A receptor-like kinase mutant with absent endodermal diffusion barrier displays selective nutrient homeostasis defects.

The endodermis represents the main barrier to extracellular diffusion in plant roots, and it is central to current models of plant nutrient uptake. Despite this, little is known about the genes setting up this endodermal barrier. In this study, we report the identification and characterization of a strong barrier mutant, schengen3 (sgn3). We observe a surprising ability of the mutant to maintain nutrient homeostasis, but demonstrate a major defect in maintaining sufficient levels of the macronutrient potassium. We show that SGN3/GASSHO1 is a receptor-like kinase that is necessary for localizing CASPARIAN STRIP DOMAIN PROTEINS (CASPs)--major players of endodermal differentiation--into an uninterrupted, ring-like domain. SGN3 appears to localize into a broader band, embedding growing CASP microdomains. The discovery of SGN3 strongly advances our ability to interrogate mechanisms of plant nutrient homeostasis and provides a novel actor for localized microdomain formation at the endodermal plasma membrane.

High-level molecular diversity of copper-zinc superoxide dismutase genes among and within species of arbuscular Mycorrhizal fungi.

In the ecologically important arbuscular mycorrhizal fungi (AMF), Sod1 encodes a functional polypeptide that confers increased tolerance to oxidative stress and that is upregulated inside the roots during early steps of the symbiosis with host plants. It is still unclear whether its expression is directed at scavenging reactive oxygen species (ROS) produced by the host, if it plays a role in the fungus-host dialogue, or if it is a consequence of oxidative stress from the surrounding environment. All these possibilities are equally likely, and molecular variation at the Sod1 locus can possibly have adaptive implications for one or all of the three mentioned functions. In this paper, we analyzed the diversity of the Sod1 gene in six AMF species, as well as 14 Glomus intraradices isolates from a single natural population. By sequencing this locus, we identified a large amount of nucleotide and amino acid molecular diversity both among AMF species and individuals, suggesting a rapid divergence of its codons. The Sod1 gene was monomorphic within each isolate we analyzed, and quantitative PCR strongly suggest this locus is present as a single copy in G. intraradices. Maximum-likelihood analyses performed using a variety of models for codon evolution indicated that a number of amino acid sites most likely evolved under the regime of positive selection among AMF species. In addition, we found that some isolates of G. intraradices from a natural population harbor very divergent orthologous Sod1 sequences, and our analysis suggested that diversifying selection, rather than recombination, was responsible for the persistence of this molecular diversity within the AMF population.

Predicting root defence against herbivores during succession

P>1. Root herbivores and pathogens interfere with basic below-ground plant function, and can thereby affect plant fitness and spatial and temporal patterns in natural plant communities. However, there has been little development of concepts and theories on below-ground plant defence, a deficit that is in contrast to the abundance of theorizing for above-ground plant parts.2. A review of the past 10 years of research on below-ground plant-herbivore interactions has revealed that, similar to above-ground tissues, root defences can be expressed constitutively or induced upon herbivore attack, and can be classified into direct and indirect traits, tolerance, and escape. Indeed, it has been shown that roots tolerate herbivory by outgrowing or re-growing lost tissues, or resist it by producing secondary metabolites that are toxic to herbivores or attract natural enemies of herbivores.3. We propose that, similar to above-ground plant-herbivore theories, the partition of abiotic and biotic factors over ecological succession can serve as the basis for predicting investment in defence strategies below-ground.4. Investigation of herbivore pressure and root responses along primary and secondary successional gradients suggests that: (i) roots are often fast growing, thinner and softer in early compared to later succession. (ii) Insect and nematode herbivore pressure increases until mid-succession and later decreases. (iii) Mycorrhizal abundance increases with succession, and the composition of fungal species changes through succession, often shifting from arbuscular mycorrhizae to ecto-mycorrhizae.5. Based on these findings, and on classical (above-ground) plant defence theory, we suggest the following set of testable hypotheses for below-ground plant defence: (i) During succession, early plants invest most of their resources in growth and less in defences (associated with a general lack of herbivores and pathogens, and with limited availability of resources in the system), therefore relying more on re-growth (tolerance) strategies. (ii) During mid-succession, a buildup of herbivore pressure facilitates replacement by plant species that exhibit greater direct and indirect defence strategies. (iii) Constitutive and inducible levels of defences may trade-off, and early successional plants should rely more on induction of defences after herbivore attack, whereas late successional plants will increasingly rely on constitutively produced levels of physical and chemical defence. (iv) Successional changes in microbial associations have consequences for root defence by improving plant nutrition and defence expression as well as directly competing for root space; however, toxic or impenetrable root defences may also limit association with root symbionts, and so may constrain the expression of root defence.

