[Effects of acid and alkali stress on ginsenoside content and histochemical localization of ginsenoside in adventitious root of Panax ginseng]

To investigate the effect of acid and alkali stress on ginsenoside content of Panax ginseng, adventitious roots culture in bioreactors were incubated for 30 d and pH value was adjusted. Ginsenoside content increased by reducing or raising the pH in culture medium, the muxium ginsenoside content was determined on the 5th days after acid treatment and on the 7th days after alkali treatment. The result of histochemical localization of ginsenoside revealed that the red color from light to dark were found in the adventitious root tissue, and ginsenoside mainly located in the pericycle cells where appeared the dark red color.

Aluminum inhibits root growth and induces hydrogen peroxide accumulation in Plantago algarbiensis and P. almogravensis seedlings

We have evaluated the impact of aluminum (Al) on germination, relative root growth, Al accumulation in roots tips, H2O2 levels, plasma membrane integrity, pigment levels, protein content, and the activities of superoxide dismutase (SOD) and catalase (CAT) in seedlings of the endangered Portuguese species Plantago algarbiensis and Plantago almogravensis. We found that up to 400 μM Al had no impact on the germination percentage in either species but inhibited root growth in a concentration-dependent manner (more severely in P. algarbiensis). Al accumulation in the root tips of both species was concentration dependent up to 200 μM but declined thereafter despite the absence of membrane damage. We observed a concentration-dependent induction of SOD activity but no change in CAT activity resulting in the accumulation of H2O2 (a known growth inhibitor), although its impact in P. almogravensis may be partially ameliorated by the accumulation of carotenoid pigments. Our data suggest an association between Al uptake, H2O2 production, and the inhibition of root growth during early seedling development in P. algarbiensis and P. almogravensis, although the latter is more tolerant towards higher concentrations of the metal.

WRKY46 functions as a transcriptional repressor of ALMT1, regulating aluminum-induced malate secretion in Arabidopsis

Aluminum (Al) toxicity is the major limiting factor for crop production on acid soils, but the transcriptional regulation of Al tolerance genes is largely unknown. Here, we found that the expression of a WRKY domain-containing transcription factor WRKY46 is inhibited by Al and expressed in root stele, whereas the expression of ALMT1, which encodes a malate efflux transporter, is induced by Al stress and spatially co-localized with WRKY46 in root stele, indicating the possible interaction between WRKY46 and ALMT1 in Arabidopsis. Mutation of WRKY46 by T-DNA insertion leads to better root growth under Al stress, and lower root Al content compared with the wild-type Col-0. The wrky46 mutant shows increased root malate secretion, which is consistent with the higher ALMT1 expression in the mutant. Transient expression analysis using truncated promoter of ALMT1 showed that ALMT1 expression can be inhibited by WRKY46 in tobacco leaves. The yeast one-hybrid assay and ChIP-qPCR analysis revealed that WRKY46 directly binds to ALMT1 promoter through specific W-boxes. Taken together, we demonstrated that WRKY46 is a negative regulator of ALMT1, mutation of WRKY46 leads to increased malate secretion and reduced Al accumulation in root apices, and thus confers higher Al resistance.

Lupinus albus plants acquire mercury tolerance when inoculated with an Hg-resistant Bradyrhizobium strain

One strain of Bradyrhizobium canariense (L-7AH) was selected for its metal-resistance and ability to nodulate white lupin (Lupinus albus L.) plants, from a collection of rhizobial strains previously created from soils of the Almadén mining district (Spain) with varying levels of Hg contamination. Plants were inoculated with either strain L-7AH (Hg-tolerant) or L-3 (Hg-sensitive, used as control), and watered with nutrient solutions supplemented with various concentrations (0-200 μM) of HgCl2 in a growth chamber. L. albus inoculated with L-7AH were able to nodulate even at the highest concentration of Hg while those inoculated with L-3 had virtually no nodules at Hg concentrations above 25 μM. Plants inoculated with L-7AH, but not those with the control strain, were able to accumulate large amounts of Hg in their roots and nodules. Nodulation with L-7AH allowed plants to maintain constant levels of both chlorophylls and carotenoids in their leaves and a high photosynthetic efficiency, whereas in those inoculated with L-3 both pigment content and photosynthetic efficiency decreased significantly as Hg concentration increased. Nitrogenase activity of plants nodulated with L-7AH remained fairly constant at all concentrations of Hg used. Results suggest that this symbiotic pair may be used for rhizoremediation of Hg-contaminated soils.

