Showing papers in "New Phytologist in 2003"

Phosphorus acquisition and use: critical adaptations by plants for securing a nonrenewable resource

Abstract: Contents I. Introduction 424 II. The phosphorus conundrum 424 III. Adaptations to low P 424 IV. Uptake of P 424 V. P deficiency alters root development and function 426 VI. P deficiency modifies carbon metabolism 431 VII. Acid phosphatase 436 VIII. Genetic regulation of P responsive genes 437 IX. Improving P acquisition 439 X. Synopsis 440 Summary Phosphorus (P) is limiting for crop yield on > 30% of the world's arable land and, by some estimates, world resources of inexpensive P may be depleted by 2050. Improvement of P acquisition and use by plants is critical for economic, humanitarian and environmental reasons. Plants have evolved a diverse array of strategies to obtain adequate P under limiting conditions, including modifications to root architecture, carbon metabolism and membrane structure, exudation of low molecular weight organic acids, protons and enzymes, and enhanced expression of the numerous genes involved in low-P adaptation. These adaptations may be less pronounced in mycorrhizal-associated plants. The formation of cluster roots under P-stress by the nonmycorrhizal species white lupin (Lupinus albus), and the accompanying biochemical changes exemplify many of the plant adaptations that enhance P acquisition and use. Physiological, biochemical, and molecular studies of white lupin and other species response to P-deficiency have identified targets that may be useful for plant improvement. Genomic approaches involving identification of expressed sequence tags (ESTs) found under low-P stress may also yield target sites for plant improvement. Interdisciplinary studies uniting plant breeding, biochemistry, soil science, and genetics under the large umbrella of genomics are prerequisite for rapid progress in improving nutrient acquisition and use in plants.

Mycorrhizas and nutrient cycling in ecosystems – a journey towards relevance?

Abstract: Progress towards understanding the extent to which mycorrhizal fungi are involved in the mobilization of nitrogen (N) and phosphorus (P) from natural substrates is reviewed here. While mycorrhiza research has emphasized the role of the symbiosis in facilitation of capture of these nutrients in ionic form, attention has shifted since the mid-1980s to analysing the mycorrhizal fungal abilities to release N and P from the detrital materials of microbial faunal and plant origins, which are the primary sources of these elements in terrestrial ecosystems. Ericoid, and some ectomycorrhizal fungi have the potential to be directly involved in attack both on structural polymers, which may render nutrients inaccessible, and in mobilization of N and P from the organic polymers in which they are sequestered. The advantages to the plant of achieving intervention in the microbial mobilization-immobilization cycles are stressed. While the new approaches may initially lack the precision achieved in studies of readily characterized ionic forms of N and P, they do provide insights of greater ecological relevance. The results support the hypothesis that selection has favoured ericoid and ectomycorrhizal systems with well developed saprotrophic capabilities in those ecosystems characterized by retention of N and P as organic complexes in the soil. The need for further investigation of the abilities of arbuscular mycorrhizal fungi to intervene in nutrient mobilization processes is stressed.

Nitric oxide signalling in plants

Abstract: Contents Summary 11 1Introduction 12 2Why does NO make a good signal? 12 3NO biosynthesis 13 4NO biology 17 5NO signal transduction 23 6Conclusion 30 Acknowledgements 31 References 31 Summary Recently nitric oxide (NO) has emerged as a key signalling molecule in plants. Here we review the potential sources of endogenous NO, outline the biological processes likely to be mediated by NO, and discuss the downstream signalling processes by which NO exerts its cellular effects. It will be important to develop methods to quantify intracellular NO synthesis and release. Clasification of the biosynthetic origins of NO is also required. NO can be synthesised from nitrite via nitrate reductase (NR) and although biochemical and immunological data indicate the presence of enzyme(s) similar to mammalian nitric oxide synthase (NOS), no NOS genes have been identified. NO can induce various processes in plants, including the expression of defence-related genes and programmed cell death (PCD), stomatal closure, seed germination and root development. Intracellular signalling responses to NO involve generation of cGMP, cADPR and elevation of cytosolic calcium, but in many cases, the precise biochemical and cellular nature of these responses has not been detailed. Research priorities here must be the reliable quantification of downstream signalling molecules in NO-responsive cells, and cloning and manipulation of the enzymes responsible for synthesis and degradation of these molecules.

