéis dos FHs na regulação de MAs são discutidos.

Medicago truncatula and Glomus intraradices gene expression in cortical cells harboring arbuscules in the arbuscular mycorrhizal symbiosis.

BackgroundMost vascular flowering plants have the capacity to form symbiotic associations with arbuscular mycorrhizal (AM) fungi. The symbiosis develops in the roots where AM fungi colonize the root cortex and form arbuscules within the cortical cells. Arbuscules are enveloped in a novel plant membrane and their establishment requires the coordinated cellular activities of both symbiotic partners. The arbuscule-cortical cell interface is the primary functional interface of the symbiosis and is of central importance in nutrient exchange. To determine the molecular events the underlie arbuscule development and function, it is first necessary to identify genes that may play a role in this process. Toward this goal we used the Affymetrix GeneChip® Medicago Genome Array to document the M. truncatula transcript profiles associated with AM symbiosis, and then developed laser microdissection (LM) of M. truncatula root cortical cells to enable analyses of gene expression in individual cell types by RT-PCR.ResultsThis approach led to the identification of novel M. truncatula and G. intraradices genes expressed in colonized cortical cells and in arbuscules. Within the arbuscule, expression of genes associated with the urea cycle, amino acid biosynthesis and cellular autophagy was detected. Analysis of gene expression in the colonized cortical cell revealed up-regulation of a lysine motif (LysM)-receptor like kinase, members of the GRAS transcription factor family and a symbiosis-specific ammonium transporter that is a likely candidate for mediating ammonium transport in the AM symbiosis.ConclusionTranscript profiling using the Affymetrix GeneChip® Medicago Genome Array provided new insights into gene expression in M. truncatula roots during AM symbiosis and revealed the existence of several G. intraradices genes on the M. truncatula GeneChip®. A laser microdissection protocol that incorporates low-melting temperature Steedman's wax, was developed to enable laser microdissection of M. truncatula root cortical cells. LM coupled with RT-PCR provided spatial gene expression information for both symbionts and expanded current information available for gene expression in cortical cells containing arbuscules.

GFP sheds light on the infection process of avocado roots by Rosellinia necatrix

In order to monitor Rosellinia necatrix infection of avocado roots, we generated a plasmid vector (pCPXHY1eGFP) constitutively expressing EGFP and developed a protoplast transformation protocol. Using this protocol, four R. necatrix isolates were efficiently transformed and were shown to stably express EGFP homogeneously while not having any observable effect on pathogenicity. Confocal laser scanning microscopy (CLSM) images of avocado roots infected with the highly virulent isolate CH53-GFP demonstrated that fungal penetration of avocado roots occurs simultaneously at several random sites, but it occurs preferentially in the crown region as well as throughout the lenticels and in the junctions between epidermal cells. Not only were R. necatrix hyphae observed invading the epidermal and cortical root cells, but they were also able to penetrate the primary and secondary xylem. Scanning electron microscopy (SEM) images allowed detailed visualisation of the hyphal network generated by invasion of R. necatrix through the epidermal, cortical and vascular cells, including hyphal anastomosis and branching points. To our knowledge, this is the first report describing the construction of GFP-tagged strains belonging to the genus Rosellinia for monitoring white root rot using CLSM and SEM.

Metal uptake by medicinal plant species grown in soils contaminated by a smelter

The hypothesis tested in this study was if medicinal plants could be grown as alternative crops in heavy metal polluted soils without contamination of the final marketable produce. Furthermore, medicinal crops may offer a phytoremediation option for mildly heavy metal polluted agricultural soils. The effect of metal-enriched soils was evaluated in five medicinal species ( Bidens tripartita L., Leonurus cardiaca L., Marrubium vulgare L., Melissa officinalis L. and Origanum heracleoticum L.). Soils were sampled in the vicinities of the Non-Ferrous Metals Combine (Pb–Zn smelter) near Plovdiv, Bulgaria, from plots at 0.5 km (soil 1), 3 km (soil 2), 6 km (soil 3) and 9 km (control soil) from the smelter. Cadmium, Pb and Zn concentration in soil 1 were above the critical total (HNO 3-extractable) concentrations for these elements in soils. Generally, heavy metals in soil 1 decreased dry mater yields of the five species relative to the control. However, the essential oil content of M. vulgare, M. officinalis and O. heracleoticum was within the usual range for respective species and was not affected by the treatments. The overall metal uptake was in the order: B. tripartita > M. vulgare > O. heracleoticum > L. cardiaca > M. officinalis for Cd, L. cardiaca = M. vulgare > B. tripartita = M. officinalis = O. heracleoticum for Pb, L. cardiaca = M. vulgare > O. heracleoticum > B. tripartita = M. officinalis for Cu and B. tripartita > L. cardiaca = M. vulgare > M. officinalis = O. heracleoticum for Mn and Zn . Overall, metal concentration in plant parts was in the order: roots > leaves > flowers > stems for Cd, Pb and Cu, leaves > roots > flowers > stems for Mn and Zn. The concentration of Cd, Pb, Cu and Zn in plant tissue correlated to the exchangeable (EXCH) and the carbonate (CARB) bound fractions of metals in soil. Heavy metals caused disruptions of the plasma membrane of some root cortical cells and alterations in chloroplasts thylakoids in plants grown in soil 1. Metal content in teas prepared from the species was negligible, the essential oils were free of metals. Generally, the transfer factor (TF) was less than 1, indicating the tested species did not have a significant phytoextraction potential. This study demonstrated the three essential oil species M. vulgare, M. officinalis and O. heracleoticum can be grown as alternative high-value crops in metal polluted agricultural soils around the smelter and provide metal-free marketable produce.