Glutamate dehydrogenase isoenzyme 3 (GDH3) of Arabidopsis thaliana is regulated by a combined effect of nitrogen and cytokinin

In higher plants, NAD(H)-glutamate dehydrogenase (GDH; EC 1.4.1.2) is an abundant enzyme that exists in different isoenzymic forms. In Arabidopsis thaliana, three genes (Gdh1, Gdh2 and Gdh3) encode three different GDH subunits (β, α and γ) that randomly associate to form a complex array of homo- and heterohexamers. The modification of the GDH isoenzyme pattern and its regulation was studied during the development of A. thaliana in the gdh1, gdh2 single mutants and the gdh1-2 double mutant, with particular emphasis on GDH3. Investigations showed that the GDH3 isoenzyme could not be detected in closely related Arabidopsis species. The induction and regulation of GDH3 activity in the leaves and roots was investigated following nitrogen deprivation in the presence or absence of sucrose or kinetin. These experiments indicate that GDH3 is likely to play an important role during senescence and nutrient remobilization.

Analysis of rice ER-resident J-proteins reveals diversity and functional differentiation of the ER-resident Hsp70 system in plants

The heat shock protein 70 (Hsp70) chaperone system participates in protein folding and quality control of unfolded proteins. To examine the roles of co-chaperones in the rice Hsp70 chaperone system in the endoplasmic reticulum (ER), the functions of six ER-resident J-proteins (OsP58A, OsP58B, OsERdj2, OsERdj3A, OsERdj3B, and OsERdj7) in rice were investigated. The expression of OsP58B, OsERdj3A, and OsERdj3B was predominantly up-regulated in roots subjected to ER stress. This response was mediated by signalling through ATF6 orthologues such as OsbZIP39 and OsbZIP60, but not through the IRE1/OsbZIP50 pathway. A co-immunoprecipitation assay demonstrated that OsP58A, OsP58B, and OsERdj3B preferentially interact with the major OsBiP, OsBiP1, while OsERdj3A interacts preferentially with OsBiP5, suggesting that there are different affinities between OsBiPs and J-proteins. In the endosperm tissue, OsP58A, OsP58B, and OsERdj2 were mainly localized in the ER, whereas OsERdj2 was localized around the outer surfaces of ER-derived protein bodies (PB-Is). Furthermore, OsERdj3A was not expressed in wild-type seeds but was up-regulated in transgenic seeds accumulating human interleukin-7 (hIL-7). Since ERdj3A-green fluorescent protein (GFP) was also detected in vacuoles of callus cells under ER stress conditions, OsERdj3A is a bona fide vacuole-localized protein. OsP58A, OsP58B and OsERdj3A were differentially accumulated in transgenic plants expressing various recombinant proteins. These results reveal the functional diversity of the rice ER-resident Hsp70 system.

