Analyse de photosynthèse sur plante entière avec le LI-COR LI6400. Construction et couplage d'une chambre

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L'analyseur de photosynthèse Li-Cor LI-6400 : applications in vitro

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Inhibition of aminotransferases by aminoethoxyvinylglycine triggers a nitrogen limitation condition and deregulation of histidine homeostasis that impact root and shoot development and nitrate uptake

International audienceBackground and Aims: Although AVG (aminoethoxyvinylglycine) is intensely used to decipher signaling in ethylene/indol-3-acetic acid (IAA) interactions on root morphogenesis, AVG is not a specific inhibitor of aminocyclopropane-1-carboxylate synthase (ACS) and tryptophan aminotransferase (TAA) and tryptophan aminotransferase related (TAR) activities since it is able to inhibit several aminotransferases involved in N metabolism. Indeed, 1 mM glutamate (Glu) supply to the roots in plants treated with 10 mu M AVG partially restores the root growth. Here, we highlight the changes induced by AVG and AVG + Glu treatments on the N metabolism impairment and root morphogenetic program. Methods: Root nitrate uptake induced by AVG and AVG + Glu treatments was measured by a differential labeling with (NO3)-N-15 (-) and (15)Nglutamate. In parallel a profiling of amino acids (AA) was performed to decipher the impairment of AA metabolism. Key Results: 10 mu M AVG treatment increases (KNO3)-N-15 uptake and N-15 translocation during root growth inhibition whereas 10 mu M AVG + 1 mM (15)Nglutamate treatment inhibits (KNO3)-N-15 uptake and increases (15)Nglutamate uptake during partial root growth restoration. This is explained by a nitrogen (N) limitation condition induced by AVG treatment and a N excess condition induced by AVG + Glu treatment. AA levels were mainly impaired by AVG treatment in roots, where levels of Ser, Thr, alpha-Ala, beta-Ala, Val, Asn and His were significantly increased. His was the only amino acid for which no restoration was observed in roots and shoots after glutamate treatment suggesting important control of His homeostasis on aminotransferase network. Results were discussed in light of recent findings on the interconnection between His homeostasis and the general amino acid control system (GAAC) in eukaryotes. Conclusions: These results demonstrate that AVG concentration above 5 mu M is a powerful pharmacological tool for unraveling the involvement of GAAC system or new N sensory system in morphological and metabolic changes of the roots in leguminous and non-leguminous plants

Combined allosteric responses explain the bifurcation in non-linear dynamics of 15 N root fluxes under nutritional steady-state conditions for nitrate

International audienceWith regard to thermodynamics out of equilibrium, seedlings are open systems that dissipate energy towards their environment. Accordingly, under nutritional steady-state conditions, changes in external concentrations of one single ion provokes instability and reorganization in the metabolic and structure/architecture of the seedling that is more favorable to the fluxes of energy and matter. This reorganization is called a bifurcation and is described in mathematics as a non-linear dynamic system. In this study, we investigate the non-linear dynamics of 15 N fluxes among cellular compartments of B. napus seedlings in response to a wide range of external 15 NO − 3 concentrations (from 0.05 to 20 mM): this allows to determine whether any stationary states and bifurcations could be found. The biphasic behavior of the root 15 NO − 3 uptake rate (v in) was explained by the combined cooperative properties between the v app (N uptake, storage and assimilation rate) and v out (N translocation rate) 15 N fluxes that revealed a unique and stable stationary state around 0.28 mM nitrate. The disappearance of this stationary state around 0.5 mM external nitrate concentrations provokes a dramatic bifurcation in 15 N flux pattern. This bifurcation in the v in and v out 15 N fluxes fits better with the increase of BnNPF6.3/NRT1.1 expression than BnNRT2.1 nitrate transporter genes, confirming the allosteric property of the BnNPF6/ NRT1.1 transporter, as reported in the literature between low and high nitrate concentrations. Moreover, several statistically significant power-law equations were found between variations in the shoots tryptophan concentrations (i.e., IAA precursor) with changes in the v app and v out 15 N fluxes as well as a synthetic parameter of plant N status estimated from the root/shoot ratio of total free amino acids concentrations. These relationships designate IAA as one of the major biological parameters related to metabolic and structural-morphological reorganization coupled with the N and water fluxes induce

Ethylene modifies architecture of root system in response to stomatal opening and water allocation changes between root and shoot

Ethylene plays a key role in the elongation of exploratory and root hair systems in plants, as demonstrated by pharmacological modulation of the activity of ethylene biosynthesis enzymes: ACC synthase (ACS) and ACC oxidase (ACO). Thus, treatments with high concentrations (10 µM) of aminoethoxyvinylglycine (AVG, inhibitor of ACS) and 1-aminocyclopropane carboxylic acid (ACC, ethylene precursor, ACO activator) severely decrease the elongation of the exploratory root system but induce opposite effects on the root hair system: root hair length and numbers were increased in seedlings treated with ACC, whereas they were reduced in seedlings treated with AVG. Until now, such elongation changes of root architecture had not been questioned in terms of nitrate uptake. In the march issue of Plant Physiology we report that N uptake and nitrate transporter BnNrt2.1 transcript level were markedly reduced in ACC treated seedlings, but were increased in AVG treated seedlings compared to the control.1 Because recent studies have revealed that ethylene can also modulate stomatal opening as well as root hair cell elongation, we have examined whether pharmacological modulation of ethylene biosynthesis could affect, in an integrated manner, and at a whole-plant level, the exploratory and root hair systems, through changes of stomatal conductance and water allocation between the root and shoot

In low transpiring conditions, nitrate and water fluxes for growth of [i]B. napus[/i] plantlets correlate with changes in BnNrt2.1 and BnNrt1.1 nitrate transporter expression