Comparative vegetative anatomy and systematics of Laeliinae (Orchidaceae)

Abstract Vanilla is a pantropical genus of green-stemmed vines bearing clasping (aerial) and absorbing (terrestrial) roots. Most vanillas bear normal, thick foliage leaves; others produce fugacious bracts. Seventeen species, including both types were studied. Foliage leaves of Vanilla are glabrous, have abaxial, tetracytic stomatal apparatuses, and a homogeneous mesophyll. Species may or may not have a uniseriate hypodermis. Crystals occur in the foliar epidermises of some species, but all species have crystalliferous idioblasts with raphides in the mesophyll. Vascular bundles in leaves are collateral and occur in a single series alternating large and small. Sclerenchyma may or may not be associated with the vascular bundles. Scale leaves may be crescent or C-shaped and usually have abaxial stomatal apparatuses. A hypodermis may or may not be present; the mesophyll contains raphide bundles in idioblasts. Vascular bundles are collateral and occur in a single row sometimes aligned close to the adaxial surface. They may or may not be associated with sclerenchyma. Stems of leafy vanillas show a sclerenchyma band separating cortex from ground tissue; stems of leafless vanillas do not show a sclerenchyma band. Ground tissue of the stem may consist solely of assimilatory cells or mixed assimilatory and water-storage cells. In some species centrally located assimilatory cells are surrounded by layers of water-storage cells. A uniseriate hypodermis is present in all stems. Sclerenchyma may completely surround the scattered collateral vascular bundles, occur only on the phloem side, or be absent. Both aerial and terrestrial roots are notable for their uniseriate velamen the cell walls of which may be unmarked or ornamented with anticlinal strips. Exodermis is uniseriate; the cells vary from barely thickened to strongly thickened. Only the outer and radial walls are thickened. Cortical cells of aerial roots generally have chloroplasts that are lacking from the same tissue of terrestrial roots. Raphide bundles occur in thin-walled cortical idioblasts. Endodermis and pericycle are uniseriate; pericycle cells are all ○ -thickened opposite the phloem. Cells of the endodermis are either ○- or U-thickened opposite the phloem. Vascular tissue may be embedded in thin- or thick-walled sclerenchyma or in parenchyma. Metaxylem cells are always wider in terrestrial than in aerial roots of the same species. Pith cells are generally parenchymatous but sclerotic in a few species.

Light‐induced transpiration alters cell water relations in figleaf gourd (Cucurbita ficifolia) seedlings exposed to low root temperatures