Differential changes in galactolipid and phospholipid species in soybean leaves and roots under nitrogen deficiency and after nodulation

The availability of nitrogen (N) to plants has a profound impact on carbohydrate and protein metabolism, but little is known about its effect on membrane lipid species. This study examines the changes in galactolipid and phospholipid species in soybean as affected by the availability of N, either supplied to soil or obtained through Bradyrhizobium japonicum nodulation. When N was limited in soil, the content of galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacyglycerol (DGDG), decreased drastically in leaves, while a smaller decrease of DGDG was observed in roots. In both leaves and roots, the overall content of different phospholipid classes was largely unchanged by N limitation, although some individual phospholipid molecular species did display significant changes. Nodulation with Bradyrhizobium of soybean grown in N-deficient soil resulted in a large increase in levels of plastidic lipid classes, MGDG, DGDG, and phosphatidylglycerol, along with smaller increases in non-plastidic phospholipids in leaves. Nodulation also led to higher levels of phospholipids in roots without changes in root levels of MGDG and DGDG. Overall, N availability alters lipid content more in leaves than roots and more in galactolipids than phospholipids. Increased N availability leads to increased galactolipid accumulation in leaves, regardless of whether N is supplied from the soil or symbiotic fixation.

Leaf and fine root carbon stocks and turnover are coupled across Arctic ecosystems

Estimates of vegetation carbon pools and their turnover rates are central to understanding and modelling ecosystem responses to climate change and their feedbacks to climate. In the Arctic, a region containing globally important stores of soil carbon, and where the most rapid climate change is expected over the coming century, plant communities have on average sixfold more biomass below ground than above ground, but knowledge of the root carbon pool sizes and turnover rates is limited. Here, we show that across eight plant communities, there is a significant positive relationship between leaf and fine root turnover rates (r(2) = 0.68, P < 0.05), and that the turnover rates of both leaf (r(2) = 0.63, P < 0.05) and fine root (r(2) = 0.55, P < 0.05) pools are strongly correlated with leaf area index (LAI, leaf area per unit ground area). This coupling of root and leaf dynamics supports the theory of a whole-plant economics spectrum. We also show that the size of the fine root carbon pool initially increases linearly with increasing LAI, and then levels off at LAI = 1 m(2) m(-2), suggesting a functional balance between investment in leaves and fine roots at the whole community scale. These ecological relationships not only demonstrate close links between above and below-ground plant carbon dynamics but also allow plant carbon pool sizes and their turnover rates to be predicted from the single readily quantifiable (and remotely sensed) parameter of LAI, including the possibility of estimating root data from satellites.

Combined effect and mechanism of acidity and lead ion on soybean biomass

Heavy metal pollution and soil acidification are serious global environmental issues. The combined pollution from acidification and heavy metal has become a new environmental issue in regions where the two issues simultaneously occur. However, studies on combined pollution are still limited. In the current study, we investigated the combined effect and mechanism of acidity and heavy metal [lead ion (Pb(2+))] on soybean biomass as well as on growth, nitrogen nutrition, and antioxidant system in soybean roots. Results showed that the combined treatment with acidity and Pb(2+) decreased the soybean biomass. At pH 4.5, the soybean biomass in the combined treatment with acidity and 0.9 mmol L(-1) Pb(2+) was lower than that in the combined treatment with acidity and Pb(2+) at 0.3 or 1.5 mmol L(-1). This result was also observed at pH 3.5 and 3.0. The combined treatment with acidity and Pb(2+) also resulted in the following consequences: root growth inhibition; decrease in nitrate, ammonium, and malondialdehyde contents; increase in nitrite reductase activity; and decrease in peroxidase activity. The extent at which the test indexes decreased/increased in the combined treatment was higher than that in the single acidity treatment. The correlation analysis results indicated that the decrease in the soybean biomass in the combined treatment with acidity and Pb(2+) resulted from the decrease in the root growth, nitrate-nitrogen assimilation, and peroxidase activity.

Auxin and the integration of environmental signals into plant root development

BACKGROUND

Auxin is a versatile plant hormone with important roles in many essential physiological processes. In recent years, significant progress has been made towards understanding the roles of this hormone in plant growth and development. Recent evidence also points to a less well-known but equally important role for auxin as a mediator of environmental adaptation in plants.

