Kinetics of NO3(-) net fluxes in Pinus pinaster, Rhizopogon roseolus and their ectomycorrhizal association, as affected by the presence of NO3(-) and NH4+

The impact of mineral N supply, N-free or NO3(-) with or without NH4+, on the subsequent uptake of NO3(-) by maritime pine seedlings associated with the ectomycorrhizal fungus Rhizopogon roseolus was studied using ion-selective microelectrodes. NO3(-) net fluxes into N-starved non-mycorrhizal short roots (NMSRs) were low and measurable only over the NO3(-) concentration range of 0-70 microM. The simple kinetics observed in those roots may reflect the dominant operation of a high-affinity NO3(-) transport system (HATS) which is constitutive. NO3(-) pretreatment increased the NO3(-) net fluxes and led to a complex kinetics that may reflect the operation of other HATS. A simple kinetics was observed in plants pre-incubated at high NH4+ concentration. In contrast, NO3(-) uptake kinetics presented only one saturation phase in the fungus, whether associated with the plant or not. NO3(-) uptake was greater after a pretreatment in N-free or NO3 (-) solution, but NH4+ pretreatment led to a threefold reduction in NO3 (-) uptake. These results suggest that the regulation of NO3(-) transport systems varies between the host and the fungal partner. This variation is likely to contribute to the positive effect of mycorrhizal association on N uptake in plants when the N supply is low and fluctuating.     

Phosphorus acquisition from phytate depends on efficient bacterial grazing, irrespective of the mycorrhizal status of Pinus pinaster

Background and aims

Phosphorus from phytate, although constituting the main proportion of organic soil P, is unavailable to plants. Despite the well-known effects of rhizosphere trophic relationships on N mineralization, no work has been done yet on P mineralization. We hypothesized that the interactions between phytate-mineralizing bacteria, mycorrhizal fungi and bacterial grazer nematodes are able to improve plant P use from phytate.

Methods

We tested this hypothesis by growing Pinus pinaster seedlings in agar containing phytate as P source. The plants, whether or not ectomycorrhizal with the basidiomycete Hebeloma cylindrosporum, were grown alone or with a phytase-producing bacteria Bacillus subtilis and two bacterial-feeder nematodes, Rhabditis sp. and Acrobeloides sp. The bacteria and the nematodes were isolated from ectomycorrhizal roots and soil from P. pinaster plantations.

Results

Only the grazing of bacteria by nematodes enhanced plant P accumulation. Although plants increased the density of phytase-producing bacteria, these bacteria alone did not improve plant P nutrition. The seedlings, whether ectomycorrhizal or not, displayed a low capacity to use P from phytate.

Conclusions

In this experiment, the bacteria locked up the phosphorus, which was delivered to plant only by bacterial grazers like nematodes. Our results open an alternative route for better utilization of poorly available organic P by plants.     

Phosphatase and phytase activities in nodules of common bean genotypes at different levels of phosphorus supply

Plants grown at limited P supply can increase the activity of phosphatases in roots to hydrolyse organic-P compounds in the soil thus improving plant P acquisition, but little information is available about the role of these enzymes for internal plant metabolism at limited-P conditions. This work intended to measure the activities of acid phosphatases and phytases in nodules of common bean (Phaseolus vulgaris) genotypes at different levels of P supply. The experiment was carried out in a 5?×?5 factorial design with four replicates, comprising five bean genotypes and five P levels (20, 40, 80, 160 and 320 μmol P plant^?1 week^?1) in nutrient solution. Root seedlings were inoculated with Rhizobium tropici and plants were grown in 1-l bottles. Nodule samples were detached from 39-day-old plants and enzyme activities were determined in crude extracts. Plants were harvested at the stage of pod setting. Polynomial models fitted to data indicated maximal values at the level of 194 μmol P for shoot mass, at 206 μmol P for nodule mass and at 221 μmol P for shoot N. Whereas shoot mass was 1.7 times lower at 20 than at 160 μmol P, nodule mass was 7.5 times lower. Concentration of P in nodules increased from 40 to 320 μmol P but remained stable between 20 and 40 μmol P, suggesting a minimal threshold concentration of 3 mg P g^?1 for nodule growth. Activities of phosphatases and phytases in nodules decreased strongly as P supply was raised from 20 to 80 μmol P, remaining almost stable at higher P levels. Phosphatase activity ranged from 1,154 to 406 nmol min^?1 g^?1 (nodule fresh mass) from 20 to 80 μmol P respectively, while the phytase activity ranged from 55 to 14 nmol min^?1 g^?1 from 20 to 80 μmol P. Bean genotypes differed in shoot and nodule mass at the levels of 80 and 160 μmol P, whilst they differed in nodule enzyme activities only at the lowest P level, the relationship between nodule enzyme activities and growth of different bean genotypes was not evident. It is concluded that bean plants at P-deficient conditions increase the activities of phosphatases and phytases in nodules. This may constitute an adaptive mechanism for N₂-fixing legumes to tolerate P deficiency, by increasing the utilisation of the scarce P within the nodules.     