Soil community feedback and the coexistence of competitors: conceptual frameworks and empirical tests

Abstract: A growing body of empirical work suggests that soil organisms can exert a strong role in plant community dynamics and may contribute to the coexistence of plant species. Some of this evidence comes from examining the feedback on plant growth through changes in the composition of the soil community. Host specific changes in soil community composition can generate feedback on plant growth and this feedback can be positive or negative. Previous work has demonstrated that negative soil community feedback can contribute to the coexistence of equivalent competitors. In this paper, I show that negative soil community feedback can also contribute to the coexistence of strong competitors, maintaining plant species that would not coexist in the absence of soil community dynamics. I review the evidence for soil community feedback and find accumulating evidence that soil community feedback can be common, strongly negative, and generated by a variety of complementary soil microbial mechanisms, including host-specific changes in the composition of the rhizosphere bacteria, nematodes, pathogenic fungi, and mycorrhizal fungi. Finally, I suggest topics needing further examination.

Plant growth and competition at elevated CO2 : on winners, losers and functional groups.

Abstract: Contents Summary 175 Summary The effects of increased atmospheric CO 2 concentrations on vegetative growth and competitive performance were evaluated, using five meta-analyses. Paying special attention to functional groups, we analysed responses at three integration levels: carbon economy parameters, vegetative biomass of isolated plants, and growth in competition. CO 2 effects on seed biomass and plant-to-plant variability were also studied. Underlying the growth stimulation is an increased unit leaf rate (ULR), espe- cially for herbaceous dicots. This is mainly caused by an increase in the whole-plant rate of photosynthesis. The increased ULR is accompanied by a decrease in specific leaf area. The net result of these and other changes is that relative growth rate is only marginally stimulated. The biomass enhancement ratio (BER) of individually grown plants varies substantially across experiments/species, and size variability in the experimental populations is a vital factor in this. Fast-growing herbaceous C3 species respond more strongly than slow-growing C3 herbs or C4 plants. CAM species and woody plants show intermediate responses. When grown in competition, C4 species show lowest responses to elevated CO 2 at high nutrient conditions, whereas at low nutrient levels N 2 -fixing dicots respond relatively strongly. No systematic differences were found between slow- and fast-growing species. BER values obtained for isolated plants cannot be used to estimate BER of the same species grown in interspecific competition - the CO 2 response of monocultures may be a better predictor.

Fusarium oxysporum and its biocontrol

Abstract: Fusarium oxysporum is well represented among the rhizosphere microflora. While all strains exist saprophytically, some are well-known for inducing wilt or root rots on plants whereas others are considered as nonpathogenic. Several methods based on phenotypic and genetic traits have been developed to characterize F. oxysporum strains. Results showed the great diversity affecting the soil-borne populations of F. oxysporum. In suppressive soils, interactions between pathogenic and nonpathogenic strains result in the control of the disease. Therefore nonpathogenic strains are developed as biocontrol agents. The nonpathogenic F. oxysporum strains show several modes of action contributing to their biocontrol capacity. They are able to compete for nutrients in the soil, affecting the rate of chlamydospore germination of the pathogen. They can also compete for infection sites on the root, and can trigger plant defence reactions, inducing systemic resistance. These mechanisms are more or less important depending on the strain. The nonpathogenic F. oxysporum are easy to mass produce and formulate, but application conditions for biocontrol efficacy under field conditions have still to be determined.

Leaf size and angle vary widely across species: what consequences for light interception?

Abstract: Summary •A rchitecture can vary widely across species. Both steeper leaf angles and increased self-shading are thought to reduce potential carbon gain by decreasing total light interception. An alternative hypothesis is that steeper leaf angles have evolved to improve day-long carbon gain by emphasising light interception from low angles. • Here we relate variation in architectural properties (leaf angle and leaf size) to cross-species patterns of leaf display, light capture and simulated carbon gain in branching-units of 38 perennial species occurring at two sites in Australian forest. Architectural comparison was made possible by combining 3D-digitising with the architecture model YPLANT. • Species with shallow angled leaves had greater daily light interception and potentially greater carbon gain. Self-shading, rather than leaf angle, explained most variance between species in light capture and potential carbon gain. Species average leaf size was the most important determinant of self-shading. • Our results provide the first cross-species evidence that steeper leaf angles function to reduce exposure to excess light levels during the middle of the day, more than to maximise carbon gain.