SCOPE

This review briefly discusses recent findings on how plants utilize auxin signalling and transport to modify their root system architecture when responding to diverse biotic and abiotic rhizosphere signals, including macro- and micro-nutrient starvation, cold and water stress, soil acidity, pathogenic and beneficial microbes, nematodes and neighbouring plants. Stress-responsive transcription factors and microRNAs that modulate auxin- and environment-mediated root development are also briefly highlighted.

CONCLUSIONS

The auxin pathway constitutes an essential component of the plant's biotic and abiotic stress tolerance mechanisms. Further understanding of the specific roles that auxin plays in environmental adaptation can ultimately lead to the development of crops better adapted to stressful environments.

Rapid auxin-induced root growth inhibition requires the TIR and AFB auxin receptors

We investigated the relation between auxin-induced gene expression and the rapid auxin-induced growth inhibition in Arabidopsis thaliana roots. The natural auxin indole-3-acetic acid (IAA) induced a strong activation of gene expression as visualized by the DR5rev::GFP reporter gene technique. This effect was specific for active auxins and was abolished in knockout mutants of the F-box auxin receptors. We measured the IAA-induced growth inhibition at high time resolution and show that the F-box auxin receptor mutants failed to display this effect. We conclude that the F-box auxin receptors are needed for the response. In hypocotyls, auxin induces an increase in elongation growth, and this effect has been earlier shown to be independent of the F-box receptors. Based on these findings, we discuss differences in the growth control modes in roots and shoots. We demonstrate that the rapid auxin-induced root growth inhibition, unlike the induction of growth in hypocotyls, requires the presence of the F-box auxin receptors.

Phytochrome-induced SIG2 expression contributes to photoregulation of phytochrome signalling and photomorphogenesis in Arabidopsis thaliana

Chloroplast-localized sigma factor (SIG) proteins promote specificity of the plastid-encoded RNA polymerase. SIG2 function appears to be necessary for light-grown Arabidopsis thaliana plants. Specific photoreceptors or light-dependent factors that impact the light-induced accumulation of SIG2 have not been reported. A molecular link between phytochromes and nuclear-encoded SIG2, which impacts photomorphogenesis specifically under red (R) and far-red (FR) light, is described here. Both phyA and phyB promote SIG2 transcript accumulation. Disruption of SIG2 results in R- and FR-specific defects in the inhibition of hypocotyl elongation and cotyledon expansion, although no impairments in these responses are detected for sig2 mutants under blue (B) or white (W) light. SIG2 also impacts root elongation under W and R, and the R-dependent expression of PIF4, encoding a phytochrome-interacting factor, and HY2, which encodes a phytochrome chromophore biosynthetic enzyme. Whereas SIG2 apparently impacts the accumulation of the phytochromobilin (PΦB) phytochrome chromophore, sig2 mutants differ significantly from PΦB mutants, primarily due to wavelength-specific defects in photomorphogenesis and disruption of a distinct subset of phytochrome-dependent responses. The molecular link between phytochromes and SIG2 is likely to be an important part of the co-ordination of gene expression to maintain stoichiometry between the nuclear-encoded phytochrome apoprotein and plastid-derived PΦB, which combine to form photoactive phytochromes, and/or light-dependent SIG2 accumulation is involved in an inductive light signalling pathway co-ordinating components between nucleus and plastids.

Inoculation with arbuscular mycorrhizae does not improve 137Cs uptake in crops grown in the Chernobyl region

