Nitrogen Remobilization Efficiency Research Articles

Chronic ozone exposure affects nitrogen remobilization in wheat at key growth stages

The interaction between nitrogen storage and translocation, senescence, and late phase photosynthesis is critical to the post-anthesis grain fill period in wheat, but ozone's effect on nitrogen dynamics within the wheat plant is not well understood. This study used solardomes to expose a widely grown elite spring wheat cultivar, cv. Skyfall, to four levels of ozone (30 ppb, 45 ppb, 70 ppb, 85 ppb) for 11 weeks, with two levels of nitrogen fertilization, 140 kg ha−1 and 160 kg ha−1, the higher rate including an additional 20 kg N ha−1 at anthesis. Chronic ozone exposure triggered earlier senescence in the 4th, 3rd and 2nd leaves but not the flag leaf, with a similar pattern of reduced chlorophyll content in the lower, older leaf cohorts, which started before senescence became visible. At anthesis there was no evidence of any effect of ozone on nitrogen storage in upper plant parts. However, high ozone increased levels of residual nitrogen found within plant parts at harvest, with concomitant reductions in C:N ratios and Nitrogen Remobilization Efficiency. Extra nitrogen fertilization applied at anthesis appeared to ameliorate the effect of ozone on nitrogen content and nitrogen translocation. The application of 15N ammonium nitrate at anthesis confirmed that the majority of post-anthesis nitrogen uptake had been translocated to the ear/grain by harvest, with no effect of ozone on the translocation of nitrogen around the plant. These data can inform future modelling of ozone's effect on nitrogen dynamics and global wheat yields.

Genetic portrait of polyamine transporters in barley: insights in the regulation of leaf senescence.

Nitrogen (N) is one of the most expensive nutrients to supply, therefore, improving the efficiency of N use is essential to reduce the cost of commercial fertilization in plant production. Since cells cannot store reduced N as NH3 or NH4 +, polyamines (PAs), the low molecular weight aliphatic nitrogenous bases, are important N storage compounds in plants. Manipulating polyamines may provide a method to increase nitrogen remobilization efficiency. Homeostasis of PAs is maintained by intricate multiple feedback mechanisms at the level of biosynthesis, catabolism, efflux, and uptake. The molecular characterization of the PA uptake transporter (PUT) in most crop plants remains largely unknown, and knowledge of polyamine exporters in plants is lacking. Bi-directional amino acid transporters (BATs) have been recently suggested as possible PAs exporters for Arabidopsis and rice, however, detailed characterization of these genes in crops is missing. This report describes the first systematic study to comprehensively analyze PA transporters in barley (Hordeum vulgare, Hv), specifically the PUT and BAT gene families. Here, seven PUTs (HvPUT1-7) and six BATs (HvBAT1-6) genes were identified as PA transporters in the barley genome and the detailed characterization of these HvPUT and HvBAT genes and proteins is provided. Homology modeling of all studied PA transporters provided 3D structures prediction of the proteins of interest with high accuracy. Moreover, molecular docking studies provided insights into the PA-binding pockets of HvPUTs and HvBATs facilitating improved understanding of the mechanisms and interactions involved in HvPUT/HvBAT-mediated transport of PAs. We also examined the physiochemical characteristics of PA transporters and discuss the function of PA transporters in barley development, and how they help barley respond to stress, with a particular emphasis on leaf senescence. Insights gained here could lead to improved barley production via modulation of polyamine homeostasis.

The Genetic Architecture of Nitrogen Use Efficiency in Switchgrass (Panicum virgatum L.).

