No abstract

In a low-input agricultural context, plants facing temporal nutrient deficiencies need to be efficient. By comparing the effects of NO(3)(-)-starvation in two lines of Arabidopsis thaliana (RIL282 and 432 from the Bay-0xShahdara population), this study aimed to screen the physiological mechanisms allowing one genotype to withstand NO(3)(-)-deprivation better than another and to rate the relative importance of processes such as nitrate uptake, storage, and recycling. These two lines, chosen because of their contrasted shoot N contents for identical shoot biomass under N-replete conditions, underwent a 10 d nitrate starvation after 28 d of culture at 5 mM NO(3)(-). It was demonstrated that line 432 coped better with NO(3)(-)-starvation, producing higher shoot and root biomass and sustaining maximal growth for a longer time. However, both lines exhibited similar features under NO(3)(-)-starvation conditions. In particular, the nitrate pool underwent the same drastic and early depletion, whereas the protein pool was increased to a similar extent. Nitrate remobilization rate was identical too. It was proportional to nitrate content in both shoots and roots, but it was higher in roots. One difference emerged: line 432 had a higher nitrate content at the beginning of the starvation phase. This suggests that to overcome NO(3)(-)-starvation, line 432 did not directly rely on the N pool composition, nor on nitrate remobilization efficiency, but on higher nitrate storage capacities prior to NO(3)(-)-starvation. Moreover, the higher resistance of 432 corresponded to a higher nitrate uptake capacity and a 2-9-fold higher expression of AtNRT1.1, AtNRT2.1, and AtNRT2.4 genes, suggesting that the corresponding nitrate transporters may be preferentially involved under fluctuating N supply conditions.

International audienc

No abstract

In chicory, we examined how NO 3 A supply aected NO 3 A uptake, N partitioning between shoot and root and N accumulation in the tuberized root throughout the vegetative period. Plants were grown at two NO 3 A concentrations: 0.6 and 3 mM. We used 15 N-labelling/ chase experiments for the quanti®cation of N¯uxes between shoot and root and for determining whether N stored in the tuberized root originates from N remobilized from the shoot or from recently absorbed NO 3A . The rate of 15 NO 3 A uptake was decreased by low NO 3 A availability at all stages of growth. In young plants (10±55 days after sowing; DAS), in both NO 3 A treatments the leaves were the strongest sink for 15 N. In mature (tuberizing) plants, (55±115 DAS), the rate of 15 NO 3 A uptake increased as well as the amount of exogenous N allocated to the root. In N-limited plants, N allocation to the tuberized root relied essentially on recent N absorption, while in N-replete plants, N remobilized from the shoot contributed more to N-reserve accumulation in the root. In senescing plants (115±170 DAS) the rate of 15 NO 3 A uptake decreased mainly in N-replete plants whereas it remained almost unchanged in N-limited plants. In both NO 3 A treatments the tuberized root was the strongest sink for recently absorbed N. Remobilization of previously absorbed N from shoot to tuberized root increased greatly in N-limited plants, whereas it increased slightly in N-replete plants. As a consequence, accumulation of the N-storage compounds vegetative storage protein (VSP) and arginine was delayed until later in the vegetative period in N-limited plants. Our results show that although the dynamics of N storage was aected by NO 3 A supply, the ®nal content of total N, VSP and arginine in roots was almost the same in N-limited and N-replete plants. This indicates that chicory is able to build up a store of available N-reserves, even when plants are grown on low N. We also suggest that in tuberized roots there is a maximal capacity for N accumulation, which was reached earlier (soon after 100 DAS) in N-replete plants. This hypothesis is supported by the fact that in N-replete plants despite NO 3 A availability, N accumulation ceased and signi®cant amounts of N were lost due to N eux.Abbreviations: DAS = days after sowing; 2-D = two-dimensional; VSP = vegetative storage protein Correspondence to: A.M. Limami; Fax: 33 (1) 30 83 30 96