Water relation parameters including elastic modulus (epsilon), half-times of water exchange (T(w)(1/2)), hydraulic conductivity and turgor pressure (P) were measured in individual root cortical and cotyledon midrib cells in intact figleaf gourd (Cucurbita ficifolia) seedlings, using a cell pressure probe. Transpiration rates (E) of cotyledons were also measured using a steady-state porometer. The seedlings were exposed to low ambient (approximately 10 micromol m(-2) s(-1)) or high supplemental irradiance (approximately 300 micromol m(-2) s(-1) PPF density) at low (8 degrees C) or warm (22 degrees C) root temperatures. When exposed to low irradiance, all the water relation parameters of cortical cells remained similar at both root temperatures. The exposure of cotyledons to supplemental light at warm root temperatures, however, resulted in a two- to three-fold increase in T(w)(1/2) values accompanied with the reduced hydraulic conductivity in both root cortical (Lp) and cotyledon midrib cells (Lp(c)). Low root temperature (LRT) further reduced Lp(c) and E, whether it was measured under low or high irradiance levels. The reductions of Lp as the result of respective light and LRT treatments were prevented by the application of 1 microM ABA. Midrib cells required higher concentrations of ABA (2 microM) in order to prevent the reduction in Lp(c). When the exposure of cotyledons to light was accompanied by LRT, however, ABA proved ineffective in reversing the inhibition of Lp. LRT combined with high irradiance triggered a drastic 10-fold reduction in water permeability of cortical and midrib cells and increased epsilon and T(w)(1/2) values. Measurement of E indicated that the increased water demand by the transpiring plants was fulfilled by an increase in the apoplastic pathway as principal water flow route. The importance of water transport regulation by transpiration affecting the hydraulic conductivity of the roots is discussed.

Rhythmic plant morphogenesis: Recurrent patterns of idioblast cell production

Most plants are constructed from repeating modular units such as phytomers, merophytes, and cell packets. Even an organism as simple as the filamentous cyanobacterium Anabaena shows recurrent patterns of differentiated cellular structures, notably with respect to its heterocysts. These examples reflect the inherent rhythms established within developmental processes of living organisms. In the present article, attention is paid to repetitious production of idioblasts—isolated cells, or clusters of cells, with an identity different to that of neighbouring cells from which they are derived. In higher plant root tissues, idioblasts are contained within cell packets that grow up from mother cells during the course of a number of cycles of cell production. The heterocysts of Anabaena are also discussed; they, too, are a type of idioblast. The idioblasts of root tissues originate as small cells which result from unequal cell divisions. Such divisions are usually the final ones within a cell packet which has already undergone a number of division cycles and are characteristically located at one or both ends of a packet. The packet end walls are suggested to have a role in regulating division asymmetry. Idioblastic systems discussed are root cortical trichosclereids and diaphragm cells; in their earliest stage, the cells from which lateral root primordia arise are also considered as clusters of idioblasts because they, too, are the products of asymmetric divisions of pericyclic mother cells. The division patterns of all these idioblastic systems were modelled in a consistent way using L-systems, with the assumption that the age of a cell-packet end wall plays a special role in cell determination.

Colonization of cucumber plants by the biocontrol fungus Clonostachys rosea f. catenulata

Clonostachys rosea f. catenulata ( Gliocladium catenulatum) strain J1446 (formulated as Prestop WP) suppressed Fusarium root and stem rot caused by Fusarium oxysporum f. sp. radicis-cucumerinum ( Forc) on cucumber plants grown hydroponically in rockwool medium. Sixty days following application at seeding, the biocontrol agent had proliferated through the rockwool blocks and was present on cucumber roots and the crown region of the stem at populations >1 × 10 5 CFU/g fresh weight. Scanning electron micrographs showed that C. rosea had rapidly colonized the root surface and was associated with root hairs and epidermal cell junctions. Following transformation of the fungus with Agrobacterium tumefaciens strain AGL-1 containing the hygromycin resistance ( hph) and β-glucuronidase ( uidA) genes, blue-stained mycelia could be seen growing on the surface and within epidermal and cortical cells of roots, stems and shoots 3 weeks after treatment. Quantification of GUS activity by fluorometric assays showed that fungal biomass was highest in the roots and crown area, while the extent of colonization of upper stems and true leaves was variable. Higher population levels resulted following application to rockwool blocks compared to seed treatment. Application of C. rosea preceding inoculation with Forc significantly reduced pathogen populations on roots compared to plants inoculated with Forc alone. Colonization of infection sites in the root zone reduced pathogen development and disease incidence. Densities of the biocontrol agent appeared to increase in the presence of the pathogen.

Histological study of syncytia induced in cereals by the Mediterranean cereal cyst nematode Heterodera latipons

