Amino acid concentration in the rhizosphere results from fluxes between plant roots, soil and microorganisms. In this context, root amino acid exudation process, composed of both efflux and influx, remains unclear. One main issue is to understand the selectivity of amino acid exudation resulting mainly in high proportions of glycine and serine in exudates compared to low proportions inside the root. To reach this point, a quantitative analysis of exudation with dissociated measurements of efflux from influx is needed. We measured efflux and influx by supplying 15 N-labelled glycine or serine for a short time of exposure at ecologically relevant concentrations to plants of white clover (Trifolium repens L.), perennial ryegrass (Lolium perenne L.), maize (Zea mays L.), oilseed rape (Brassica napus L.), tomato (Lycopersicon esculentum Mill.) and alfalfa (Medicago sativa L.). Efflux was estimated by the increase of 14 N content of amino acids in root exudates and influx was estimated by the increase of 15 N content in plant tissue. Glycine efflux exceeded influx for all six species and was much higher in Fabaceae than in Poaceae. Serine efflux exceeded influx in alfalfa, white clover and rape. We conclude that presence of glycine and serine in root bathing solutions results from high glycine and serine efflux rates, observed in all six species studied here. The physiological and ecological significances of these high efflux rates are discussed in the context of N metabolism and plantsoil-microorganisms interactions.

The relationship between nitrate influx, BnNrt2 nitrate transporter gene expression and amino acid composition of phloem exudate was investigated during N-deprivation (short-term experiment) and over a growth cycle (longterm experiment) in Brassica napus L. The data showed a positive correlation between g g g g -aminobutyric acid (GABA) in phloem exudate and nitrate uptake in the short-and the long-term experiments. The hypothesis that this non-protein amino acid could up-regulate nitrate uptake via a longdistance signalling pathway was tested by providing an exogenous GABA supply to the roots. The effect of GABA was compared with the effects of Gln, Glu and Asn, each known to be inhibitors of nitrate uptake. The results showed that GABA treatment induced a significant increase of BnNrt2 mRNA expression, but had less effect on nitrate influx. By contrast, Gln, Glu and Asn significantly reduced nitrate influx and BnNrt2 mRNA expression compared with the control plants. This study provides the first evidence that GABA may act as a putative long-distance inter-organ signal molecule in plants in conjunction with negative control exerted by Gln. The upregulation effect of GABA on nitrate uptake is discussed in the context of its role in N metabolism, nutritional stress and the recent discovery of a putative role of GABA as a signal molecule in plant development.

SummaryA novel endosperm-specific gene named Esr (embryo _surrounding region) has been isolated by differential display between early developmental stages of maize endosperms and embryos. It is expressed in a restricted region of the endosperm, surrounding the entire embryo at early stages (4 to 7 days after pollination, DAP) and everdecreasing parts of the suspensor at subsequent stages. The expression starts at 4 DAP and is maintained until at least 28 DAP. A minimum of three Esr genes are present in the maize genome and at least two of them map to the short arm of chromosome 1 at position 56. The Esr genes contain no introns and show no significant nucleotide or amino acid sequence homologies to sequences in the databases. The open reading frames encode basic proteins of 14 kDa with presumptive signal peptides at their Ntermini followed by a hypervariable and a conserved region. The gene product may play a role in the nutrition of the developing embryo or in the establishment of a physical barrier between embryo and endosperm.

The use of kinetic equations of NO 3 Ϫ transport systems in oilseed rape (Brassica napus), determined by 15 NO 3 Ϫ labeling under controlled conditions, combined with experimental field data from the INRA-Châlons rape database were used to model NO 3 Ϫ uptake during the plant growth cycle. The quantitative effects of different factors such as day/night cycle, ontogenetic stages, root temperature, photosynthetically active radiation, and soil nitrate availability on different components of the constitutive high-affinity transport systems, constitutive low-affinity transport systems, inducible low-affinity transport systems, and inducible high-affinity transport systems of nitrate were then determined to improve the model's predictions. Simulated uptake correlated well with measured values of nitrogen (N) uptake under field conditions for all N fertilization rates tested. Model outputs showed that the high-affinity transport system accounted for about 89% of total NO 3 Ϫ uptake (18% and 71% for constitutive high-affinity transport systems and inducible high-affinity transport systems, respectively) when no fertilizer was applied. The low-affinity transport system accounted for a minor proportion of total N uptake, and its activity was restricted to the early phase of the growth cycle. However, N fertilization in spring increased the duration of its contribution to total N uptake. Overall, data show that this mechanistic and environmentally regulated approach is a powerful means to simulate total N uptake in the field with the advantage of taking both physiologically regulated processes at the overall plant level and specific nitrate transport system characteristics into account.Winter oilseed rape (Brassica napus) is an important crop in northern Europe because of its varied utilizations (oil and biofuel). However, yields remain highly variable. As a consequence, oilseed rape has been extensively studied to identify key components of yield and to improve them by more effective nitrogen (N) application with the target of reducing environmental impacts such as N leaching and improving N use efficiency for seed filling (Boelcke et al., 1991;Habekotté, 1993;Schjoerring et al., 1995;Sieling and Christen, 1997;Vos and van der Putten, 1997). Many mathematical models have been built to simulate crop growth, development, and yield (BRASNAP-PH, Habekotté, 1997a; and LINTUL BRASNAP, Habekotté, 1997b). Some of these (DAISY, Petersen et al., 1995; and CERES-Rape, Gabrielle et al., 1998) have been devoted mainly to predicting ecological impacts of N losses from winter oilseed rape. When N nutrition has been taken into account, N uptake usually has been based on the balance of demand and supply. In this context, N availability in the soil solution is modeled using mass flow and NO 3 Ϫ diffusion equations (CERES-Rape, and DAISY), and N demand is often determined using the critical dilution curve determined by Colnenne et al. (1998) for oilseed rape (CERES-rape). In these models, the root system is considered as a "black box." ...