Switchgrass (Panicum virgatum L.) has immense potential as a bioenergy crop with the aim of producing biofuel as an end goal. Nitrogen (N)-related sustainability traits, such as nitrogen use efficiency (NUE) and nitrogen remobilization efficiency (NRE), are important factors affecting switchgrass quality and productivity. Hence, it is imperative to develop nitrogen use-efficient switchgrass accessions by exploring the genetic basis of NUE in switchgrass. For that, we used 331 diverse field-grown switchgrass accessions planted under low and moderate N fertility treatments. We performed a genome wide association study (GWAS) in a holistic manner where we not only considered NUE as a single trait but also used its related phenotypic traits, such as total dry biomass at low N and moderate N, and nitrogen use index, such as NRE. We have evaluated the phenotypic characterization of the NUE and the related traits, highlighted their relationship using correlation analysis, and identified the top ten nitrogen use-efficient switchgrass accessions. Our GWAS analysis identified 19 unique single nucleotide polymorphisms (SNPs) and 32 candidate genes. Two promising GWAS candidate genes, caffeoyl-CoA O-methyltransferase (CCoAOMT) and alfin-like 6 (AL6), were further supported by linkage disequilibrium (LD) analysis. Finally, we discussed the potential role of nitrogen in modulating the expression of these two genes. Our findings have opened avenues for the development of improved nitrogen use-efficient switchgrass lines.

Combined physiological, transcriptome, and genetic analysis reveals a molecular network of nitrogen remobilization in maize.

In plants, nitrogen remobilization from source to sink organs is an important process regulated by complex transcriptional regulatory networks. However, the relationship between nitrogen remobilization and leaf senescence and the molecular regulatory network that controls them are unknown in maize. Here, using 15N labeling and a transcriptome approach, a dynamic analysis of the nitrogen remobilization process was conducted in two elite maize inbred lines (PH4CV and PH6WC) with contrasting leaf senescence. PH4CV showed higher nitrogen remobilization efficiency (NRE) than PH6WC, mainly in the middle and lower leaves from 15 d to 35 d after silking. The co-expression network analysis revealed that ethylene and cytokinin metabolism-related genes triggered the onset of nitrogen remobilization, while abscisic acid and jasmonic acid biosynthesis-related genes controlled the progression of nitrogen remobilization. By integrating genetic analysis, functional annotation, and gene expression, two candidate genes underlying a major quantitative trait locus of NRE were identified, namely an early senescence acting gene (ZmASR6) and an ATP-dependent Clp protease gene (GRMZM2G172230). Hormone-coupled transcription factors and downstream target genes reveal a gene regulatory network for the nitrogen remobilization process after silking in maize. These results uncovered a sophisticated regulatory mechanism for nitrogen remobilization, and further provided characterization of valuable genes for genetic improvement of nitrogen use efficiency in maize.

Nitrogen dynamic in wheat (Triticum aestivum L.) agroecosystem as influenced by intra- and interspecific competition

Abstract Background: The study of both intra- and interspecific competitions in Wheat (Triticum aestivum L.) agroecosystem is quite complex and it is essential to understand the influence of nitrogen in these competitions. Objective: The aim of this study was to evaluate the effect of different competitive models on nitrogen balance in wheat ecosystem agriculture (Chamran cultivar). Methods: A field experiment was performed using split-plot based on randomized complete block design (RCBD) with three replications. The main and sub-plots consisted of different nitrogen rates (control, 50, 100 and 150 kg nitrogen ha-1) and different competitive patterns (no, intraspecific, interspecific competition and intra- and interspecific in combination) were applied to the sub-plots, respectively. Weed density in the interspecies competition was applied through the equal planting of two narrow leaves (Avena sativa L.) and broadleaf (Sinapis arvensis L.) weed species. Results: The dry matter (DM) accumulation and nitrogen content (NC) at anthesis was higher than that at maturity stage for all competition treatments as well as nitrogen rates. The trend of nitrogen translocation efficiency (NTE) and nitrogen harvest index (NHI) was different from dry matter translocation efficiency (DMTE), and both were highest in interspecific competition treatments. Moreover, the highest grain yield in no competition treatment resulted in a lower dry matter and NTE. Conclusions: It was concluded that the capacity for nitrogen accumulation in the stem, was associated with a high nitrogen uptake, nitrogen assimilation and high post-anthesis nitrogen remobilization efficiency. In additions, high NHI could be used in the development of cultivars with the desired N balance.