Occurrence and molecular diversity of the Fit insect toxin locus in plant-beneficial pseudomonads

The application of plant-beneficial pseudomonads provides a promising alternative to chemical pest management in agriculture. The fact that Pseudomonas fluorescens CHA0 and Pf-5, both well-known biocontrol agents of fungal root diseases, exhibit also potent insecticidal activity is of particular interest, as these plant-beneficial bacteria naturally colonize the rhizosphere of important crop plants. Insecticidal activity in these strains depends on a novel locus encoding the production of a protein toxin termed Fit (for P. fluorescens insecticidal toxin). To gain a better understanding of the ecological relevance of the Pseudomonas anti-insect activity, we have begun to investigate the occurrence and molecular diversity of the Fit toxin genes among root-associated pseudomonads. To this end, we have screened a large world-wide collection of fluorescent Pseudomonas sp. isolated from the roots of different plant species using molecular fingerprinting techniques. The strains are already well characterized for exoproduct patterns and disease-suppressive ability and are currently being tested for insecticidal activity in a greater wax moth larvae assay system.

PH01 gene family in Arabidopsis thaliana and Physcomitrella patens

Summary Inorganic phosphate (Pi) is a main limiting nutrient to the growth and production yield of plants in many agro-ecosystems. Plants have evolved a series of metabolic and developmental adaptations to cope with low Pi availability. PH01 has been identified as a protein involved in the loading of Pi into the xylem of roots in Arabidopsis. In this study, the PHO1 gene family in both higher plant Arabidopsis and lower plant Physcomitrella was characterized. Additional ten PHO1 homologues in Arabidopsis and three homologues in Physcomitrella were identified. All proteins harbor a SPX tripartite domain in the N-terminal hydrophilic portion and an EXS domain in the highly conserved C-terminal hydrophobic portion. RT-PCR analysis of the Arabidopsis PHO1 genes revealed a broad pattern of expression in leaves, roots, stems, and flowers for most genes, although two genes are expressed exclusively in flowers, indicating their potential roles not only in Pi transport but also in Pi homeostasis within the Arabidopsis plant. The regulation of gene expression by different nutrient-starvations showed that some genes are strongly up-regulated by elements other than Pi, e.g. by NO3, Mg, and Zn starvation. Northern blot and RT-PCR analysis showed distinct expression patterns of the three Physcomitrella PHO1 genes. The investigation of Pi starvation effects on some Pi-deprivation responsive genes demonstrates that Physcomitrella has evolved a similar mechanism as higher plants to respond to Pi deficiency. Promoter activity analysis for the Physcomitrella PHO1 family genes using promoter-GUS fusions revealed their expression in protonemata and gametophores but at different levels and with different patterns, suggesting these genes may play distinct roles in Pi transport and/or Pi homeostasis in the moss plant. Single knockout mutants of the three genes were generated by gene targeting and one of them displayed a reduced Pi content in the protonemata under Pi starvation. The evolution of the PHO1 family in land plants was also discussed. Together, these findings indicate that the PHO1 family genes, present in a broad range of plant species from lower plants to flowering plants, play important roles in Pi transport and homeostasis. Résumé Le phosphate inorganique (Pi) est un nutriment essentiel à la croissance des plantes et au rendement de la production végétale. Dans beaucoup d'agro-écosystèmes, ce nutriment est limitant. Les plantes ont développé des adaptations métaboliques et développementales pour palier à la faible disponibilité du Pi. Il a été démontré que la protéine PHOI est indispensable au transfert du Pi dans le xylème des racines d' Arabidopsis. Cette étude porte sur la famille de gènes définie par PHO1 ; ceci, dans deux organismes modèles : la plante Arabidopsis pour les végétaux supérieurs, et la mousse Physcomitrella pour les végétaux inférieurs. Dix homologues à PHOI dans Arabidopsis et trois homologues dans Physcomitrella ont été identifiés. Toutes les protéines encodées présentent un domaine tripartite SPX dans leur partie N terminale hydrophile et un domaine EXS dans la partie C terminale hydrophobe hautement conservée d'entre eux. L'analyse par RT-PCR de l'expression des gènes PHO1 dans Arabidopsis révèle une expression ectopique pour la plupart, à l'exception de deux gènes dont l'expression est uniquement florale ; ceci suggère l'implication de cette famille non seulement dans le transport mais aussi dans l'homéostasie du Pi dans Arabidopsis. L'observation de l'expression de ces gènes en fonction de l'absence de différents nutriments montre que certains gènes sont fortement régulés lors de carences en NO3, Mg et Zn. L'analyse par northern blot et RT-PCR met en évidence des profils d'expression distincts pour les trois gènes de Physcomitrella. Les effets de la carence en Pi sur Physcomitrella ont été étudiés par le biais de gènes dépendants connus pour Arabidopsis, les résultats suggèrent un mode de réponse à cette carence conservé entre les végétaux inférieurs et supérieurs. La localisation tissulaire de l'expression de la famille PHO1 dans la mousse a été étudiée au moyen du gène rapporteur GUS fusionné aux différents promoteurs. Ceci a révélé leur expression dans les protonemata et les gametophores, mais à des intensités et avec des profils différents, ce qui suggère des implications distinctes dans le transport et/ou l'homéostasie du Pi dans la mousse. Des simples mutants knockout ont été générés pour chaque gène de mousse ; l'un d'eux présente une diminution du contenu protonemal en Pi lorsque soumis à une carence en Pi. L'évolution de la famille PHO1 dans les plantes terrestres est également discutée. Ensemble, ces résultats indiquent que les gènes de la famille PHO1 sont présents dans une large gamme de plantes allant des végétaux inférieurs aux supérieurs, et cette étude démontre que leur conservation se justifie potentiellement par le fait qu'ils sont probablement impliqués dans des mécanismes conservés de transport et d'homéostasie du Pi.