Nitrogen is known to modulate plant development and resistance to pathogens. Four selected lines (Alg, NS1, NR1 and NR2) of chicory (Cichorium intybus L.) were grown on low (0.6 mM) and high (3 mM) NO(-)(3) nutrition in order to study the effect of N on the expression of three traits, namely, shoot/root ratio, chicon morphology and resistance to soft rot caused by Erwinia sp. For all genotypes, increasing N supply led to a higher shoot/root ratio, resulting from an increased shoot biomass but with no effect on root growth. In contrast, the effect of N on chicon morphology and resistance to bacteria was genotype-dependent and we distinguished two groups of lines according to their phenotypic characteristics. In the group consisting of NR1 and NR2, increasing NO(-)(3) supply during the vegetative phase made the chicon morphology switch from an opened to a closed type while resistance to bacteria was not affected by N supply. In the NS1 and Alg group, the effect of N on chicon morphology was the opposite to that observed in the NR1-NR2 group while NS1 and Alg exhibited a partial resistance to Erwinia sp. , only expressing soft-rot disease when the N supply reached 3 mM. Characterization by DNA amplification fingerprinting (DAF) allowed the generation of 110 polymorphic bands and confirmed that the lines NR1 and NR2, on the one hand, and NS1 and Alg, on the other hand, belong to two distinct genetic groups. The DAF results indicate that chicon morphology and partial resistance to Erwinia sp. are complex traits which would be amenable to quantitative trait loci analysis. The split growth phase of chicory means that any changes in chicon related to N supply during vegetative growth were mediated by a root-originating signal. No variation in root carbon content among genotypes and NO(-)(3) treatments was observed. In contrast, differences in root N content revealed the same grouping of the chicory lines, NR1 and NR2 being systematically richer in amino acids and NO(-)(3) than NS1 and Alg. However, no correlation existed between N compounds and chicon morphology or pathology if all genotypes were considered together. Thus, the effect of N on plant development and pathology as well as putative identified signals might be specific for a genotype. Our study indicates that it is necessary to consider the genetic variability within a species in any signalling-pathway research.

The stolons of white clover (Trifolium repens L.) contain a predominant 17.3-kDa protein, previously characterized as a vegetative storage protein (VSP), which accumulates under autumn and winter conditions. Its full-length complementary DNA, TrVsp, was obtained and its 157 amino acid sequence deduced. This VSP has common characteristics to stress-responsive proteins (high neutral amino acid content and potential alpha helices in its secondary structure) and shows high homologies to abscisic acid-responsive and pathogenesis-related-10 proteins. The lack of any common amino acid sequence domains with known dehydrins or late embryogenesis abundant proteins suggests that clover VSP is not related to these proteins. Antibodies raised against the protein were produced and used in light and electron microscopic studies to show that it is localized to the cytosol of cortical parenchyma cells. This is in agreement with the VSP sequence, which does not contain any transit peptide signal. The accumulation of the transcript and the protein in roots is quickly induced by root chilling, suggesting a direct transcriptional regulation of TrVsp in response to low temperatures. Altogether, these results suggest that the 17.3-kDa protein may have an additional or alternative function to its role in nitrogen storage and may confer putative tolerance to chilling in white clover.

Root system architecture adapts to low nitrogen (N) nutrition. Some adaptations may be mediated by modifications of carbon (C) fluxes. The objective of this study was to test the hypothesis that changes in root system architecture under different N regimes may be accounted for by using simple hypotheses of C allocation within the root system of Arabidopsis thaliana. With that purpose, a model during vegetative growth was developed that predicted the main traits of root system architecture (total root length, lateral root number, and specific root length). Different experimental data sets crossing three C levels and two N homogenous nutrition levels were generated. Parameters were estimated from an experiment carried out under medium C and high N conditions. They were then checked under other CxN conditions. It was found that the model was able to simulate correctly C effects on root architecture in both high and low N nutrition conditions, with the same parameter values. It was concluded that C flux modifications explained the major part of root system adaptation to N supply, even if they were not sufficient to simulate some changes, such as specific root length.