Correlation between nitrogen fruiting efficiency and nitrogen utilization and remobilization in winter wheat at different sowing dates

To understand the correlation between nitrogen fruiting efficiency and nitrogen utilization and remobilization in winter wheat, the differences and mutual relationships of nitrogen fruiting efficiency, nitrogen utilization and remobilization of winter wheat at different sowing dates (S1:24 September, S2:1 October, S3:8 October, S4:15 October and S5:22 October) were analyzed in two growing seasons from 2014 to 2016. The results showed that there was no significant difference in grain yield and grain number per unit area among different sowing dates. Delayed sowing date decreased nitrogen accumulation in both shoots and spikes, and then reduced nitrogen uptake efficiency and increased nitrogen utilization efficiency and nitrogen fruiting efficiency. Nitrogen fruiting efficiency was positively correlated with nitrogen utilization efficiency, negatively correlated with nitrogen uptake efficiency, but not significantly correlated with nitrogen use efficiency. Nitrogen nutrition index tended to be optimum with delayed sowing dates, showed synchronicity with the improvement of nitrogen fruiting efficiency. Pre-anthesis nitrogen remobilization amount in vegetative organs and post-anthesis nitrogen accumulation amount significantly decreased with postpone of sowing dates, but pre-anthesis nitrogen remobilization efficiency remarkably rise. There was a positive correlation between nitrogen fruiting efficiency and nitrogen remobilization efficiency, indicating that the improvement in nitrogen remobilization efficiency facilitated the increment in nitrogen frui-ting efficiency. Taken together, properly delayed sowing date reduced nitrogen uptake, but increased nitrogen utilization efficiency and nitrogen fruiting efficiency and improved nitrogen supply status, which provided a theoretical basis for the implementation of reducing input and improving nitrogen utilization efficiency in wheat production in this region.

A Genotypic Comparison Reveals That the Improvement in Nitrogen Remobilization Efficiency in Oilseed Rape Leaves Is Related to Specific Patterns of Senescence-Associated Protease Activities and Phytohormones.

Oilseed rape (Brassica napus L.) is an oleoproteaginous crop characterized by low N use efficiency (NUE) that is mainly related to a weak Nitrogen Remobilization Efficiency (NRE) during the sequential leaf senescence of the vegetative stages. Based on the hypothesis that proteolysis efficiency is crucial for the improvement of leafNRE, our objective was to characterize key senescence-associated proteolytic mechanisms of two genotypes (Ténor and Samouraï) previously identified with contrasting NREs. To reach this goal, biochemical changes, protease activities and phytohormone patterns were studied in mature leaves undergoing senescence in two genotypes with contrasting NRE cultivated in a greenhouse under limiting or ample nitrate supply. The genotype with the higher NRE (Ténor) possessed enhanced senescence processes in response to nitrate limitation, and this led to greater degradation of soluble proteins compared to the other genotype (Samouraï). This efficient proteolysis is associated with (i) an increase in serine and cysteine protease (CP) activities and (ii) the appearance of new CP activities (RD21-like, SAG12-like, RD19-like, cathepsin-B, XBCP3-like and aleurain-like proteases) during senescence induced by N limitation. Compared to Samouraï, Ténor has a higher hormonal ratio ([salicylic acid] + [abscisic acid])/([cytokinins]) that promotes senescence, particularly under low N conditions, and this is correlated with the stronger protein degradation and serine/CP activities observed during senescence. Short statement: The improvement in N recycling during leaf senescence in a genotype of Brassica napus L. characterized by a high nitrogen remobilization efficiency is related to a high phytohormonal ratio ([salicylic acid] + [abscisic acid])/([cytokinins]) that promotes leaf senescence and is correlated with an increase or the induction of specific serine and cysteine protease activities.

Physiological mechanisms underlying post‐silking nitrogen use efficiency of high‐yielding maize hybrids differing in nitrogen remobilization efficiency