International audienceWe analyzed how changes in BnNrt nitrate transporter gene expression induced by nitrate are associated with morphological changes in plantlets and osmotic water flow for growth. We hypothesized that in a Petri dish system, reduction in transpiration should induce conditions where nitrate and water fluxes for growth depend directly on nitrate transporter activity and nitrate signaling. Rape seedlings growing on agar plates were supplied with increasing external K15NO3 concentrations from 0.05 to 20 mM. After 5 d of treatment, morphological switches in plantlet growth were observed between 0.5 and 5 mM nitrate supply. Root elongation was reduced by 50% while the cotyledon surface area was doubled. These morphological switches were strongly associated with increases in 15NO3- and water uptake rates as well as 15N and water allocation to the shoot. These switches were also highly correlated with the upregulation of BnNrt1.1 and BnNrt2.1 in the root. However, while root expression of BnNrt2.1 was correlated linearly with a shoot growth-associated increase in 15N and water uptake, BnNrt1.1 expression was correlated exponentially with both 15N and water accumulation. In low transpiring conditions, the tight control exercised by nitrate transporters on K15NO3 uptake and allocation clearly demonstrates that they modulated the nitrate-signaling cascade involved in cell growth and as a consequence, water uptake and allocation to the growing organs. Deciphering this signaling cascade in relation to acid growth theory seems to be the most important challenge for our understanding of the nitrate-signaling role in plant growth

Effects of altered source–sink relationships on N allocation and vegetative storage protein accumulation in Brassica napus L.

Growth, N allocation and involvement of vegetative storage protein (VSP) in the temporary N accumulation were evaluated infield conditions of oilseed rape grown with altered source-sink relationships (by leaf halving or removal of stem, flowers or pods). As compared to control plants, removal of half the leaves early in the vegetative period-induced delayed flowering, a lower accumulation of the taproot 23 kDa VSP and finally a decreased pod production. Stem removal, when flower buds were still covered by leaves, increased taproot biomass and its N concentration but prevented taproot VSP accumulation. Continuous removal of flowers or pods increased the shoot growth and N uptake duration (for deflowered plants). When foliar senescence finally occurred in plants continuously subjected to flower or pod removal, the N was allocated in a large proportion to buffering organs (taproot and stem) and it led to an increased and late VSP accumulation without subsequent mobilization. In control plants, N stored as VSP was remobilized during late pod filling concomitant with stem and taproot sink to source transition. A summarized scheme of N source-sink relationships during the growth cycle is presented where the 23 kDa VSP may act as an N storage buffer in response to an asynchronism between mobilization of foliar N and N requirements for seed formation. (C) 2004 Elsevier Ireland Ltd. All rights reserved

Changes in (NO3-)-N-15 availability and transpiration rate are associated with a rapid diurnal adjustment of anion contents as well as N-15 and water fluxes between the roots and shoots

International audienceBackground and Aims: Understanding interactions between water and nitrate fluxes in response to nitrate availability and transpiration rate is crucial to select more efficient plants for the use of water and nitrate. Methods: Some of these interactions were investigated in intact Brassica napus plants by combining a non-destructive gravimetric device with (NO3-)-N-15 labeling. The set-up allowed high-resolution measurement of the effects of a cross-combination of two concentrations of KNO3 or KCl (0.5 and 5 mM) with two different rates of transpiration controlled by the relative humidity during a day-night cycle. Key Results: Results show that (1) high external nitrate concentrations increased root water uptake significantly whatever the transpiration rate, (2) nitrate translocation depended both on the rate of nitrate uptake and loading into xylem (3) dilution-concentration effect of nitrate in the xylem was mainly modulated by both external nitrate availability and transpiration rate, (4) dynamic changes in N-15 translocation in the xylem modified shoot growth and capacitance, and (5) variations in tissue concentrations of NO3- induced by the experimental conditions were balanced by changes in concentrations of chloride and sulfate ions. These effects were even more amplified under low transpiration condition and 0.5 mM external nitrate concentration. Conclusion: Taken together, these results highlight the fine and rapid adjustment of anion contents, nitrate and water flows to changes in transpiration rate and nitrate availability during a day-night cycle. The use of this non-invasive gravimetric device is therefore a powerful tool to assess candidates genes involved in nitrogen and water use efficiency

Light Restriction Delays Leaf Senescence in Winter Oilseed Rape (Brassica napus L.)

International audienceOilseed rape (Brassica napus L.) is a crop with a complex aerial architecture that can cause self-shading leading to a vertical light gradient over the foliage. Mutual shading between neighboring plants at a high sowing density also results in an alteration of photosynthetically active radiation (PAR) absorption by lower leaves. The aim of this study was to analyze the impact that light restriction on lower leaves has on shoot architecture, biomass production and allocation, nitrogen (N) fluxes, and progression of sequential senescence. Field-grown plants were collected at the end of the vegetative rest period and grown in hydroponic conditions until pod maturity. A shading treatment corresponding to a 43.4 % reduction of PAR was applied at the early flowering stage. N uptake and fluxes of N allocation and remobilization were determined by supplying (KNO3)-N-15 in the nutrient solution. Photosynthesis and expression of SAG12 and Cab genes (indicators of leaf senescence progression) were also analyzed on different leaf ranks. The results showed that shading enhanced leaf development on the main stem and ramifications to optimize light capture. The expression pattern of the SAG12/Cab molecular indicator suggested a delay in leaf senescence that allowed leaf life span to be extended resulting in a more efficient leaf compound remobilization, with lower N residual contents in fallen leaves under shading. N uptake increased and N remobilization fluxes were enhanced from source organs (leaves and stem) toward sink organs (flowers). Profuse branching and late senescing varieties would be of interest for further selection programs under high sowing densities