Does wheat genetically modified for disease resistance affect root-colonizing pseudomonads and arbuscular mycorrhizal fungi?

This study aimed to evaluate the impact of genetically modified (GM) wheat with introduced pm3b mildew resistance transgene, on two types of root-colonizing microorganisms, namely pseudomonads and arbuscular mycorrhizal fungi (AMF). Our investigations were carried out in field trials over three field seasons and at two locations. Serial dilution in selective King's B medium and microscopy were used to assess the abundance of cultivable pseudomonads and AMF, respectively. We developed a denaturing gradient gel electrophoresis (DGGE) method to characterize the diversity of the pqqC gene, which is involved in Pseudomonas phosphate solubilization. A major result was that in the first field season Pseudomonas abundances and diversity on roots of GM pm3b lines, but also on non-GM sister lines were different from those of the parental lines and conventional wheat cultivars. This indicates a strong effect of the procedures by which these plants were created, as GM and sister lines were generated via tissue cultures and propagated in the greenhouse. Moreover, Pseudomonas population sizes and DGGE profiles varied considerably between individual GM lines with different genomic locations of the pm3b transgene. At individual time points, differences in Pseudomonas and AMF accumulation between GM and control lines were detected, but they were not consistent and much less pronounced than differences detected between young and old plants, different conventional wheat cultivars or at different locations and field seasons. Thus, we conclude that impacts of GM wheat on plant-beneficial root-colonizing microorganisms are minor and not of ecological importance. The cultivation-independent pqqC-DGGE approach proved to be a useful tool for monitoring the dynamics of Pseudomonas populations in a wheat field and even sensitive enough for detecting population responses to altered plant physiology.

Biological control of soil-borne pathogens by fluorescent pseudomonads.