AbstractIt is a great challenge to increase nitrogen (N) remobilization from leaves while ensuring high grain yield of maize (Zea mays L.). One possible way is to increase photosynthetic N use efficiency (PNUE) of leaves during post‐silking. This study aimed to understand the strategies required to maintain high PNUE during post‐silking in a N‐remobilizing high‐yielding maize hybrid. In this study, the high‐yielding hybrid XY335 with high N remobilization efficiency (NRE) and the high‐yielding hybrid ZD958 with low NRE were used in a three‐year field experiment under normal N levels. The results show that specific leaf N contents were similar in lower, middle, and upper leaves of the two hybrids from silking stage to maturity. Throughout the grain filling period, net photosynthetic rate (PN), stomatal conductance (gs), intercellular CO2 concentration, and PNUE of XY335 were similar to that in ZD958 in lower leaves, but higher in middle and upper leaves. Stomatal density and maximum rate of carboxylation in upper leaves and maximum rate of electron transport in middle leaves was higher in XY335 than that in ZD958. It is concluded that maintaining the PNUE of lower leaves together with increasing the PNUE of middle and upper leaves at canopy‐level are the possible regulatory mechanisms to improve NRE without penalty in whole‐plant photosynthesis during post‐silking in modern high‐yielding cultivars. The higher PNUE is at least partly related to the higher proportion of N allocation to the carboxylation and bioenergetics systems and the higher stomatal density and greater gs. Therefore, genetic improvement in N allocation to the carboxylation and bioenergetics systems and stomatal density and gs might increase PNUE.

Nitrogen supply and sink demand modulate the patterns of leaf senescence in maize

Senescence is a key physiological process that can regulate crop grain yield. Patterns of leaf senescence and its association with grain yield for a short maturity maize hybrid were investigated in a factorial combination of two tillage treatments (conventional and no-till), three amounts of stubble (0, 3 and 5 t ha−1) and three N rates (0, 80 and 120 kg N ha−1) over three seasons in 2015 long rains, 2015/2016 short rains and 2016 long rains. Leaf senescence from flowering to harvest was assessed at (a) the whole-plant scale by the visual scoring of dry leaves and (b) the canopy-layer scale by measuring leaf greenness with a SPAD 502 chlorophyll meter. A bilinear model was used to quantify the patterns of senescence at the whole-plant scale. A logistic function was fitted to estimate the traits of senescence at three canopy layers (top, mid, bottom), including minimum and maximum SPAD, onset of senescence (EC90), time to loss of 50% maximum SPAD (EC50) and the rate of senescence in each layer. Nitrogen rate effect on patterns and traits of senescence were large and its interactions with stubble were more frequent than interactions between other treatments. Tillage and stubble amount had marginal effects. EC50 was delayed in the unfertilized controls compared with fertilized crops and was negatively correlated with grain yield. Rate of senescence was faster in fertilised crops compared with unfertilized controls at both whole-plant and canopy-layer scales. Grain yield, grain number and nitrogen remobilization efficiency were associated with faster rates of senescence in the top and mid leaves but with slower rates of senescence in the bottom layer leaves. We advance a sink-driven leaf senescence ideotype for high yield and efficient use of nitrogen for short maturity maize.

Proteomic Investigations of Proteases Involved in Cotyledon Senescence: A Model to Explore the Genotypic Variability of Proteolysis Machinery Associated with Nitrogen Remobilization Efficiency during the Leaf Senescence of Oilseed Rape.

Oilseed rape is characterized by a low nitrogen remobilization efficiency during leaf senescence, mainly due to a lack of proteolysis. Because cotyledons are subjected to senescence, it was hypothesized that contrasting protease activities between genotypes may be distinguishable early in the senescence of cotyledons. To verify this assumption, our goals were to (i) characterize protease activities in cotyledons between two genotypes with contrasting nitrogen remobilization efficiency (Ténor and Samouraï) under limiting or ample nitrate supply; and (ii) test the role of salicylic acid (SA) and abscisic acid (ABA) in proteolysis regulation. Protease activities were measured and identified by a proteomics approach combining activity-based protein profiling with LC-MS/MS. As in senescing leaves, chlorophyll and protein contents decrease in senescing cotyledons and are correlated with an increase in serine and cysteine protease activities. Two RD21-like and SAG-12 proteases previously associated with an efficient proteolysis in senescing leaves of Ténor are also detected in senescing cotyledons. The infiltration of ABA and SA provokes the induction of senescence and several cysteine and serine protease activities. The study of protease activities during the senescence of cotyledons seems to be a promising experimental model to investigate the regulation and genotypic variability of proteolysis associated with efficient N remobilization.