Methods for cleaning up radioactive contaminated soils are urgently needed. In this study we investigated whether the use of arbuscular mycorrhizal (AM) fungi can improve (137)Cs uptake by crops. Barley, cucumber, perennial ryegrass, and sunflower were inoculated with AM fungi and grown in low-level radionuclide contaminated soils in a field experiment 70 km southwest of Chernobyl, Ukraine, during two successive years (2009-2010). Roots of barley, cucumber and sunflower plants were slightly or moderately infected with AM fungus and root infection frequency was negatively or non-correlated with (137)Cs uptake by plants. Roots of ryegrass were moderately infected with AM fungus and infection frequency was moderately correlated with (137)Cs uptake by ryegrass. The application of AM fungi to soil in situ did not enhance radionuclide plant uptake or biomass. The responsiveness of host plants and AM fungus combination to (137)Cs uptake varied depending on the soil, although mycorrhization of soil in the field was conditional and did not facilitate the uptake of radiocesium. The total amount of (137)Cs uptake by plants growing on inoculated soil was equal to amounts in plant cultivated on non-inoculated soil. Thus, the use of AM fungi in situ for bioremediation of soil contaminated with a low concentration of (137)Cs could not be recommended.

Multi-nutrient vs. nitrogen-only effects on carbon sequestration in grassland soils

Human activities have greatly increased the availability of biologically active forms of nutrients [e.g., nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg)] in many soil ecosystems worldwide. Multi-nutrient fertilization strongly increases plant productivity but may also alter the storage of carbon (C) in soil, which represents the largest terrestrial pool of organic C. Despite this issue is important from a global change perspective, key questions remain on how the single addition of N or the combination of N with other nutrients might affect C sequestration in human-managed soils. Here, we use a 19-year old nutrient addition experiment on a permanent grassland to test for nutrient-induced effects on soil C sequestration. We show that combined NPKMg additions to permanent grassland have 'constrained' soil C sequestration to levels similar to unfertilized plots whereas the single addition of N significantly enhanced soil C stocks (N-only fertilized soils store, on average, 11 t C ha(-1) more than unfertilized soils). These results were consistent across grazing and liming treatments suggesting that whilst multi-nutrient additions increase plant productivity, soil C sequestration is increased by N-only additions. The positive N-only effect on soil C content was not related to changes in plant species diversity or to the functional composition of the plant community. N-only fertilized grasslands show, however, increases in total root mass and the accumulation of organic matter detritus in topsoils. Finally, soils receiving any N addition (N only or N in combination with other nutrients) were associated with high N losses. Overall, our results demonstrate that nutrient fertilization remains an important global change driver of ecosystem functioning, which can strongly affect the long-term sustainability of grassland soil ecosystems (e.g., soils ability to deliver multiple ecosystem services).

Phosphatidic acid interacts with a MYB transcription factor and regulates its nuclear localization and function in Arabidopsis

Phosphatidic acid (PA) has emerged as a class of cellular mediators involved in various cellular and physiological processes, but little is known about its mechanism of action. Here we show that PA interacts with werewolf (WER), a R2R3 MYB transcription factor involved in root hair formation. The PA-interacting region is confined to the end of the R2 subdomain. The ablation of the PA binding motif has no effect on WER binding to DNA, but abolishes its nuclear localization and its function in regulating epidermal cell fate. Inhibition of PA production by phospholipase Dζ also suppresses WER's nuclear localization, root hair formation, and elongation. These results suggest a role for PA in promoting protein nuclear localization.

CEP genes regulate root and shoot development in response to environmental cues and are specific to seed plants

The manifestation of repetitive developmental programmes during plant growth can be adjusted in response to various environmental cues. During root development, this means being able to precisely control root growth and lateral root development. Small signalling peptides have been found to play roles in many aspects of root development. One member of the CEP (C-TERMINALLY ENCODED PEPTIDE) gene family has been shown to arrest root growth. Here we report that CEP genes are widespread among seed plants but are not present in land plants that lack true branching roots or root vasculature. We have identified 10 additional CEP genes in Arabidopsis. Expression analysis revealed that CEP genes are regulated by environmental cues such as nitrogen limitation, increased salt levels, increased osmotic strength, and increased CO2 levels in both roots and shoots. Analysis of synthetic CEP variants showed that both peptide sequence and modifications of key amino acids affect CEP biological activity. Analysis of several CEP over-expression lines revealed distinct roles for CEP genes in root and shoot development. A cep3 knockout mutant showed increased root and shoot growth under a range of abiotic stress, nutrient, and light conditions. We demonstrate that CEPs are negative regulators of root development, slowing primary root growth and reducing lateral root formation. We propose that CEPs are negative regulators that mediate environmental influences on plant development.