Source–sink balance and carbon allocation below ground in plants exposed to ozone

Abstract: Contents Summary 213 I. Introduction 213 II. Source–sink model: carbohydrate signaling 214 III. Effect of ozone on above-ground sources and sinks 216 IV. Decreased allocation below ground 218 V. Carbon flux to soils 220 VI. Soil food web 223 VII. Summary, conclusions and future research 223 Acknowledgements 223 References 223 Summary The role of tropospheric ozone in altering plant growth and development has been the subject of thousands of publications over the last several decades. Still, there is limited understanding regarding the possible effects of ozone on soil processes. In this review, the effects of ozone are discussed using the flow of carbon from the atmosphere, through the plant to soils, and back to the atmosphere as a framework. A conceptual model based on carbohydrate signaling is used to illustrate physiological changes in response to ozone, and to discuss possible feedbacks that may occur. Despite past emphasis on above-ground effects, ozone has the potential to alter below-ground processes and hence ecosystem characteristics in ways that are not currently being considered.

Extreme events as shaping physiology, ecology, and evolution of plants: toward a unified definition and evaluation of their consequences

Abstract: Here we consider how extreme events, particularly climatic and biotic, affect the physiology, development, ecology and evolution of organisms, focusing on plants. The marked effects on organisms are of increasing interest for ecological prediction, given the natural and anthropogenic changes in spectra of extreme events being induced by global change. Yet there is currently a paucity of knowledge or even a common world-view of how extreme events shape individuals, communities and ecosystems. We propose that extreme events need be defined in terms of organismal responses of acclimation and of de-acclimation or hysteresis. From this definition we proceed to develop a number of hypotheses, including that fitness effects of extreme events occur primarily during recovery. We review evidence that, on the evolutionary time scale, selection is virtually absent except during extreme events; these drive strong directional selection, even to trait fixation and speciation. We describe a number of new tools, both conceptual and technological, that are now at hand or that merit rapid development. Contents I. Introduction 22 II. Moving to an organismally based definition of extreme events 22 III. Features to discern in extreme events 26 IV. Additional challenges in the study of extreme events 27 V. Evolutionary dimensions 29 VI. The mandate for new conceptual tools for ecological and evolutionary prediction 34 VII. Tools in hand, and tools needed, to study extreme events 35 VIII. Conclusions 37 Acknowledgements 37 References 38.

Fine scale distribution of ectomycorrhizal fungi and roots across substrate layers including coarse woody debris in a mixed forest

Abstract: Summary • Ectomycorrhizal (ECM) fungi are widespread plant root symbionts in boreal forests, but information is lacking on the fine scale distribution of roots and fungi in substrate patches of different quality, including coarse woody debris (CWD). • Wood and soil cores were taken systematically both horizontally and vertically through decayed logs and underlying soil layers in a mixed forest. Root tips were counted and ECM fungi identified by morphotyping and sequencing. • The abundance of root tips and ECM fungi was highly variable on a 5-cm scale. Most species were replaced on a 50-cm scale. Detrended correspondence analysis demonstrated strong preference of resupinate thelephoroid and athelioid fungi and Sebacinaceae for CWD, whereas ascomycetes and euagarics appeared more frequently in mineral soil. Clavulicium delectabile was determined to be an ECM fungus for the first time. • ECM fungi occupy different niches and show variable distribution patterns. CWD plays an important role as a habitat both for roots and ECM fungi. We suggest sampling larger soil cores and selecting random root tips in future studies. Sequencing is a powerful tool in ECM community studies.

Phenazines and their role in biocontrol by Pseudomonas bacteria.

Abstract: Various rhizosphere bacteria are potential (micro)biological pesticides which are able to protect plants against diseases and improve plant yield. Knowledge of the molecular mechanisms that govern these beneficial plant-microbe interactions enables optimization, enhancement and identification of potential synergistic effects in plant protection. The production of antifungal metabolites, induction of systemic resistance, and the ability to compete efficiently with other resident rhizobacteria are considered to be important prerequisites for the optimal performance of biocontrol agents. Intriguing aspects in the molecular mechanisms of these processes have been discovered recently. Phenazines and phloroglucinols are major determinants of biological control of soilborne plant pathogens by various strains of fluorescent Pseudomonas spp. This review focuses on the current state of knowledge on biocontrol by phenazine-producing Pseudomonas strains and the action, biosynthesis, and regulation mechanisms of the production of microbial phenazines.