Particular bacterial strains in certain natural environments prevent infectious diseases of plant roots. How these bacteria achieve this protection from pathogenic fungi has been analysed in detail in biocontrol strains of fluorescent pseudomonads. During root colonization, these bacteria produce antifungal antibiotics, elicit induced systemic resistance in the host plant or interfere specifically with fungal pathogenicity factors. Before engaging in these activities, biocontrol bacteria go through several regulatory processes at the transcriptional and post-transcriptional levels.

Essays in general equilibrium asset pricing

ABSTRACT : Research in empirical asset pricing has pointed out several anomalies both in the cross section and time series of asset prices, as well as in investors' portfolio choice. This dissertation aims to discover the forces driving some of these "puzzling" asset pricing dynamics and portfolio decisions observed in the financial market. Through the dissertation I construct and study dynamic general equilibrium models of heterogeneous investors in the presence of frictions and evaluate quantitatively their implications for financial-market asset prices and portfolio choice. I also explore the potential roots of puzzles in international finance. Chapter 1 shows that, by introducing jointly endogenous no-default type of borrowing constraints and heterogeneous beliefs in a dynamic general-equilibrium economy, many empirical features of stock return volatility can be reproduced. While most of the research on stock return volatility is empirical, this paper provides a theoretical framework that is able to reproduce simultaneously the cross section and time series stylized facts concerning stock returns and their volatility. In contrast to the existing theoretical literature related to stock return volatility, I don't impose persistence or regimes in any of the exogenous state variables or in preferences. Volatility clustering, asymmetry in the stock return-volatility relationship, and pricing of multi-factor volatility components in the cross section all arise endogenously as a consequence of the feedback between the binding of no-default constraints and heterogeneous beliefs. Chapters 2 and 3 explore the implications of differences of opinion across investors in different countries for international asset pricing anomalies. Chapter 2 demonstrates that several international finance "puzzles" can be reproduced by a single risk factor which captures heterogeneous beliefs across international investors. These puzzles include: (i) home equity preference; (ii) the dependence of firm returns on local and foreign factors; (iii) the co-movement of returns and international capital flows; and (iv) abnormal returns around foreign firm cross-listing events in the local market. These are reproduced in a setup with symmetric information and in a perfectly integrated world with multiple countries and independent processes producing the same good. Chapter 3 shows that by extending this framework to multiple goods and correlated production processes; the "forward premium puzzle" arises naturally as a compensation for the heterogeneous expectations about the depreciation of the exchange rate held by international investors. Chapters 2 and 3 propose differences of opinion across international investors as the potential resolution of several international finance `puzzles'. In a globalized world where both capital and information flow freely across countries, this explanation seems more appealing than existing asymmetric information or segmented markets theories aiming to explain international finance puzzles.

The mycorrhizal contribution to plant productivity, plant nutrition and soil structure in experimental grassland.

Previous studies have shown that arbuscular mycorrhizal fungi (AMF) can influence plant diversity and ecosystem productivity. However, little is known about the effects of AMF and different AMF taxa on other important community properties such as nutrient acquisition, plant survival and soil structure. We established experimental grassland microcosms and tested the impact of AMF and of different AMF taxa on a number of grassland characteristics. We also tested whether plant species benefited from the same or different AMF taxa in subsequent growing seasons. AMF enhanced phosphorus acquisition, soil aggregation and survival of several plant species, but AMF did not increase total plant productivity. Moreover, AMF increased nitrogen acquisition by some plant species, but AMF had no effect on total N uptake by the plant community. Plant growth responses to AMF were temporally variable and some plant species obtained the highest biomass with different AMF in different years. Hence the results indicate that it may be beneficial for a plant to be colonized by different AMF taxa in different seasons. This study shows that AMF play a key role in grassland by improving plant nutrition and soil structure, and by regulating the make-up of the plant community.

Characterization of the PHO1 gene family and the responses to phosphate deficiency of Physcomitrella patens.