Ethylene is a plant hormone that plays a major role in the elongation of both exploratory and root hair systems. Here, we demonstrate in Brassica napus seedlings that treatments with the ethylene precursor, aminocyclopropane carboxylic acid (ACC) and the ethylene biosynthesis inhibitor, aminoethoxyvinylglycine (AVG), cause modification of the dynamic processes of primary root and root hair elongation in a dose-dependent way. Moreover, restoration of root elongation in AVG-treated seedlings by 1 mM L-glutamate suggested that high concentrations of AVG affect root elongation through nonoverlapping ethylene metabolic pathway involving pyridoxal 5#-P-dependent enzymes of nitrate (N) metabolism. In this respect, treatments with high concentrations of ACC and AVG (10 mM) over 5 d revealed significant differences in relationships between root growth architecture and N uptake capacities. Indeed, if these treatments decreased severely the elongation of the exploratory root system (primary root and lateral roots) they had opposing effects on the root hair system. Although ACC increased the length and number of root hairs, the rate of N uptake and the transcript level of the N transporter BnNrt2.1 were markedly reduced. In contrast, the decrease in root hair length and number in AVG-treated seedlings was overcompensated by an increase of N uptake and BnNrt2.1 gene expression. These root architectural changes demonstrated that BnNrt2.1 expression levels were more correlated to the changes of the exploratory root system than the changes of the root hair system. The difference between treatments in N transporters BnNrt1.1 and BnNrt2

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SummaryTwo bursts of H 2 O 2 production have been detected by in situ 3,3 H -diaminobenzidine (DAB) staining after cutting of Lolium perenne L. leaf blades. The ®rst burst, which occurred immediately after wounding was inhibited by Na-diethydithiocarbamate (DIECA), a Cu/Zn±superoxide dismutase (SOD) inhibitor. The second burst, which was initiated several hours later, coincided with the induction of oxalate oxidase (G-OXO) activity detected in vitro or visualized in situ by the a-chloronaphtol assay. Four genes encoding G-OXO have been identi®ed from cDNA obtained from wounded L. perenne L. leaf blades. Comparison of protein sequences revealed more than 91% homology in the coding region between G-OXOs of the true cereals and G-OXOs of ryegrass, which is a Gramineae belonging to the tribe of Festucaceae. The wound-dependent increase of G-OXO activity in¯oated cut leaf blades was the result of differential induction of the four g-oxo genes. The involvement of G-OXOs in wound-induced H 2 O 2 production coincided with the presence in leaf tissues of oxalate throughout the period of increase of G-OXO synthesis. Moreover, expression of g-oxo genes was enhanced by an exogenous supply of H 2 O 2 or methyljasmonate (MeJa). Expression of the four g-oxo genes was also induced after in planta stinging of leaf blades. The pattern of their expression in planta was identical to that occuring in senescing leaf sheaths. These results emphasize the importance of G-OXOs in H 2 O 2 production in oxalate-producing plant species such as ryegrass. G-OXOs might be crucial during critical events in the life of plants such as cutting and senescence by initiating H 2 O 2 -mediated defences against pathogens and foraging animals.

Changes in the activity of oxalate oxidase (OxO) and of the concentrations of oxalate and H 2 O 2 were investigated during the ageing of leaf sheaths of ryegrass ( Lolium perenne L.) stubble. The accumulation of H 2 O 2 during ageing coincides with the increases of both oxalate level and OxO activity. Western and Northern blot analyses using protein and RNA extracts of the different categories of leaf sheaths suggested that OxO gene expression, as well as Ca-oxalate synthesis, are crucial events of ageing for leaf sheaths. Immunocytochemistry experiments have revealed that OxO, which is an extracellular enzyme, is nearly always present in the parenchymatous cells surrounding the vascular bundles and in the cells of the lower epidermis. Overall, results suggest that in ryegrass that synthesizes both Caoxalate and OxO, the production of H 2 O 2 and Ca 2+ during ageing of stubble might be involved in the constitutive defences against pathogens, thus allowing the phloem mobilization of nutrient reserves from the leaf sheaths towards elongating leaf bases of ryegrass.