Control of pollen tube growth: role of ion gradients and fluxes

Abstract: Pollen tube growth attracts our attention as a model system for studying cell elongation in plants. The process is fast, it is confined to the tip of the tube, and it is crucial for sexual reproduction in plants. In the enclosed review we focus on the control of pollen tube growth, giving special attention to the role of ions, especially calcium and protons. During the last decade technical advances have made it possible to detect localized intracellular gradients, and extracellular fluxes of calcium and protons in the apical domain. Other ions, notably potassium and chloride, are also receiving attention. An important development has been the realization that pollen tube growth oscillates in rate; in addition, the ion gradients and fluxes oscillate in magnitude. Although all the ionic oscillations show the same period as that of the growth rate, with the exception of extracellular chloride efflux, they are not in phase with growth. Considerable effort is devoted to the elucidation of these different phase relationships, with the view that a hierarchical order may provide clues about those events that are primary vs. secondary in growth control. Attention is also given to the targets for the ions, for example, the secretory system, the cytoskeleton, the cell wall, in an attempt to provide a global understanding of pollen tube growth. Contents Summary 539 I. Introduction 540 II. Ion gradients and flux patterns 541 III. Oscillations 544 IV. The need for a Ca2+ store 547 V. Intracellular targets for Ion activity 549 VI. Extracellular targets for ions: the cell wall 552 VII. Ions in navigation 554 VIII. Role of ions in self-incompatibility 555 IX. The plasma membrane; site of global coordination and control 556 X. A model for pollen tube growth 557 IX. Conclusions 558 Acknowledgements 559 References 559.

Thlaspi caerulescens, an attractive model species to study heavy metal hyperaccumulation in plants.

Abstract: Studying heavy metal hyperaccumulation is becoming more and more interesting for ecological, evolutionary, nutritional, and environmental reasons. One model species, especially in the era of high throughput genomics, transcriptomics, proteomics and metabolomics technologies, would be very advantageous. Although there are several hyperaccumulator species known, there is no single model species yet. The Zn, Cd and Ni hyperaccumulator species Thlaspi caerulescens has been studied to a great extent, especially for Zn and Cd hyperaccumulation and tolerance. Its physiological, morphological and genetic characteristics, and its close relationship to Arabidopsis thaliana, the general plant reference species, make it an excellent candidate to be the plant heavy metal hyperaccumulation model species.

Vertical distribution of ectomycorrhizal fungal taxa in a podzol soil profile

Abstract: Summary • Studies of ectomycorrhizal fungal communities in forest soils are usually restricted to the uppermost organic horizons. Boreal forest podzols are highly stratified and little is known about the vertical distribution of ectomycorrhizal communities in the underlying mineral horizons. • Ectomycorrhizal root tips were sampled from seven horizons in three continuous columns of a 52-cm deep podzol profile. Root tips were sorted into morphological groups and the colonising fungi identified by sequencing of the rDNA ITS region. The vertical distribution of mycorrhizal taxa was examined. • A relationship between ectomycorrhizal species composition and soil horizon was found. Tomentellopsis submollis, three Piloderma species and Dermocybe spp. were found predominantly in the upper horizons while Suillus luteus, Lactarius utilis and three undescribed Piloderma species were associated with the mineral horizons. • Two thirds of the root tips were found in the mineral soil and half of the taxa were restricted to the mineral horizons. The results highlight the need to include the mineral soil in order to gain a more accurate representation of the ectomycorrhizal community.

Ectomycorrhizal fungal communities in young forest stands regenerating after clearcut logging

Abstract: Contents Summary399 I. Introduction400 II. Population biology and inoculum potential of ectomycorrhizal fungi401 III. Ectomycorrhiza development on seedlings regenerating after clearcut logging402 IV. Which is the most important factor driving changes in the ECM fungal community after clearcut logging: inoculum loss or change in the below-ground environment?406 V. Possible consequences for regenerating stands of species shifts in ectomycorrhizal fungi414 VI. Conclusions416 Summary The effects on the ectomycorrhizal fungal community of clearcut logging, which is used to harvest millions of hectares of ectomycorrhizal forest annually, has been studied for a number of years. Here, we review current knowledge of inoculum sources for ectomycorrhizal fungi in forests and then re-examine earlier studies of ectomycorrhizas on young trees in regenerating stands. We conclude that, taken separately from the effects of site preparation, the major impact of clearcut logging is to change the species composition of the ectomycorrhizal fungal community rather than to reduce the percentage of roots colonized. A thorough examination of site preparation treatments suggests that the changes in fungal species composition are driven by changes in the biology and chemistry of the soil environment after clearcutting as much as they are by loss or change in fungal inoculum. This is an important conclusion because it implies that these new ectomycorrhizal fungal communities are better adapted to the new conditions than the ones in the forest would have been. The shift in fungal species composition and diversity will have implications for seedling establishment and competition. The effects of individual fungi or diverse assemblages of fungi on seedling growth, and effects of changes in the ability of young trees to associate with a common mycelium are discussed.