AbstractFeeding sites of Heterodera latipons in wheat, barley and Lolium rigidum were studied and compared with those of Heterodera avenae in wheat. Juveniles of H. latipons penetrated mainly differentiated roots and chose a root cortical cell as their initial syncytium cell (ISC) in wheat and barley, whilst H. avenae chose a vascular parenchymal cell as an ISC. The cell associated with females in wheat root underwent hypertrophy, and incorporated endodermis, pericycle and parenchyma cells of the vascular cylinder by cell wall dissolution. Syncytia of H. latipons contained a high number of organelles, including endoplasmic reticulum, mitochondria, plastids and amoeboid nuclei, whilst those of H. avenae were highly vacuolated. The syncytium partly occupied both the cortex and the vascular cylinder in contrast to those of H. avenae, which occupied most of the space within the vascular cylinder near the infection site. The cortical cell associated with H. latipons males did not undergo hypertrophy, but had dense cytoplasm, and incorporation of other cells by cell wall dissolution was rare. A similar syncytium formation process was observed with barley, but a cortical cell, several cell layers away from the endodermis, was chosen for the ISC. In resistant L. rigidum, H. latipons juveniles chose an endodermal cell or its neighbouring cortical cell as an ISC. Partial dissolution of cell wall was observed in neighbouring parenchymal cells, although the nematode did not develop into adults.

Toxicities of soluble Al, Cu, and La include ruptures to rhizodermal and root cortical cells of cowpea

Elevated levels of many metals are toxic to plant roots, but their modes of action are not well understood. We investigated the toxicities of aluminium (Al), copper (Cu), and lanthanum (La) in solution on the growth and external morphology of 3-d-old cowpea (Vigna unguiculata L.) roots for periods of up to 48 h. Root elongation rate decreased by 50% at ca. 30 µM Al, 0.3 µM Cu, or 2.0 µM La, accompanied by a decrease in the distance from the root tip to the proximal lateral root. Kinks developed in some roots 2.0 ± 0.4 mm from the root apex on exposure to Al or La (but not Cu). Light and scanning electron microscopy showed that soluble Al, Cu, or La caused similar transverse ruptures to develop > 1 mm from the root apex through the breaking and separation of the rhizodermis and outer cortex from inner-layers. The metals differed, however, in the range in concentration at which they had this effect; developing in solutions containing 54 to‑600 µM Al, but only from 0.85 to 1.8 µM Cu or 2.0 to 5.5 µM La. These findings suggest that Al, Cu, and La bind to the walls of cells, causing increased cell wall rigidity and eventual cell rupturing of the rhizodermis and outer cortex in the elongating zone. We propose that this is a major toxic effect of Al, and that Cu and La also have additional toxic effects.

Riboregulators in plant development

npcRNA (non-protein-coding RNAs) are an emerging class of regulators, so-called riboregulators, and include a large diversity of small RNAs [miRNAs (microRNAs)/siRNAs (small interfering RNAs)] that are involved in various developmental processes in plants and animals. In addition, several other npcRNAs encompassing various transcript sizes (up to several kilobases) have been identified using different genomic approaches. Much less is known about the mechanism of action of these other classes of riboregulators also present in the cell. The organogenesis of nitrogen-fixing nodules in legume plants is initiated in specific root cortical cells that express the npcRNA MtENOD40 (Medicago truncatula early nodulin 40). We have identified a novel RBP (RNA-binding protein), MtRBP1 (M. truncatula RBP 1), which interacts with the MtENOD40 RNA, and is exported into the cytoplasm during legume nodule development in the region expressing MtENOD40. A direct involvement of the MtENOD40 RNA in the relocalization of this RBP into cytoplasmic granules could be demonstrated, revealing a new RNA function in the cell. To extend these results, we searched for npcRNAs in the model plant Arabidopsis thaliana whose genome is completely known. We have identified 86 novel npcRNAs from which 27 corresponded to antisense RNAs of known coding regions. Using a dedicated 'macroarray' containing these npcRNAs and a collection of RBPs, we characterized their regulation in different tissues and plants subjected to environmental stresses. Most of the npcRNAs showed high variations in gene expression in contrast with the RBP genes. Recent large-scale analysis of the sRNA component of the transcriptome revealed an enormous diversity of siRNAs/miRNAs in the Arabidopsis genome. Bioinformatic analysis revealed that 34 large npcRNAs are precursors of siRNAs/miRNAs. npcRNAs, which are a sensitive component of the transcriptome, may reveal novel riboregulatory mechanisms involved in post-transcriptional control of differentiation or environmental responses.

