Marie-Noëlle Binet

Agroecology: the key role of arbuscular mycorrhizas in ecosystem services

The beneficial effects of arbuscular mycorrhizal (AM) fungi on plant performance and soil health are essential for the sustainable management of agricultural ecosystems. Nevertheless, since the ‘first green revolution’, less attention has been given to beneficial soil microorganisms in general and to AM fungi in particular. Human society benefits from a multitude of resources and processes from natural and managed ecosystems, to which AM make a crucial contribution. These resources and processes, which are called ecosystem services, include products like food and processes like nutrient transfer. Many people have been under the illusion that these ecosystem services are free, invulnerable and infinitely available; taken for granted as public benefits, they lack a formal market and are traditionally absent from society’s balance sheet. In 1997, a team of researchers from the USA, Argentina and the Netherlands put an average price tag of US

Evidence for specific, high-affinity binding sites for a proteinaceous elicitor in tobacco plasma membrane

33 trillion a year on these fundamental ecosystem services. The present review highlights the key role that the AM symbiosis can play as an ecosystem service provider to guarantee plant productivity and quality in emerging systems of sustainable agriculture. The appropriate management of ecosystem services rendered by AM will impact on natural resource conservation and utilisation with an obvious net gain for human society.

Structure and expression of sunflower ubiquitin genes

Binding of cryptogein, a proteinaceous elicitor, was studied on tobacco plasma membrane. The binding of the [125I]cryptogein was saturable, reversible and specific with an apparent K d of 2 nM. A single class of cryptogein binding sites was found with a sharp optimum pH for binding at about pH 7.0. The high‐affinity correlates with cryptogein concentrations required for biological activity in vivo.

Colonization of Flax Roots and Early Physiological Responses of Flax Cells Inoculated with Pathogenic and Nonpathogenic Strains of Fusarium oxysporum

Two ubiquitin genes, designatedUbB1 andUbB2, were isolated from a sunflower genomic library. They encode polyubiquitin transcripts corresponding to six repeats of the monomer. Northern blot analysis identified several different transcript size classes: bothUbB1 andUbB2 transcripts are found in the most abundant 1.6 kb class. In contrast to the previously isolatedUbF transcript which is present at high levels in flowers,UbB1 andUbB2 are expressed constitutively at low levels in different tissues. The levels of the two transcripts increase after heat stress. The two genes exhibit strong homology suggesting that they may result from duplication and conversion. Surprisingly,UbB1 gene shows structural similarities with the chicken ubiquitin heat shock gene, in particular the presence of an intron located just in front of the first ATG.

Analysis of a sunflower polyubiquitin promoter by transient expression

ABSTRACT Fusarium oxysporum includes nonpathogenic strains and pathogenic strains that can induce necrosis or tracheomycosis in plants. The objective of this study was to compare the abilities of a pathogenic strain (Foln3) and a nonpathogenic strain (Fo47) to colonize flax roots and to induce early physiological responses in flax cell culture suspensions. Both strains colonized the outer cortex of the root; however, plant defense reactions, i.e., the presence of wall appositions, osmiophilic material, and collapsed cells, were less frequent and less intense in a root colonized by Foln3 than by Fo47. Early physiological responses were measured in flax cell suspensions confronted with germinated microconidia of both strains. Both pathogenic (Foln3) and nonpathogenic strains (Fo47) triggered transient H2O2 production in the first few minutes of the interaction, but the nonpathogenic strain also induced a second burst 3 h postinoculation. Ca2+ influx was more intense in cells inoculated with Fo47 than in cells inoculated with Foln3. Similarly, alkalinization of the extracellular medium was higher with Fo47 than with Foln3. Inoculation of the fungi into flax cell suspensions induced cell death 10 to 20 h postinoculation, with a higher percentage of dead cells observed with Fo47 than with Foln3 beginning at 14 h. This is the first report showing that early physiological responses of flax cells can be used to distinguish pathogenic and nonpathogenic strains of the soil-borne fungus F. oxysporum.