Rhizobacterial effects on nickel extraction from soil and uptake by Alyssum murale

Abstract: Summary • Alyssum murale has a remarkable ability to hyperaccumulate Ni from soils containing mostly nonlabile Ni. Here, rhizobacteria are shown to play an important role in increasing the availability of Ni in soil, thus enhancing Ni accumulation by A. murale. • Three bacteria, originally isolated from the rhizosphere of A. murale, were examined for their ability to solubilize Ni in soil and for their effect on Ni uptake into Alyssum. Sphingomonas macrogoltabidus, Microbacterium liquefaciens, and Microbacterium arabinogalactanolyticum were added to both sterile and nonsterile Ni-rich Serpentine soil from OR, USA. • Sphingomonas macrogoltabidus significantly reduced Ni extraction by 10 mM Sr(NO3)2 from soil, M. arabinogalactanolyticum significantly increased Ni extraction, whereas M. liquefaciens had no effect. Extractability of few other metals was affected by inoculation. When these bacteria were added to surface-sterilized seeds of A. murale grown in nonsterile soil, they increased Ni uptake into the shoot by 17% (S. macrogoltabidus), 24% (M. liquefaciens), and 32.4% (M. arabinogalactanolyticum), compared with uninoculated controls. • These results show that bacteria are important for Ni uptake, which from a commercial perspective, could significantly increase revenue generated during phytomining of Ni from soil.

Arsenite transport into paddy rice ( Oryza sativa ) roots

Abstract: Summary • Here the mechanism of arsenite transport into paddy rice ( Oryza sativa ) roots, uptake of which is described by Michaelis–Menten kinetics, is reported. A recent study on yeast ( Saccharomyces cerevisiae ) showed that undissociated arsenite (its pK a is 9.2) was transported across the plasma membrane via a glycerol transporting channel. To investigate whether the same mechanism of transport was involved for rice, competitive studies with glycerol, which is transported into cells via aquaporins, were performed. • Glycerol competed with arsenite for transport in a dose-dependent manner, indicating that arsenite and glycerol uptake mechanisms were the same. Arsenate transport was unaffected by glycerol, confirming that arsenate and arsenite are taken up into cells by different mechanisms. • Antimonite, an arsenite analogue that is transported into S. cerevisiae cells by aquaporins, also competed with arsenite transport in a dose-dependent manner, providing further evidence that arsenite is transported into rice roots via glycerol transporting channels. Mercury (Hg 2+ ) inhibited both arsenite and arsenate uptake, suggesting that inhibition of influx was due to general cellular stress rather than the specific action of Hg 2+ on aquaporins. • Arsenite uptake by pea ( Pisum sativum ) and wheat ( Triticum aestivum ) was also described by Michaelis–Menten kinetics.

Different arbuscular mycorrhizal fungi alter coexistence and resource distribution between co-occurring plant

Abstract: Summary • It is often thought that the coexistence of plants and plant diversity is determined by resource heterogeneity of the abiotic environment. However, the presence and heterogeneity of biotic plant resources, such as arbuscular mycorrhizal fungi (AMF), could also affect plant species coexistence. • In this study, Brachypodium pinnatum and Prunella vulgaris were grown together in pots and biotic resource heterogeneity was simulated by inoculating these pots with one of three different AMF taxa, with a mixture of these three taxa, or pots remained uninoculated. • The AMF acted as biotic plant resources since the biomass of plants in pots inoculated with AMF was on average 11.8 times higher than uninoculated pots. The way in which the two plant species coexisted, and the distribution of phosphorus and nitrogen between the plant species, varied strongly depending on which AMF were present. The results showed that the composition of AMF communities determines how plant species coexist and to which plant species nutrients are allocated. • Biotic plant resources such as AMF should therefore be considered as one of the factors that determine how plant species coexist and how soil resources are distributed among co-occurring plant species.