PHO1 was previously identified in Arabidopsis (Arabidopsis thaliana) as a protein involved in loading inorganic phosphate (Pi) into the xylem of roots and its expression was associated with the vascular cylinder. Seven genes homologous to AtPHO1 (PpPHO1;1-PpPHO1;7) have been identified in the moss Physcomitrella patens. The corresponding proteins harbor an SPX tripartite domain in the N-terminal hydrophilic portion and an EXS domain in the conserved C-terminal hydrophobic portion, both common features of the plant PHO1 family. Northern-blot analysis showed distinct expression patterns for the PpPHO1 genes, both at the tissue level and in response to phosphate deficiency. Transgenic P. patens expressing the beta-glucuronidase reporter gene under three different PpPHO1 promoters revealed distinct expression profiles in various tissues. Expression of PpPHO1;1 and PpPHO1;7 was specifically induced by Pi starvation. P. patens homologs to the Arabidopsis PHT1, DGD2, SQD1, and APS1 genes also responded to Pi deficiency by increased mRNA levels. Morphological changes associated with Pi deficiency included elongation of caulonemata with inhibition of the formation of side branches, resulting in colonies with greater diameter, but reduced mass compared to Pi-sufficient plants. Under Pi-deficient conditions, P. patens also increased the synthesis of ribonucleases and of an acid phosphatase, and increased the ratio of sulfolipids over phospholipids. These results indicate that P. patens and higher plants share some common strategies to adapt to Pi deficiency, although morphological changes are distinct, and that the PHO1 proteins are well conserved in bryophyte despite the lack of a developed vascular system.

Mechanism, regulation, and ecological role of bacterial cyanide biosynthesis.

A few bacterial species are known to produce and excrete hydrogen cyanide (HCN), a potent inhibitor of cytochrome c oxidase and several other metalloenzymes. In the producer strains, HCN does not appear to have a role in primary metabolism and is generally considered a secondary metabolite. HCN synthase of proteobacteria (especially fluorescent pseudomonads) is a membrane-bound flavoenzyme that oxidizes glycine, producing HCN and CO2. The hcnABC structural genes of Pseudomonas fluorescens and P. aeruginosa have sequence similarities with genes encoding various amino acid dehydrogenases/oxidases, in particular with nopaline oxidase of Agrobacterium tumefaciens. Induction of the hcn genes of P. fluorescens by oxygen limitation requires the FNR-like transcriptional regulator ANR, an ANR recognition sequence in the -40 region of the hcn promoter, and nonlimiting amounts of iron. In addition, expression of the hcn genes depends on a regulatory cascade initiated by the GacS/GacA (global control) two-component system. This regulation, which is typical of secondary metabolism, manifests itself during the transition from exponential to stationary growth phase. Cyanide produced by P. fluorescens strain CHA0 has an ecological role in that this metabolite accounts for part of the biocontrol capacity of strain CHA0, which suppresses fungal diseases on plant roots. Cyanide can also be a ligand of hydrogenases in some anaerobic bacteria that have not been described as cyanogenic. However, in this case, as well as in other situations, the physiological function of cyanide is unknown.

Uncoupling phosphate deficiency from its major effects on growth and transcriptome via PHO1 expression in Arabidopsis.

Inorganic phosphate (Pi) is one of the most limiting nutrients for plant growth in both natural and agricultural contexts. Pi-deficiency leads to a strong decrease in shoot growth, and triggers extensive changes at the developmental, biochemical and gene expression levels that are presumably aimed at improving the acquisition of this nutrient and sustaining growth. The Arabidopsis thaliana PHO1 gene has previously been shown to participate in the transport of Pi from roots to shoots, and the null pho1 mutant has all the hallmarks associated with shoot Pi deficiency. We show here that A. thaliana plants with a reduced expression of PHO1 in roots have shoot growth similar to Pi-sufficient plants, despite leaves being strongly Pi deficient. Furthermore, the gene expression profile normally triggered by Pi deficiency is suppressed in plants with low PHO1 expression. At comparable levels of shoot Pi supply, the wild type reduces shoot growth but maintains adequate shoot vacuolar Pi content, whereas the PHO1 underexpressor maintains maximal growth with strongly depleted Pi reserves. Expression of the Oryza sativa (rice) PHO1 ortholog in the pho1 null mutant also leads to plants that maintain normal growth and suppression of the Pi-deficiency response, despite the low shoot Pi. These data show that it is possible to unlink low shoot Pi content with the responses normally associated with Pi deficiency through the modulation of PHO1 expression or activity. These data also show that reduced shoot growth is not a direct consequence of Pi deficiency, but is more likely to be a result of extensive gene expression reprogramming triggered by Pi deficiency.