Biosynthesis of the carbohydrate moieties of arabinogalactan proteins by membrane-bound β-glucuronosyltransferases from radish primary roots

A membrane fraction from etiolated 6-day-old primary radish roots (Raphanus sativus L. var hortensis) contained β-glucuronosyltransferases (GlcATs) involved in the synthesis of the carbohydrate moieties of arabinogalactan proteins (AGPs). The GlcATs transferred [(14)C]GlcA from UDP-[(14)C]GlcA on to β-(1 → 3)-galactan as an exogenous acceptor substrate, giving a specific activity of 50-150 pmol min(-1) (mg protein)(-1). The enzyme specimen also catalyzed the transfer of [(14)C]GlcA on to an enzymatically modified AGP from mature radish root. Analysis of the transfer products revealed that the transfer of [(14)C]GlcA occurred preferentially on to consecutive (1 → 3)-linked β-Gal chains as well as single branched β-(1 → 6)-Gal residues through β-(1 → 6) linkages, producing branched acidic side chains. The enzymes also transferred [(14)C]GlcA residues on to several oligosaccharides, such as β-(1 → 6)- and β-(1 → 3)-galactotrioses. A trisaccharide, α-L-Araf-(1 → 3)-β-Gal-(1 → 6)-Gal, was a good acceptor, yielding a branched tetrasaccharide, α-L-Araf-(1 → 3)[β-GlcA-(1 → 6)]-β-Gal-(1 → 6)-Gal. We report the first in vitro assay system for β-GlcATs involved in the AG synthesis as a step toward full characterization and cloning.

Rapid ammonia gas transport accounts for futile transmembrane cycling under NH3/NH4+ toxicity in plant roots

Futile transmembrane NH3/NH4(+) cycling in plant root cells, characterized by extremely rapid fluxes and high efflux to influx ratios, has been successfully linked to NH3/NH4(+) toxicity. Surprisingly, the fundamental question of which species of the conjugate pair (NH3 or NH4(+)) participates in such fluxes is unresolved. Using flux analyses with the short-lived radioisotope (13)N and electrophysiological, respiratory, and histochemical measurements, we show that futile cycling in roots of barley (Hordeum vulgare) seedlings is predominately of the gaseous NH3 species, rather than the NH4(+) ion. Influx of (13)NH3/(13)NH4(+), which exceeded 200 µmol g(-1) h(-1), was not commensurate with membrane depolarization or increases in root respiration, suggesting electroneutral NH3 transport. Influx followed Michaelis-Menten kinetics for NH3 (but not NH4(+)), as a function of external concentration (Km = 152 µm, Vmax = 205 µmol g(-1) h(-1)). Efflux of (13)NH3/(13)NH4(+) responded with a nearly identical Km. Pharmacological characterization of influx and efflux suggests mediation by aquaporins. Our study fundamentally revises the futile-cycling model by demonstrating that NH3 is the major permeating species across both plasmalemma and tonoplast of root cells under toxicity conditions.

Experimental evidence for diel δ15N-patterns in different tissues, xylem and phloem saps of castor bean (Ricinus communis L.)

Nitrogen isotope signatures in plants might give insights in the metabolism and allocation of nitrogen. To obtain a deeper understanding of the modifications of the nitrogen isotope signatures, we determined δ(15)N in transport saps and in different fractions of leaves, axes and roots during a diel course along the plant axis. The most significant diel variations were observed in xylem and phloem saps where δ(15)N was significantly higher during the day compared with during the night. However in xylem saps, this was observed only in the canopy, but not at the hypocotyl positions. In the canopy, δ(15)N was correlated fairly well between phloem and xylem saps. These variations in δ(15)N in transport saps can be attributed to nitrate reduction in leaves during the photoperiod as well as to (15)N-enriched glutamine acting as transport form of N. δ(15)N of the water soluble fraction of roots and leaves partially affected δ(15)N of phloem and xylems saps. δ(15)N patterns are likely the result of a complex set of interactions and N-fluxes between plant organs. Furthermore, the natural nitrogen isotope abundance in plant tissue is not constant during the diel course - a fact that needs to be taken into account when sampling for isotopic studies.