Calmodulins and related potential calcium sensors of Arabidopsis

Abstract: Summary • Calcium (Ca2+) signaling is thought to orchestrate responses to cellular stimuli. The efficacy of Ca2+ signaling requires mediation by Ca2+-binding proteins. • The determination of the Arabidopsis genome sequence enables the identification of genes encoding potential Ca2+ sensors. • Six Arabidopsis loci are defined as calmodulin (CAM) genes. Fifty additional genes are CAM-like (CML) genes, encoding proteins composed mostly of EF-hand Ca2+-binding motifs, have no other identifiable functional domains, and at least 16% identical with CaM. Number and structural diversity of the EF hands are evaluated. Intron/exon boundaries, phylogenetic tree and chromosomal distribution data for the CAMs and CMLs are presented. • Arabidopsis has 6 CAM genes, encoding only 3 isoforms. Maintenance of these genes suggests that they are unlikely to be fully redundant in function. Furthermore, the repeated EF hand motif is incorporated into at least 50 additional loci. The CaM relatives have altered EF hand number, organization, and predicted functional capacity. Additional structural differences and expression behaviors also indicate that the CML family has likely evolved distinct roles from the CAMs.

Differential metal-specific tolerance and accumulation patterns among Thlaspi caerulescens populations originating from different soil types.

Abstract: Summary • Here, Thlaspi caerulescens populations from contrasting soil types (serpentine, calamine and nonmetalliferous) were characterized with regard to tolerance, uptake and translocation of zinc (Zn), cadmium (Cd) and nickel (Ni) in hydroponic culture. • Results showed that high-level tolerances were apparently metal-specific and confined to the metals that were enriched at toxic levels in the soil at the population site. •W ith regard to metal accumulation, results suggested that, unlike Zn hyperaccumulation, Cd and Ni hyperaccumulation were not constitutive at the species level in T. caerulescens . • In general, the populations under study exhibited a pronounced uncorrelated and metal-specific variation in uptake, root to shoot translocation, and tolerance of Zn, Cd and Ni. The distinct intraspecific variation of these characters provides excellent opportunities for further genetic and physiological dissection of the hyperaccumulation trait.

15N and 13C natural abundance of autotrophic and myco‐heterotrophic orchids provides insight into nitrogen and carbon gain from fungal association

Abstract: Summary • Whereas mycorrhizal fungi are acknowledged to be the sources of nitrogen (N) and carbon (C) in achlorophyllous (myco-heterotrophic) orchids, the sources of these elements in autotrophic orchids are unknown. We have determined the stable isotope abundance of N and C to quantify their gain from different sources in these two functional groups and in non-orchids of distinctive mycorrhizal types. • Leaves of each plant were collected from four forest and four grassland sites in Europe. The N and C isotope abundance, and total N concentrations of their tissues and of associated soils were determined. • Myco-heterotrophic orchids were significantly more enriched in 15N (ɛMHO-R= 11.5‰) and 13C (ɛMHO-R= 8.4‰) than co-occurring non-orchids. δ15N and δ13C signatures of autotrophic orchids ranged from values typical of non-orchids to those more representative of myco-heterotrophic orchids. • Utilization of fungi-derived N and C probably explains the relative 15N and 13C enrichment in the myco-heterotrophs. A linear two-source isotopic mixing model was used to estimate N and C gain of autotrophic orchids from their fungal associates. Of the putatively autotrophic species, Cephalanthera damasonium obtained the most N and C by the fungal route, but several other species also fell into the partially myco-heterotrophic category.

On the unreliability of published DNA sequences

Abstract: Summary • Here, the reliability of published fungal nucleic acid sequences is tested by the critical re-evaluation of 206 named sequences obtained from public-access databases. • Sequences from the ribosomal RNA (rRNA) gene cluster were examined as these are commonly used to establish fungal phylogeny and evolution, and are also increasingly employed in the identification of fungi from nonculture based studies. • Fifty-one rRNA internal transcribed spacer (ITS) sequences were obtained for species of Amanita, 55 ITS sequences were obtained for species of Phoma and 100 rRNA small subunit sequences were obtained from representative genera of the order Helotiales. In each case, the fungal group was selected partly on the basis of sequences deposited by three or more laboratories in order to avoid sample bias. The results suggest that up to 20% of the sequences available for each group may be unreliable, and this proportion is supported by additional informal observations.