Temperature tolerances for stems and roots of two cultivated cacti, Nopalea cochenillifera and Opuntia robusta: Acclimation, light, and drought

Whether environmental conditions influence the sensitivity to extreme temperatures, which determines where species occur naturally or can be cultivated, was investigated for Nopalea cochenillifera and Opuntia robusta. These cacti tolerated high-temperatures, as LT 50 (the temperature killing 50% of the cells, based on uptake of the vital stain neutral red) averaged 57 °C for stem chlorenchyma cells and root cortical cells for plants at day/night air temperatures of 25/20 °C; the plants were relatively susceptible to low temperatures, for which LT 50 averaged –7 °C. As air temperatures were reduced by 15 °C (10/5 °C), LT 50 for low temperatures decreased 1.1 °C for stems and roots; as they were raised by 20 °C (45/40 °C), LT 50 for high temperatures increased 4.2 °C for stems and 3.4 °C for roots. This acclimation leads to advantageous seasonal adjustments to ambient temperatures. Variations of the total daily photosynthetic flux density incident on the opaque stems for 35 days did not influence the tolerance to extreme temperatures, nor did shading the plants by 90%, indicating no effect of locally adverse photosynthetic conditions on temperature sensitivity. In contrast, the low-temperatures tolerated decreased 2.1 °C for stems and 1.3 °C for roots during 100 days of drought; concomitantly, the high temperatures tolerated increased 2.5 °C for stems, although roots were unaffected. The increased low-temperature tolerance during drought was accompanied by a decreased stem water potential and water content.

Cortical Aerenchyma formation in hypocotyl and adventitious roots of Luffa cylindrica subjected to soil flooding.

Aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Although sponge gourd is an annual vegetable upland crop, in response to flooding the hypocotyl and newly formed adventitious roots create aerenchyma that is neither schizogenous nor lysigenous, but is produced by radial elongation of cortical cells. The aim of this study is to characterize the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation, and to analyse the relative amount of aerenchyma formed. Plants were harvested at 16 d after the flooding treatment was initiated. The root system was observed, and sections of fresh materials (hypocotyl, tap root and adventitious root) were viewed with a light or fluorescence microscope. Distributions of porosity along adventitious roots were estimated by a pycnometer method. Under flooded conditions, a considerable part of the root system consisted of new adventitious roots which soon emerged and grew quickly over the soil surface. The outer cortical cells of these roots and those of the hypocotyl elongated radially and contributed to the development of large intercellular spaces. The elongated cortical cells of adventitious roots were clearly T-shaped, and occurred regularly in mesh-like lacunate structures. In these positions, slits were formed in the epidermis. In the roots, the enlargement of the gas space system began close to the apex in the cortical cell layers immediately beneath the epidermis. The porosity along these roots was 11-45 %. In non-flooded plants, adventitious roots were not formed and no aerenchyma developed in the hypocotyl or tap root. Sponge gourd aerenchyma is produced by the unique radial elongation of cells that make the expansigeny. These morphological changes seem to enhance flooding tolerance by promoting tissue gas exchange, and sponge gourd might thereby adapt to flooding stress.

Regulation of Root Nitrate Uptake at the NRT2.1 Protein Level in Arabidopsis thaliana

In Arabidopsis the NRT2.1 gene encodes a main component of the root high-affinity nitrate uptake system (HATS). Its regulation has been thoroughly studied showing a strong correlation between NRT2.1 expression and HATS activity. Despite its central role in plant nutrition, nothing is known concerning localization and regulation of NRT2.1 at the protein level. By combining a green fluorescent protein fusion strategy and an immunological approach, we show that NRT2.1 is mainly localized in the plasma membrane of root cortical and epidermal cells, and that several forms of the protein seems to co-exist in cell membranes (the monomer and at least one higher molecular weight complex). The monomer is the most abundant form of NRT2.1, and seems to be the one involved in NO(3)(-) transport. It strictly requires the NAR2.1 protein to be expressed and addressed at the plasma membrane. No rapid changes in NRT2.1 abundance were observed in response to light, sucrose, or nitrogen treatments that strongly affect both NRT2.1 mRNA level and HATS activity. This suggests the occurrence of post-translational regulatory mechanisms. One such mechanism could correspond to the cleavage of NRT2.1 C terminus, which results in the presence of both intact and truncated proteins in the plasma membrane.

How does a symbiotic fungus modulate expression of its host‐plant nitrite reductase?

* In the mycorrhizal association, changes in the metabolic activities expressed by the plant and fungal partners could result from modulations in the quantity and nature of nutrients available at the plant-fungus interface. This hypothesis was tested for the nitrite reductase gene in the association Hebeloma cylindrosporumxPinus pinaster. * Transcripts from plant and fungal nitrite reductases and a fungal ammonium transporter were quantified in control uninoculated roots, extraradical mycelia and mycorrhizas formed by either wild-type or nitrate reductase deficient fungal strains. * The fungal genes were downregulated in mycorrhizas compared with extraradical hyphae. The plant nitrite reductase was induced only transiently by NO(3)(-) in the association with a wild-type strain, but permanently expressed at a high level in mycorrhizas formed by the deficient mutant. * These results suggest that reduced nitrogen compounds transferred from the fungus to the root cortical cells repress the plant nitrite reductase, thus highlighting a plant gene regulation by the nutrients available in the Hartig net.