Effects of pollination-prevention on leaf senescence and post-silking nitrogen accumulation and remobilization in maize hybrids released in the past four decades in China

In order to understand the regulation of source-sink relationship on leaf senescence in maize, we investigated the effect of pollination prevention on leaf senescence, post-silking dry matter and nitrogen accumulation and remobilization in different vegetative organs in one landrace and ten representative maize hybrids released between 1973 and 2000 in China. When pollination was prevented by covering silk with paper bags at silking stage, leaf senescence of Nongda 60, Yedan 13, Shendan 7 and Zhengdan 958 was delayed as shown by the remaining green leaf area per plant at maturity. However, pollination-prevention did not have obvious effect on the leaf senescence of Baimaya, Danyu 6, Yedan 2 and Xianyu 335. By contrast, pollination-prevention accelerated the leaf senescence of maize hybrids Zhongdan 2, Danyu13 and Yedan 4. It was found that leaf senescence of the late-released (since 1985) Chinese hybrids tended to be delayed by pollination-prevention, except XY335. From silking to physiological maturity, nitrogen content increased in the stem (plus the sheath, cob, husk, and tassel) and root of the non-pollinated plants. However, there was still a net reduction in leaf nitrogen content. We also found that pollination-prevention reduced leaf nitrogen remobilization efficiency, with genotypic difference and variation between the two years of testing. The results suggested that the response of leaf senescence to pollination-prevention is at least partially due to the change of leaf nitrogen remobilization efficiency. Leaf senescence tended to be delayed if leaf nitrogen remobilization efficiency is highly reduced by pollination-prevention.

Nitrogen remobilization during leaf senescence: lessons from Arabidopsis to crops.

As a result of climate changes, land use and agriculture have to adapt to new demands. Agriculture is responsible for a large part of the greenhouse gas (GHG) emissions that have to be urgently reduced in order to protect the environment. At the same time, agriculture has to cope with the challenges of sustainably feeding a growing world population. Reducing the use of the ammonia-nitrate fertilizers that are responsible for a large part of the GHGs released and that have a negative impact on carbon balance is one of the objectives of precision agriculture. One way to reduce N fertilizers without dramatically affecting grain yields is to improve the nitrogen recycling and remobilization performances of plants. Mechanisms involved in nitrogen recycling, such as autophagy, are essential for nutrient remobilization at the whole-plant level and for seed quality. Studies on leaf senescence and nutrient recycling provide new perspectives for improvement. The aim of this review is to give an overview of the mechanisms involved in nitrogen recycling and remobilization during leaf senescence and to present the different approaches undertaken to improve nitrogen remobilization efficiency using both model plants and crop species.

Nitrogen use efficiency in modern wheat cultivars

ABSTRACT The nitrogen use efficiency (NUE) is defined as the capacity of a given genotype in take advantage of the applied nitrogen (N) and transform it in biomass and grains. The objective of this study was to evaluate 12 wheat cultivars as to the NUE and its components. The experiment was conducted in a controlled environment, in a randomized block design with three replications. Twelve wheat cultivars were submitted to four N supply levels (0, 80, 160 and 240 kg of N∙ha–1). The data were submitted to analysis of variance, means multiple comparison, polynomial regression, and path analysis. The nitrogen remobilization efficiency (NRE) was the main NUE component of the evaluated cultivars, in both low and high conditions of nitrogen fertilization. In the cultivars average, the nitrogen utilization efficiency (NUtE) presented reduction tendency as the N supply was increased, tending to stabilization at the dose of 231 kg of N∙ha–1. The wheat cultivars Mirante, TBIO Itaipu, BRS Parrudo, and TBIO Iguaçu were the most efficient on the N use, and the first two were also efficient in remobilizing the N from the phytomass to the grains.