The influence of cadmium contamination and salinity on the survival, growth and phytoremediation capacity of the saltmarsh plant Salicornia ramosissima

The major aim of this study was to evaluate the capacity of Salicornia ramosissima on Cadmium phytoremediation under distinct salinities and, consequently, the toxic effects on the plant's development. A greenhouse experiment was performed, using two Cd concentrations (50 and 100 μg l(-1)) in different salinities (0, 5 and 10). Mortality and weight variation, observed at the end of the experiment, showed significant differences between some treatments, meaning that these variables were affected by the salinity and Cd concentrations. The highest Cd accumulation was detected in the roots, and decreased with the increase of salinity and Cd concentration. S. ramosissima is a potential candidate for Cd phytoremediation at salinities close to 0 and its capabilities in Cd phytoaccumulation and phytoestabilization proved to be quite interesting. The optimization of phytoremediation processes by S. ramosissima could turn possible the use of this plant in the recovery of contaminated ecosystems.

Mechanism of aflatoxin uptake in roots of intact groundnut (Arachis hypogaea L.) seedlings

Aflatoxins are one of the most potent toxic substances that occur naturally, which enter agricultural soils through the growth of aflatoxigenic fungi in rhizhosphere and nonrhizhosphere soils. Though several reports regarding the uptake of aflatoxin by plants are available, the mechanism of aflatoxin uptake remains unknown. This study characterized the aflatoxin uptake mechanism by in vitro hydroponic experiments under variable conditions. The uptake reached saturation after 48 h of incubation for AFB1 and B2 and 60 h for AFG1 and G2. A linear increase in uptake with increasing aflatoxin concentrations was observed, and it fits both linear and nonlinear regression. AFB1 uptake was directly proportional to transpiration rate, and blocking aquaporin activity using mercuric chloride revealed its involvement in the uptake. None of the metabolic inhibitors used to block active transport had any effect on aflatoxin uptake except for sodium azide. From the present study, it could be concluded that aflatoxin uptake by groundnut roots followed mainly a passive way and is facilitated through aquaporins. The involvement of active component should be studied in detail.

Soil nitrogen, and not phosphorus, promotes cluster-root formation in a South American Proteaceae, Embothrium coccineum

PREMISE OF THE STUDY

Cluster roots are a characteristic root adaptation of Proteaceae species. In South African and Australian species, cluster roots promote phosphorus (P) acquisition from poor soils. In a South American Proteaceae species, where cluster roots have been scarcely studied and their function is unknown, we tested whether cluster-root formation is stimulated by low soil nutrition, in particular low P-availability.

METHODS

Small and large seedlings (< 6- and > 6-months old, respectively) of Embothrium coccineum and soil were collected across four different sites in Patagonia (Chile). We determined cluster-root number and relative mass, and leaf Pi concentration per mass (Pimass) and per area (Piarea) for each seedling, and tested relationships with Olsen-P (OP), sorbed-P (sP) and total nitrogen (N) using generalized linear mixed-effects models and model selection to assess the relative strength of soil and plant drivers.

KEY RESULTS

Best-fit models showed a negative logarithmic relationship between cluster-root number and soil nitrogen (N), and between cluster-root relative mass and both leaf Piarea and soil N, and a positive logarithmic relationship between cluster-root number and leaf Piarea. Cluster-root relative mass was higher in small than in large seedlings.