Geochronological constraints on post-extinction recovery of the ammonoids and carbon cycle perturbations during the Early Jurassic

This paper presents the first quantitative study of the Early Jurassic recovery of ammonoids after the end-Triassic mass extinction based on detailed U-Pb ID-TIMS (isotope dilution thermal ionization mass spectrometry) geochronology from ash bed zircons placed within a clear phylogenetical and biochronological framework at the subzonal and species level. This study was triggered by the discovery of a rich Peruvian succession of ammonites, deposited concomitantly with an unusually large number of ash beds. Two major phases of rediversification are observed during the Psiloceras spelae and Angulaticeras zones that correspond to positive peaks in the delta C-13(org) curve, providing a possible link between biodiversity and the global carbon cycle. In the case of the post-extinction recovery, the development of the earliest Hettangian ammonites occurs within the genus Psiloceras, which begins with the occurrence of P. spelae and then explodes into worldwide development of smooth psiloceratids of the Psiloceras planorbis group s.l. This rapid biodiversification likely occurred less than 100 ka after the end-Triassic crisis; the genus Psiloceras occupied all the possible ecological niches worldwide, from the Pacific deep waters to the NW European shallow deposits and also in some rare Tethyan occurrences like at Germig in Tibet. This global dispersion allowed the differentiation of the group in several major phyla, the Schlotheimiidae, Discamphiceratinae, Arietitidae and Lytocerataceae, which were the roots of all other Jurassic and Cretaceous ammonites. (C) 2012 Elsevier B.V. All rights reserved.

Expression analyses of three members of the AtPHO1 family reveal differential interactions between signaling pathways involved in phosphate deficiency and the responses to auxin, cytokinin, and abscisic acid.

The PHO1 protein is involved in loading inorganic phosphate (Pi) to the root xylem. Ten genes homologous to AtPHO1 are present in the Arabidopsis thaliana (L.) Heyn genome. From this gene family, transcript levels of only AtPHO1, AtPHO1;H1 and AtPHO1;H10 were increased by Pi-deficiency. While the up-regulation of AtPHO1;H1 and AtPHO1;H10 by Pi deficiency followed the same rapid kinetics and was dependent on the PHR1 transcription factor, phosphite only strongly suppressed the expression of AtPHO1;H1 and had a minor effect on AtPHO1;H10. Addition of sucrose was found to increase transcript levels of both AtPHO1 and AtPHO1;H1 in Pi-sufficient or Pi-deficient plants, but to suppress AtPHO1:H10 under the same conditions. Treatments of plants with auxin or cytokinin had contrasting effect depending on the gene and on the Pi status of the plants. Thus, while both hormones down-regulated expression of AtPHO1 independently of the plant Pi status, auxin and cytokinin up-regulated AtPHO1;H1 and AtPHO1;H10 expression in Pi-sufficient plants and down-regulated expression in Pi-deficient plants. Treatments with abscisic acid inhibited AtPHO1 and AtPHO1;H1 expression in both Pi-sufficient and Pi-deficient plants, but increased AtPHO1;H10 expression under the same conditions. The inhibition of expression by abscisic acid of AtPHO1 and AtPHO1;H1, and of the Pi-starvation responsive genes AtPHT1;1 and AtIPS1, was dependant on the ABI1 type 2C protein phosphatase. These results reveal that various levels of cross talk between the signal transduction pathways to Pi, sucrose and phytohormones are involved in the regulation of expression of the three AtPHO1 homologues.

Effect of abscisic acid on maize root growth. A critical examination

The effect of abscisic acid (ABA) on the growth of maize roots maintained in the dark is investigated in relation to the root varieties and the root age, the mode of application, the concentration used and the duration of both the treatment and the culture. In all the assays, when ABA produces a significant change in root elongation, it shows an inhibitory effect which is enhanced with increasing ABA concentration. The data strongly support the hypothesis that ABA could be one of the growth inhibitors which are formed in or released from the root cap.