English

Nitrogen use efficiency (NUE) is an important characteristic for increasing the yield and quality of rice grains and reducing the use of nitrogen fertilizers. This study evaluated parameters that contribute to NUE in two tropical japonica rice varieties contrasting in nitrogen remobilization efficiency (NRE) and nitrate-uptake kinetics: IAC-47 (bred for high-input farming), and Piaui (a landrace from the state of Maranhao, Brazil). The two varieties were grown with and without N supplementation at anthesis stage. Both varieties received urea equivalent to 60 kg N ha -1 in the first ten days after planting (DAP) and a subset of the two varieties received a supplementation with 40 kg N ha -1 at anthesis stage, composing a treatment with supplementation (Ts). The control treatment (Tc) was constituted of the plants that did not receive nitrogen supplementation at anthesis. We analyzed the nitrogen soluble fractions, soluble sugars, dry matter production, total N, crude protein in grains, NUE parameters, and expression and activity of glutamine synthetase (GS). The N uptake after anthesis affected the total N content in the shoots and grains of both rice varieties. In treatments with and without N supplementation, the nitrogen uptake efficiency (NUpE) was higher in IAC-47 than in Piaui plants. The NRE, a desirable characteristic for plants grown with low N input, was around two times higher in the Piaui plants that were not supplemented with N during anthesis. panicles of piaui plants grouwn without N supplementation presented higher expression of the glutamine synthetase gene OsGS1.3 post-anthesis. The results indicate that NRE is the primary factor contributing to NUE in rice varieties adapted to a low N supply, whereas improved varieties exhibit a higher NUpE.

Evidence of Salicylic Acid-Mediated Protein Degradation and Amino Acid Transport in Mature Leaves of Brassica napus

Brassica napus is characterized by low nitrogen remobilization efficiency from source leaves to seed filling, which is mainly due to poor hydrolysis of foliar proteins. To investigate the roles of salicylic acid (SA) in the proteolytic process in source organ at the early pod-filling stage, the SA concentration, protein profile, protease activity, and amino acid transporter genes were measured in mature leaves sprayed with SA or control at four and 30 days after treatment. Exogenous SA application resulted in an increase in SA concentration, accompanied with a significant decrease in protein with concomitant increase in amino acid concentration. Rubisco proteins were largely degraded in the SA-applied leaves. A significant increase in protease activity (up to 98 % at day 30) in the SA-applied leaves was observed. The increased proteolytic activity was confirmed in-gel staining of protease. Three amino acid permeases, BnAAP1, BnAAP2, and BnAAP6, were identified in mature leaves. Exogenous SA application enhanced the expression of three amino acid transporter genes. The present study clearly indicates that the SA level increased by exogenous SA application could improve N remobilization efficiency by up-regulating protein degradation and amino acid transport in source leaves at the pod-filling stage.

Effects of nitrogen application rate on grain yield and grain nitrogen concentration in two maize hybrids with contrasting nitrogen remobilization efficiency

A target in crop production is to simultaneously increase grain yield (GY) and grain nitrogen concentration (GNC). In maize, nitrogen (N) and genotype are two major factors affecting GY and GNC. Both N remobilization from vegetative tissues and post-silking N uptake contribute to grain N, but their relative contributions are genotype specific, and are affected by the N application rate. It is unclear whether the responses of GY and GNC to N application differ between genotypes with different post-silking N uptake and vegetative N remobilization characteristics. We investigated the effect of N application rate on post-silking N uptake, vegetative N remobilization, GY, and GNC of two high-yielding maize hybrids, ZD958 and XY335, which have contrasting N remobilization characteristics. We tested five N application rates (0, 60, 120, 180, 240kgNha−1) in a 4-year field study (from 2010 to 2013). There was a significant year×N×genotype interaction in the amount of vegetative N remobilization and N remobilization efficiency (NRE), and residual stalk N concentration at maturity. Compared with the low-NRE cultivar ZD958, XY335 showed the same GY but higher GNC because it had higher vegetative N remobilization, NRE but lower residual stalk N concentration under the favorable weather condition in 2010. The response of GNC to increasing N levels was the same between XY335 and ZD958 and was not affected by year conditions. The N level required to obtain the highest GY was the same in the two hybrids (156±13kgha−1and 159±19kgha−1), but that required to obtain the highest GNC was greater in XY335 (216±30kgha−1) than in ZD958 (195±23kgha−1). From these results, we conclude that precise N fertilizer management as well as the selection of high-yielding hybrids with high NRE can increase GNC without negatively affecting GY or leading to surplus N storage in vegetative organs.

Leaf senescence and nitrogen remobilization efficiency in oilseed rape (Brassica napus L.).