CONCLUSIONS

Contrary to that found in South African and Australian Proteaceae, cluster roots of E. coccineum do not appear to be driven by soil P, but rather by soil N and leaf Piarea. We suggest that cluster roots are a constitutive and functional trait that allows plants to prevail in poor N soils.

Notes from the underground: receptor-like kinases in Arabidopsis root development

During plant development, the frequency and context of cell division must be controlled, and cells must differentiate properly to perform their mature functions. In addition, stem cell niches need to be maintained as a reservoir for new cells. All of these processes require intercellular signaling, whether it is a cell relaying its position to other cells, or more mature cells signaling to the stem cell niche to regulate the rate of growth. Receptor-like kinases have emerged as a major component in these diverse roles, especially within the Arabidopsis root. In this review, the functions of receptor-like kinase signaling in regulating Arabidopsis root development will be examined in the areas of root apical meristem maintenance, regulation of epidermal cell fate, lateral root development and vascular differentiation. [Figure: see text] Frans E. Tax (Corresponding author).

Proteomic and phosphoproteomic analysis of polyethylene glycol-induced osmotic stress in root tips of common bean (Phaseolus vulgaris L.)

Previous studies have shown that polyethylene glycol (PEG)-induced osmotic stress (OS) reduces cell-wall (CW) porosity and limits aluminium (Al) uptake by root tips of common bean (Phaseolus vulgaris L.). A subsequent transcriptomic study suggested that genes related to CW processes are involved in adjustment to OS. In this study, a proteomic and phosphoproteomic approach was applied to identify OS-induced protein regulation to further improve our understanding of how OS affects Al accumulation. Analysis of total soluble proteins in root tips indicated that, in total, 22 proteins were differentially regulated by OS; these proteins were functionally categorized. Seventy-seven per- cent of the total expressed proteins were involved in metabolic pathways, particularly of carbohydrate and amino acid metabolism. An analysis of the apoplastic proteome revealed that OS reduced the level of five proteins and increased that of seven proteins. Investigation of the total soluble phosphoproteome suggested that dehydrin responded to OS with an enhanced phosphorylation state without a change in abundance. A cellular immunolocalization analysis indicated that dehydrin was localized mainly in the CW. This suggests that dehydrin may play a major protective role in the OS-induced physical breakdown of the CW structure and thus maintenance of the reversibility of CW extensibility during recovery from OS. The proteomic and phosphoproteomic analyses provided novel insights into the complex mechanisms of OS-induced reduction of Al accumulation in the root tips of common bean and highlight a key role for modification of CW structure.

Differences in shoot and root terpenoid profiles and plant responses to fertilisation in Tanacetum vulgare

Intraspecific chemical diversity is a common phenomenon especially found in shoots of essential oil-accumulating plant species. Abiotic factors can influence the concentration of essential oils, but the effects are inconsistent and little is known in how far these may vary within an individual and within species between chemotypes. Tanacetum vulgare L. occurs in various chemotypes that differ in the composition of mono- and sesquiterpenoids in their shoot tissues. We investigated how far shoot chemotype grouping is mirrored in root terpenoid profiles. Furthermore, we studied whether different fertilisation amounts influence the plant growth and morphological traits as well as the constitutive terpenoid concentration of leaves and roots of three chemotypes, trans-carvyl acetate, β-thujone, and camphor, to different degrees. Shoot terpenoids were dominated by monoterpenoids, while the roots contained mainly sesquiterpenoids. The clear grouping in three chemotypes based on leaf chemistry was weakly mirrored in the root terpenoid composition. Furthermore, the leaf C/N ratio and the stem height differed between chemotypes. All plants responded to increased nutrient availability with increased total biomass and specific leaf area but decreased C/N and root/shoot ratios. Leaf terpenoid concentrations decreased with increasing fertiliser supply, independent of chemotype. In contrast to the leaves, the terpenoid concentrations of the roots were unaffected by fertilisation. Our results demonstrate that aboveground and belowground organs within a species can be under different selection pressures.