Involvement of plasma membrane proteins in plant defense responses. Analysis of the cryptogein signal transduction in tobacco

Abstract We have recently isolated two sunflower polyubiquitin genes (UbB1 and UbB2) which are induced by heat-stress and show a highly conserved region of about 250 bp upstream of the transcription initiation site. The promoter activity of this region has been analyzed by transient expression using the β-glucoronidase (GUS) gene as reporter in tobacco protoplasts. The GUS expression in absence of any external inducer is five times higher than that observed under the control of the CaMV 35S promoter. Analysis of different deletions within the 250-bp region showed that a 65-bp sequence is responsible for most of the promoter activity in tobacco protoplasts. This activity is intron-independent and is not enhanced when protoplasts are heat-stressed. Sequence homologies strongly suggest that UbB1 plays a major role in the stress response in sunflower.

Comparison of the effects of cryptogein and oligogalacturonides on tobacco cells and evidence of different forms of desensitization induced by these elicitors

Cryptogein, a 98 amino acid protein secreted by the fungus Phytophthora cryptogea, induces a hypersensitive response and systemic acquired resistance in tobacco plants (Nicotiana tabacum var Xanthi). The mode of action of cryptogein has been studied using tobacco cell suspensions. The recognition of this elicitor by a plasma membrane receptor leads to a cascade of events including protein phosphorylation, calcium influx, potassium and chloride effluxes, plasma membrane depolarization, activation of a NADPH oxidase responsible for active oxygen species (AOS) production and cytosol acidification, activation of the pentose phosphate pathway, and activation of two mitogen-activated protein kinase (MAPK) homologues. The organization of the cryptogein responses reveals that the earliest steps of the signal transduction pathway involve plasma membrane activities. Their activation generates a complex network of second messengers which triggers the specific physiological responses. This study may contribute to our understanding of plant signaling processes because elicitors and a variety of signals including hormones, Nod factors, light, gravity and stresses share some common transduction elements and pathways.

Soil legacy effects of climatic stress, management and plant functional composition on microbial communities influence the response of Lolium perenne to a new drought event

Abstract The effects of cryptogein and oligogalacturonides (OGs) were compared on tobacco cells by measuring calcium influx and calcium-dependent responses including extracellular alkalinization and H 2 O 2 production. The main difference is the higher calcium influx and the sustained H 2 O 2 production induced by cryptogein compared to OGs. Amplitude and duration of calcium signalling triggered by cryptogein or OGs may explain the necrotic effect of cryptogein, and the absence of necrosis in tobacco plants treated with OGs. We used induction of alkalinization and H 2 O 2 production to investigate cryptogein effects after a first treatment with cryptogein or OGs, and reciprocally. Results show that tobacco cells treated with OGs or cryptogein differed in their ability to respond to a subsequent stimulation: OGs-treated cells were refractory to the same stimulus but remained responsive to cryptogein whereas cryptogein-treated cells still responded weakly to a second application of cryptogein or OGs. Desensitization induced by OGs and cryptogein could correspond, respectively, to some forms of homologous or heterologous desensitization described in animal systems. Alternatively, cryptogein-activated pathways via large calcium influx could promote important cellular modifications leading to a reduction of response in general.

Disruption of Microtubular Cytoskeleton Induced by Cryptogein, an Elicitor of Hypersensitive Response in Tobacco Cells

Background and aimsDrought events, agricultural practices and plant communities influence microbial and soil abiotic parameters which can feedback to fodder production. This study aimed to determine which soil legacies influence plant biomass production and nutritional quality, and its resistance and recovery to extreme weather events.MethodsIn a greenhouse experiment, soil legacy effects on Lolium perenne were examined, first under optimal conditions, and subsequently during and after drought. We used subalpine grassland soils previously cultivated for two years with grass communities of distinct functional composition, and subjected to combinations of climatic stress and simulated management.ResultsThe soil legacy of climatic stress increased biomass production of Lolium perenne and its resistance and recovery to a new drought. This beneficial effect resulted from higher nutrient availability in soils previously exposed to climatic stresses due to lower competitive abilities and resistance of microbial communities to a new drought. This negative effect on microbial communities was strongest in soils from previously cut and fertilized grasslands or dominated by conservative grasses.ConclusionIn subalpine grasslands more frequent climatic stresses could benefit fodder production in the short term, but threaten ecosystem functioning and the maintenance of traditional agricultural practices in the long term.