Plant roots release phospholipid surfactants that modify the physical and chemical properties of soil

Abstract: Summary • Plant root mucilages contain powerful surfactants that will alter the interaction of soil solids with water and ions, and the rates of microbial processes. • The lipid composition of maize, lupin and wheat root mucilages was analysed by thin layer chromatography and gas chromatography-mass spectrometry. A commercially available phosphatidylcholine (lecithin), chemically similar to the phospholipid surfactants identified in the mucilages, was then used to evaluate its effects on selected soil properties. • The lipids found in the mucilages were principally phosphatidylcholines, composed mainly of saturated fatty acids, in contrast to the lipids extracted from root tissues. In soil at low tension, lecithin reduced the water content at any particular tension by as much as 10 and 50% in soil and acid-washed sand, respectively. Lecithin decreased the amount of phosphate adsorption in soil and increased the phosphate concentration in solution by 10%. The surfactant also reduced net rates of ammonium consumption and nitrate production in soil. • These experiments provide the first evidence we are aware of that plant-released surfactants will significantly modify the biophysical environment of the rhizosphere.

Characterization of the Arabidopsis metallothionein gene family: tissue-specific expression and induction during senescence and in response to copper

Abstract: Summary •Expression and regulation of Arabidopsis metallothionein (MT) genes were investigated to examine the functions of MTs in plants. •To examine the tissue-specific expression of MT genes, GUS reporter gene activity driven by promoters of MT1a, MT2a, MT2b and MT3 was analysed in transgenic plants. •MT1a and MT2b are expressed in the phloem of all organs and are copper (Cu)-inducible; MT2a and MT3, by contrast, are expressed predominantly in mesophyll cells and are also induced by Cu in young leaves and at root tips. Expression of MT genes is highly induced by Cu in trichomes and increases during senescence. Expression of MT4 genes is restricted to seeds. •We propose that plant MTs have distinct functions in heavy metal homeostasis, especially for Cu: MT1a and MT2b are involved in the distribution of Cu via the phloem, while MT2a and MT3 chaperone excess metals in mesophyll cells and root tips. These functional capabilities may allow MTs to play a role in mobilization of metal ions from senescing leaves and the sequestration of excess metal ions in trichomes.

Role of dynamics of intracellular calcium in aluminium-toxicity syndrome

Abstract: Contents I. Introduction 000 II. Symptoms of aluminium toxicity 000 III. Calcium – aluminium interactions 000 IV. The role of electrical properties of the plasma membrane in calcium–aluminium interactions 000 V. Oxidative stress 000 VI. Callose 000 VII. Cytoskeleton 000 VIII. Conclusions 000 Acknowledgements 000 References 000 Summary This review is concentrating on the role of aluminium (Al)-calcium (Ca) interactions in Al toxicity syndrome in plants. Disruption of cytoplasmic Ca2+ homeostasis has been suggested as a primary trigger of Al toxicity. Aluminium causes an increase in cytosolic Ca2+ activity, potentially disrupting numerous biochemical and physiological processes, including those involved in the root growth. The source of Ca2+ for the increase in cytosolic Ca2+ activity under Al exposure is partly extracellular (likely to be due to the Al-resistant portion of the flux through depolarization-activated Ca2+ channels and fluxes through Ca2+-permeable nonselective cation channels in the plasma membrane) as well as intracellular (increased cytosolic Ca2+ activity enhances the activity of Ca2+ release channels in the tonoplast and the endoplasmic reticulum membrane). The effect on increased cytosolic Ca2+ activity of possible Al-related inhibition of the plasma membrane and endo-membrane Ca2+-ATPases and Ca2+ exchangers (CaX) that sequester Ca2+ out of the cytosol is insufficiently documented at present. The relationship between Al toxicity, cytoplasmic Ca2+ homeostasis and cytoplasmic pH needs to be elucidated. Technical improvements that would allow measurements of cytosolic Ca2+ activity within the short time after exposure to Al (seconds or shorter) are eagerly awaited.

Production of external mycelium by ectomycorrhizal fungi in a norway spruce forest was reduced in response to nitrogen fertilization

Abstract: A field study was carried out to evaluate the influence of N fertilization on the growth of the external mycelium of ectomycorrhizal (EM) fungi in a Norway spruce forest in SW Sweden. Nylon mesh bags filled with sand were buried in the soil for 6-18 months and the ingrowth of mycelium was used as an estimate of EM mycelial growth. Root-isolated, trenched plots were used to estimate background growth of saprotrophic fungi. Mycelial growth of EM fungi in N-treated plots was reduced to c. 50% of that in nonfertilized plots. Local addition of apatite stimulated the EM mycelial growth in N-treated plots. The negative influence of N on the growth of external EM mycelium observed earlier in laboratory studies was confirmed in the present field study. The growth of EM mycelia was not directly related to N concentration in the soil but rather to the N status of the trees, although other factors induced by the N treatment may also have influenced EM mycelial growth. (Less)

