Summary• Processes allowing the recycling of organic nitrogen and export to young leaves and seeds are important determinants of plant yield, especially when plants are nitrate-limited. Because autophagy is induced during leaf ageing and in response to nitrogen starvation, its role in nitrogen remobilization was suspected. It was recently shown that autophagy participates in the trafficking of Rubisco-containing bodies to the vacuole.• To investigate the role of autophagy in nitrogen remobilization, several autophagy-defective (atg) Arabidopsis mutants were grown under low and high nitrate supplies and labeled with 15 NO À 3 at the vegetative stage in order to determine 15 N partitioning in seeds at harvest.Because atg mutants displayed earlier and more rapid leaf senescence than wild type, we investigated whether their defects in nitrogen remobilization were related to premature leaf cell death by studying the stay-green atg5.sid2 and atg5.NahG mutants.• Results showed that nitrogen remobilization efficiency was significantly lower in all the atg mutants irrespective of biomass defects, harvest index reduction, leaf senescence phenotypes and nitrogen conditions.• We conclude that autophagy core machinery is needed for nitrogen remobilization and seed filling.

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.

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The impact of sulphur limitation on the remobilization of endogenous S compounds during the rosette stage of oilseed rape, and the interactions with N availability on these processes, were examined using a long-term 34SO42− labelling method combined with a study of leaf senescence progression (using SAG12/Cab as a molecular indicator) and gene expression of the transporters, BnSultr4;1 and BnSultr4;2, involved in vacuolar sulphate efflux. After 51 d on hydroponic culture at 0.3 mM 34SO42− (1 atom% excess), the labelling was stopped and plants were subject for 28 d to High S-High N (HS-HN, control), Low S-High N (LS-HN) or Low S-Low N (LS-LN) conditions. Compared with the control, LS-HN plants showed delayed leaf senescence and, whilst the shoot growth and the foliar soluble protein amounts were not affected, S, 34S, and SO42− amounts in the old leaves declined rapidly and were associated with the up-regulation of BnSultr4;1. In LS-LN plants, shoot growth was reduced, leaf senescence was accelerated, and the rapid S mobilization in old leaves was accompanied by decreased 34S and SO42−, higher protein mobilization, and up-regulation of BnSultr4;2, but without any change of expression of BnSultr4;1. The data suggest that to sustain the S demand for growth under S restriction (i) vacuolar SO42− is specifically remobilized in LS-HN conditions without any acceleration of leaf senescence, (ii) SO42− mobilization is related to an up-regulation of BnSultr4;1 and/or BnSultr4;2 expression, and (iii) the relationship between sulphate mobilization and up-regulation of expression of BnSultr4 genes is specifically dependent on the N availability.

In coming decades, increasing temperatures are expected to impact crop yield and seed quality. To develop low input systems, the effects of temperature and sulfur (S) nutrition in oilseed rape, a high S demanding crop, need to be jointly considered. In this study, we investigated the effects of temperatures [High Temperature (HT), 33°C/day, 19°C/night vs. Control Temperature (Ctrl T), 20°C/day, 15°C/day] and S supply [High S (HS), 500 μm SO2−4 vs. Low S (LS), 8.7 μM SO2−4] during seed filling on (i) yield components [seed number, seed dry weight (SDW) and seed yield], (ii) grain composition [nitrogen (N) and S contents] and quality [fatty acid (FA) composition and seed storage protein (SSP) accumulation] and (iii) germination characteristics (pre-harvest sprouting, germination rates and abnormal seedlings). Abscisic acid (ABA), soluble sugar contents and seed conductivity were also measured. HT and LS decreased the number of seeds per plant. SDW was less affected due to compensatory effects since the number of seeds decreased under stress conditions. While LS had negative effects on seed composition by reducing the FA contents and increasing the ratio S-poor SSPs (12S globulins)/S-rich SSPs (2S albumins) ratio, HT had positive effects by increasing S and FA contents and decreasing the C18:2/C18:3 ratio and the 12S/2S protein ratio. Seeds produced under HT showed high pre-harvest sprouting rates along with decreased ABA contents and high rates of abnormal seedlings. HT and LS restriction significantly accelerated germination times. High conductivity, which indicates poor seed storage capacity, was higher in HT seeds. Consistently, the lower ratio of (raffinose + stachyose)/sucrose in HT seeds indicated low seed storage capacity. We demonstrated the effects of HT and LS on grain and on germination characteristics. These results suggest that hormonal changes might control several seed characteristics simultaneously.

was already a mix of the other four OMS, but rather a trophic reservoir that potentially mirrored the trophic functioning of marine ecosystems.

International audienceSubalpine grasslands are highly seasonal environments and likely subject to strong variability in nitrogen (N) dynamics. Plants and microbes typically compete for N acquisition during the growing season and particularly at plant peak biomass. During snowmelt, plants could potentially benefit from a decrease in competition by microbes, leading to greater plant N uptake associated with active growth and freeze-thaw cycles restricting microbial growth. In managed subalpine grasslands, we expect these interactions to be influenced by recent changes in agricultural land use, and associated modifications in plant and microbial communities. At several subalpine grasslands in the French Alps, we added pulses of 15 N to the soil at the end of snowmelt, allowing us to compare the dynamics of inorganic N uptake in plants and microbes during this period with that previously reported at the peak biomass in July. In all grasslands, while specific shoot N translocation (per g of biomass) of dissolved inorganic nitrogen (DIN) was two to five times greater at snowmelt than at peak biomass, specific microbial DIN uptakes were similar between the two sampling dates. On an area basis, plant communities took more DIN than microbial communities at the end of snowmelt when above-ground plant biomasses were at least two times lower than at peak biomass. Consequently, inorganic N partitioning after snowmelt switches in favor of plant communities, allowing them to support their growing capacities at this period of the year. Seasonal differences in microbial and plant inorganic N-related dynamics were also affected by past (terraced vs. unterraced) rather than current (mown vs. unmown) land use. In terraced grasslands, microbial biomass N remained similar across seasons, whereas in unterraced grasslands, microbial biomass N was higher and microbial C : N lower at the end of snowmelt as compared to peak biomass. Further investigations on microbial community composition and their organic N uptake dynamics are required to better understand the decrease in microbial DIN uptake

The extent of the influence of salmon farming on the environment and on the uptake of particulate and dissolved effluents by benthic organisms was assessed using community structure and stable isotope analyses. Sediment cores were collected in 2 directions: perpendicular and parallel to the main residual current, 0, 25 and 200 m from 2 salmon farms (Millstone and Cranford) located in Mulroy Bay, Ireland. In addition, artificial substrates were placed for 2 mo at 1 m depth 0, 25 and 200 m from one farm to trace the uptake of farm-related nutrients by fouling organisms. The extent of measurable change in benthic communities (abundance, diversity, structure, trophic composition) depended on residual current direction. Intraspecific variation in isotopic values in benthic invertebrates was mostly explained by distance from cages. Organisms collected at impacted sites exhibited a shift in isotopic composition towards that of farm wastes. A shift in δ 13 C was observed in several invertebrates, including the polychaetes Malacoceros fuligi nosus and Nephtys hombergii, Nematoda and the anemone Anthopleura balii. Fouling communities collected on artificial structures, mainly composed of the tunicate Ascidiella aspersa, showed higher δ 15 N values at fish cage sites compared to sites 200 m away. The study demonstrated that fish effluents were assimilated and became food sources for several organisms.