Boron uptake by the root cortex symplast of tomato and pea plants: evidence for low-boron-induced active transport.

The objective of this research was to test the hypothesis of the existence of an active boron (B) uptake into the cortical cells induced by low B supply. The uptake of B was characterised in two tomato (Lycopersicon esculentum Mill.) genotypes: B-efficient FER and B-inefficient mutant T3238. In addition, pea (Pisum sativum L.) was used as an anatomically appropriate model for obtaining intact root cortex. Time course uptake studies in tomato indicate that the B-inefficient mutant was defective by the absence of an active low-B-induced uptake system in the cortex. Pea roots showed up to 10-fold higher accumulation of B into the cortex symplast at low (0.5 µm) external B supply in comparison to adequate B (10 µm) supply. Also, low-B-induced uptake of B was strongly inhibited by 2,4-dinitrophenol, indicating a metabolic energy-derived active component of B uptake at low external supply. Uptake of B by the cortical cells of tomato and pea plants appears to be a combination of both passive and active components, with a passive component prevailing at higher external B. An active component of B uptake suppressed by either adequate or high B supply might indicate a downregulation of plasma membrane-associated B transporter(s) in root cortical cells.

Transcriptome Profiling of Lotus japonicus Roots During Arbuscular Mycorrhiza Development and Comparison with that of Nodulation

To better understand the molecular responses of plants to arbuscular mycorrhizal (AM) fungi, we analyzed the differential gene expression patterns of Lotus japonicus, a model legume, with the aid of a large-scale cDNA macroarray. Experiments were carried out considering the effects of contaminating microorganisms in the soil inoculants. When the colonization by AM fungi, i.e. Glomus mosseae and Gigaspora margarita, was well established, four cysteine protease genes were induced. In situ hybridization revealed that these cysteine protease genes were specifically expressed in arbuscule-containing inner cortical cells of AM roots. On the other hand, phenylpropanoid biosynthesis-related genes for phenylalanine ammonia-lyase (PAL), chalcone synthase, etc. were repressed in the later stage, although they were moderately up-regulated on the initial association with the AM fungus. Real-time RT–PCR experiments supported the array experiments. To further confirm the characteristic expression, a PAL promoter was fused with a reporter gene and introduced into L. japonicus, and then the transformants were grown with a commercial inoculum of G. mosseae. The reporter activity was augmented throughout the roots due to the presence of contaminating microorganisms in the inoculum. Interestingly, G. mosseae only colonized where the reporter activity was low. Comparison of the transcriptome profiles of AM roots and nitrogen-fixing root nodules formed with Mesorhizobium loti indicated that the PAL genes and other phenylpropanoid biosynthesis-related genes were similarly repressed in the two organs.

Binucleate Rhizoctonia (Ceratorhiza spp.) as non-mycorrhizal endophytes alter Pinus sylvestris L. seedling root architecture and affect growth of rooted cuttings

The main aim of this study was to assess critically the effects of binucleate Rhizoctonia (BnR) inoculation on Scots pine (Pinus sylvestris L.) seedling and hypocotyl root architecture and early plant growth in tree seedling nursery soil, Sphagnum peat and forest humus. The BnR isolates (251, 266, 268, 269) stimulated early seedling growth in the nitrogen-limited nursery soil 86 days postinoculation (p.i.). These seedlings exhibited significantly higher root length and reduced root width, although percentage root infection levels were <6%. At a harvest 240 days p.i., no significant plant and root growth differences were identified, although short root numbers were significantly increased. BnR infection detected in roots was characterized by the presence of intercellular fungal hyphae and subtending intracellular monilioid fungal cells located in outer cortical cells of long roots. Similar endophytic infection morphology was detected in adventitious roots generated in prerooted hypocotyl cuttings [Kaparakis, G. & Sen, R. (2006). Scandinavian Journal of Forest Research, 21] exposed to all BnR isolates in nursery peat and forest humus, although isolate-specific cutting growth was detected. In conclusion, these BnR isolates, known to be genetically related to orchid mycorrhizal Ceratorhiza spp., are non-pathogenic and have the capacity to stimulate significantly early Scots pine root development in nursery soils and forest humus.