Regulation of low-molecular weight organic acid production in fungi

Organic acids produced by fungi have been proposed to have many roles in wood-decay processes, lignocellulose degradation or plant pathogenesis involving saprotrophic or pathogenic fungi, as well as in nutrient acquisition and metal detoxification involving mycorrhizal or rhizosphere-inhabiting fungi. In comparison with other fungi, a considerable body of work has been devoted to the comprehension of biosynthesis pathways in fungi involved in industrial production of organic acids, and also in those involved in wood-decay processes and pathogenicity. In this review we therefore focus on information available from these different types of low-molecular weight organic acid (LMWOA) producing fungi in order to better understand the environmental cues involved in regulating production of LMWOAs.     

An Anti-β-Amyloid Vaccine for Treating Cognitive Deficits in a Mouse Model of Down Syndrome

In Down syndrome (DS) or trisomy of chromosome 21, the β-amyloid (Aβ) peptide product of the amyloid precursor protein (APP) is present in excess. Evidence points to increased APP gene dose and Aβ as playing a critical role in cognitive difficulties experienced by people with DS. Particularly, Aβ is linked to the late-life emergence of dementia as associated with neuropathological markers of Alzheimer’s disease (AD). At present, no treatment targets Aβ–related pathogenesis in people with DS. Herein we used a vaccine containing the Aβ 1–15 peptide embedded into liposomes together with the adjuvant monophosphoryl lipid A (MPLA). Ts65Dn mice, a model of DS, were immunized with the anti-Aβ vaccine at 5 months of age and were examined for cognitive measures at 8 months of age. The status of basal forebrain cholinergic neurons and brain levels of APP and its proteolytic products were measured. Immunization of Ts65Dn mice resulted in robust anti-Aβ IgG titers, demonstrating the ability of the vaccine to break self-tolerance. The vaccine-induced antibodies reacted with Aβ without detectable binding to either APP or its C-terminal fragments. Vaccination of Ts65Dn mice resulted in a modest, but non-significant reduction in brain Aβ levels relative to vehicle-treated Ts65Dn mice, resulting in similar levels of Aβ as diploid (2N) mice. Importantly, vaccinated Ts65Dn mice showed resolution of memory deficits in the novel object recognition and contextual fear conditioning tests, as well as reduction of cholinergic neuron atrophy. No treatment adverse effects were observed; vaccine did not result in inflammation, cellular infiltration, or hemorrhage. These data are the first to show that an anti-Aβ immunotherapeutic approach may act to target Aβ-related pathology in a mouse model of DS.     

Juvenile nitrogen uptake capacities and root architecture of two open-pollinated families of Picea abies. Effects of nitrogen source and ectomycorrhizal symbiosis

This study was carried out to find early physiological differences occurring in young seedlings between two contrasting Picea abies open-pollinated families (OPF), one with high- and one with low-growth performance in the field by, determining their N uptake capacities and their root architecture. We used three potential N-sources in forest soil solution, NO3-, NH4+ and amino acids, to establish N uptake rates by the plants, whether or not associated with a fungus isolated from the field and identified as Paxillus involutus. NO3- fluxes were determined locally at the root surface using NO3(-)-selective microelectrodes whereas NH4+ and amino acid (L-glutamate and L-aspartate) uptake rates were calculated from their depletion of the incubation solution by the whole root system. Root systems were digitised in order to determine the number and the length of different root types. In non-mycorrhizal plants, the results showed that the most distinguishing parameters between OPF were NO3- uptake rates measured in the white tip of the secondary roots and the root architecture, with higher values determined in high-growth than in low-growth field performance OPF. The presence of the mycorrhizal fungus decreased NO3- uptake rates in both OPF and had an opposite effect on root architecture by increasing it in low-growth and decreasing it in high-growth field performance OPF, respectively. In non-mycorrhizal plants, NH4+ and amino-acid uptake rates were not different between OPF. Mycorrhizal symbiosis did not change NH4+ uptake rates whereas it increased that of amino acids, specifically that of L-aspartate in the low-growth field performance OPF. Taken together these results suggest that the measurement of local fluxes in roots of young plants could be a good potential tool for the early evaluating of growth capacity of Picea abies OPF.     

Optimized assay and storage conditions for enzyme activity profiling of ectomycorrhizae

The aim of a joint effort by different research teams was to provide an improved procedure for enzyme activity profiling of field-sampled ectomycorrhizae, including recommendations on the best conditions and maximum duration for storage of ectomycorrhizal samples. A more simplified and efficient protocol compared to formerly published procedures was achieved by using manufactured 96-filter plates in combination with a vacuum manifold and by optimizing incubation times. Major improvements were achieved by performing the series of eight enzyme assays with a single series of root samples instead of two series, reducing the time needed for sample preparation, minimizing error-prone steps such as pipetting and morphotyping, and facilitating subsequent DNA analyses due to the reduced sequencing effort. The best preservation of samples proved to be storage in soil at 4?C6°C in the form of undisturbed soil cores containing roots. Enzyme activities were maintained for up to 4?weeks under these conditions. Short-term storage of washed roots and ectomycorrhizal tips overnight in water did not cause substantial changes in enzyme activity profiles. No optimal means for longer-term storage by freezing at ?20°C or storage in 100% ethanol were recommended.