Rust haustoria : nutrient uptake and beyond

Abstract: Haustoria are morphological features of an extremely successful class of plant parasites, the obligate biotrophs. The broad phylogenetic spectrum of organisms producing haustoria suggests that these structures have arisen many times in the course of evolution and represent specific adaptations of these organisms to the close interaction with their respective host plants. This close interaction and the fact that these structures cannot be produced in vitro have hampered an analysis of the roles of haustoria in biotrophy for many decades. Only recently has it become possible to analyse haustorial function at a molecular level. A picture is beginning to emerge indicating that haustoria do not only serve in nutrient uptake - a task postulated for these elements ever since their discovery. Moreover, they seem to perform enormous biosynthetic duties. They also seem to be engaged in the suppression of host defense responses and in redirecting or reprogramming the host's metabolic flow. This review intends to summarize current knowledge about the structure and function especially of rust haustoria.

How do some plants tolerate low levels of soil zinc? Mechanisms of zinc efficiency in crop plants

Abstract: Researchers are beginning to understand how some plant genotypes can maintain reasonable growth and yields under low soil Zn, a trait termed zinc efficiency (ZE). Several studies have shown no correlation between ZE and root Zn uptake, Zn translocation to shoot, or shoot Zn accumulation. Furthermore, it has not been possible to conclusively link differences in leaf subcellular Zn compartmentation with ZE. However, biochemical Zn utilization, including the ability to maintain the activity of Zn requiring enzymes in response to Zn deficiency may be a key component of ZE. The next logical step in investigations of this trait is research on the genetic and molecular basis for ZE, in order to better understand the relationship between Zn utilization and ZE, and to identify the gene(s) controlling ZE. Progress in this research area could provide the knowledge to facilitate the engineering of Zn-efficient plant varieties, which could help both crop production on marginal soils as well as possibly improve the micronutrient density of food crops to help address significant human nutrition problems related to micronutrient deficiency.

The role of phytochelatins in arsenic tolerance in the hyperaccumulator Pteris vittata

Abstract: Summary • Pteris vittata was the first identified arsenic (As) hyperaccumulator. Here we investigated whether phytochelatins (PCs) are involved in the hypertolerance of arsenic by P. vittata. • P. vittata was exposed to 0–500 µm arsenate for 5 d, or to 50 µm arsenate for 0–7 d. In addition, l-buthionine-sulphoximine (BSO), an inhibitor of γ-glutamylcysteine synthetase, was used in combination with different arsenate exposures. The relationships between As accumulation and the concentrations of PCs and glutathione (GSH) were examined. • PC synthesis was induced upon exposure to arsenate in P. vittata, with only PC2 detected in the plant. The As concentration correlated significantly with PC2 concentration in both roots and shoots, but not with GSH. The molar ratio of PC-SH to As was c. 0.09 and 0.03 for shoots and roots, respectively, suggesting that only a small proportion (1–3%) of the As in P. vittata can be complexed with PCs. In the presence of arsenate, addition of BSO decreased PC2 concentrations in roots and shoots by 89–96% and 30–33%, respectively. BSO alone was found to inhibit root growth of P. vittata markedly. • The results suggest that PCs play a limited role in the hypertolerance of As in P. vittata.

Antioxidant activities in mycorrhizal soybean plants under drought stress and their possible relationship to the process of nodule senescence

Abstract: Summary • The mechanisms by which the mycorrhizal symbiosis protects soybean ( Glycine max ) plants against premature nodule senescence induced by drought stress is investigated here by evaluating the activity of a set of antioxidant enzymes in relation to nodule senescence. • Superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) activity was determined in well watered or droughtstressed soybean plants inoculated with Bradyrrhizobium japonicum alone or in combination with Glomus mosseae . • In roots, only GR activity was higher in mycorrhizal than in non-mycorrhizal plants. The other antioxidant activities were similar, or lower (APX), in droughted, mycorrhizal plants than in the corresponding nonmycorrhizal ones. Similarly, in nodules, SOD, CAT and APX activities were lower in droughted, mycorrhizal plants than in nonmycorrhizal plants whereas, again, GR activity was higher in nodules from mycorrhizal plants. • We propose that the consistently higher GR activity in roots and nodules of mycorrhizal plants might have contributed to decreased oxidative damage to biomolecules, which are involved in premature nodule senescence. Additional drought-avoidance mechanisms induced by the AM symbiosis might also contribute to the lower oxidative stress in mycorrhizal plants.