Despite its worldwide economic importance for food (oil, meal) and non-food (green energy and chemistry) uses, oilseed rape has a low nitrogen (N) use efficiency (NUE), mainly due to the low N remobilization efficiency (NRE) observed during the vegetative phase when sequential leaf senescence occurs. Assuming that improvement of NRE is the main lever for NUE optimization, unravelling the cellular mechanisms responsible for the recycling of proteins (the main N source in leaf) during sequential senescence is a prerequisite for identifying the physiological and molecular determinants that are associated with high NRE. The development of a relevant molecular indicator (SAG12/Cab) of leaf senescence progression in combination with a (15)N-labelling method were used to decipher the N remobilization associated with sequential senescence and to determine modulation of this process by abiotic factors especially N deficiency. Interestingly, in young leaves, N starvation delayed senescence and induced BnD22, a water-soluble chlorophyll-binding protein that acts against oxidative alterations of chlorophylls and exhibits a protease inhibitor activity. Through its dual function, BnD22 may help to sustain sink growth of stressed plants and contribute to a better utilization of N recycled from senescent leaves, a physiological trait that could improve NUE. Proteomics approaches have revealed that proteolysis involves chloroplastic FtsH protease in the early stages of senescence, aspartic protease during the course of leaf senescence, and the proteasome β1 subunit, mitochondria processing protease and SAG12 (cysteine protease) during the later senescence phases. Overall, the results constitute interesting pathways for screening genotypes with high NRE and NUE.

Autophagy machinery controls nitrogen remobilization at the whole‐plant level under both limiting and ample nitrate conditions in Arabidopsis

• 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 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.

Environmental effects on associations among nitrogen use efficiency traits in wheat

Nitrogen use efficiency parameters of twelve bread wheat cultivars were studied in nine environments comprised of three nitrogen applications tested over three years. An environment-by-trait association biplot and a trait-association by environment biplot were applied to understand environmental effects on these traits and relationships among them. The following trait associations were relatively stable across environments: negative associations of straw nitrogen content vs. nitrogen remobilization efficiency, straw nitrogen content vs. nitrogen harvest index and a positive association of nitrogen remobilization efficiency vs. nitrogen harvest index. The selection of genotypes with a high nitrogen harvest index and nitrogen remobilization efficiency could be used in the development of cultivars with desired nitrogen use efficiency. A low straw nitrogen content can also be used as a reliable and cost-effective indicator in the selection of genotypes with high nitrogen use efficiency.

Decorrelating source and sink determinism of nitrogen remobilization during grain filling in wheat

Nitrogen (N) remobilization is the major source of N for grain filling in wheat, the other being N uptake after anthesis (N(up)); however, variations in remobilization efficiency are not fully understood. It is hard to tell whether the source or the sink effects predominate, because N in the culm at anthesis (N(ant)) correlates strongly with both N remobilization (N(rem)) and grain number (G(n)), respectively the main source and the main sink. A pot experiment was thus designed to assess the relative contributions of the source and sink to N(rem) regulation. Using two cultivars of winter wheat (Triticum aestivum, 'Apache' and 'Autan'), three pre-anthesis and two post-anthesis N fertilization levels were applied in order to vary the N sources, while ear trimming at anthesis reduced sink size. Unlike results observed at a scale of m(2), the equation binding N(ant) to N(rem) exhibited a negative intercept, challenging the concept of nitrogen remobilization efficiency. Before ear trimming, G(n) fitted well to N(ant), with a slope dependent on genotype. To obtain a sink variable that was less correlated with N(ant), the difference deltaG(n) was calculated between actual grain number and that which could be predicted from culm N before trimming. A multiple regression then predicted N(rem) (r(2) = 0.95) from N(ant), N(up) and deltaG(n), with fitting unbiased by fertilization treatment, trimming or genotype. In untrimmed culms, deltaG(n) had a negligible effect, so that N(rem) could be fitted to N(ant) and N(up) only: grain N filling appeared to be determined by sources only (N(ant) and N(up)), not by sink, and the reduction of N(rem) by N(up) was quantified. In these 'normal' cases, the regulation of N(rem) should thus be located within the N sources themselves. In contrast, ear-trimming needs to be considered with caution as it introduced a sink limitation on N(rem); moreover one with an important genotype effect.