Nathalie Munier-Jolain

Dynamics of Exogenous Nitrogen Partitioning and Nitrogen Remobilization from Vegetative Organs in Pea Revealed by 15N in Vivo Labeling throughout Seed Filling

The fluxes of (1) exogenous nitrogen (N) assimilation and (2) remobilization of endogenous N from vegetative plant compartments were measured by 15N labeling during the seed-filling period in pea (Pisum sativum L. cv Caméor), to better understand the mechanism of N remobilization. While the majority (86%) of exogenous N was allocated to the vegetative organs before the beginning of seed filling, this fraction decreased to 45% at the onset of seed filling, the remainder being directed to seeds. Nitrogen remobilization from vegetative parts contributed to 71% of the total N in mature seeds borne on the first two nodes (first stratum). The contribution of remobilized N to total seed N varied, with the highest proportion at the beginning of filling; it was independent of the developmental stage of each stratum of seeds, suggesting that remobilized N forms a unique pool, managed at the whole-plant level and supplied to all filling seeds whatever their position on the plant. Once seed filling starts, N is remobilized from all vegetative organs: 30% of the total N accumulated in seeds was remobilized from leaves, 20% from pod walls, 11% from roots, and 10% from stems. The rate of N remobilization was maximal when seeds of all the different strata were filling, consistent with regulation according to the N demand of seeds. At later stages of seed filling, the rate of remobilization decreases and may become controlled by the amount of residual N in vegetative tissues.

Developmental Genes Have Pleiotropic Effects on Plant Morphology and Source Capacity, Eventually Impacting on Seed Protein Content and Productivity in Pea

Increasing pea (Pisum sativum) seed nutritional value and particularly seed protein content, while maintaining yield, is an important challenge for further development of this crop. Seed protein content and yield are complex and unstable traits, integrating all the processes occurring during the plant life cycle. During filling, seeds are the main sink to which assimilates are preferentially allocated at the expense of vegetative organs. Nitrogen seed demand is satisfied partly by nitrogen acquired by the roots, but also by nitrogen remobilized from vegetative organs. In this study, we evaluated the respective roles of nitrogen source capacity and sink strength in the genetic variability of seed protein content and yield. We showed in eight genotypes of diverse origins that both the maximal rate of nitrogen accumulation in the seeds and nitrogen source capacity varied among genotypes. Then, to identify the genetic factors responsible for seed protein content and yield variation, we searched for quantitative trait loci (QTL) for seed traits and for indicators of sink strength and source nitrogen capacity. We detected 261 QTL across five environments for all traits measured. Most QTL for seed and plant traits mapped in clusters, raising the possibility of common underlying processes and candidate genes. In most environments, the genes Le and Afila, which control internode length and the switch between leaflets and tendrils, respectively, determined plant nitrogen status. Depending on the environment, these genes were linked to QTL of seed protein content and yield, suggesting that source-sink adjustments depend on growing conditions.

Quantitative effects of soil nitrate, growth potential and phenology on symbiotic nitrogen fixation of pea (Pisum sativum L.)

The influence of soil nitrate availability, crop growth rate and phenology on the activity of symbiotic nitrogen fixation (SNF) during the growth cycle of pea (Pisum sativum cv. Baccara) was investigated in the field under adequate water availability, applying various levels of fertiliser N at the time of sowing. Nitrate availability in the ploughed layer of the soil was shown to inhibit both SNF initiation and activity. Contribution of SNF to total nitrogen uptake (%Ndfa) over the growth cycle could be predicted as a linear function of mineral N content of the ploughed layer at sowing. Nitrate inhibition of SNF was absolute when mineral N at sowing was over 380 kg N ha−1. Symbiotic nitrogen fixation was not initiated unless nitrate availability in the soil dropped below 56 kg N ha−1. However, SNF could no longer be initiated after the beginning of seed filling (BSF). Other linear relationships were established between instantaneous %Ndfa and instantaneous nitrate availability in the ploughed layer of the soil until BSF. Instantaneous %Ndfa decreased linearly with soil nitrate availability and was nil above 48 and 34 kg N ha−1 for the vegetative and reproductive stages, respectively, levels after which no SNF occurred. Moreover, SNF rate was shown to be closely related to the crop growth rate until BSF. The ratio of SNF rate over crop growth rate decreased linearly with thermal time. Maximum SNF rate was about 40 mg N m−2 degree-day−1, equivalent to 7 kg N ha−1, regardless of the N treatment. From BSF to the end of the growth cycle, the high N requirements of the crop were supported by both SNF and nitrate root absorption but, of the two sources, nitrate root absorption seemed to be less affected by the presence of reproductive organs. However, since soil nitrate availability was low at the end of the growth cycle, SNF was the main source of nitrogen acquisition. The onset of SNF decrease at the end of the growth cycle seemed to be first due to nodule age and then associated to the slowing of the crop growth rate.

Effect of mineral nitrogen on nitrogen nutrition and biomass partitioning between the shoot and roots of pea (Pisum sativum L.).

The effect of mineral N availability on nitrogen nutrition and biomass partitioning between shoot and roots of pea (Pisum sativum L., cv Baccara) was investigated under adequately watered conditions in the field, using five levels of fertiliser N application at sowing (0, 50, 100, 200 and 400 kg N ha−1). Although the presence of mineral N in the soil stimulated vegetative growth, resulting in a higher biomass accumulation in shoots in the fertilised treatments, neither seed yield nor seed nitrogen concentration was affected by soil mineral N availability. Symbiotic nitrogen fixation was inhibited by mineral N in the soil but it was replaced by root mineral N absorption, which resulted in optimum nitrogen nutrition for all treatments. However, the excessive nitrogen and biomass accumulation in the shoot of the 400 kg N ha−1 treatment caused crop lodging and slightly depressed seed yield and seed nitrogen content. Thus, the presumed higher carbon costs of symbiotic nitrogen fixation, as compared to root mineral N absorption, affected neither seed yield nor the nitrogen nutrition level. However, biomass partitioning within the nodulated roots was changed. The more symbiotic nitrogen fixation was inhibited, the more root growth was enhanced. Root biomass was greater when soil mineral N availability was increased: root growth was greater and began earlier for plants that received mineral N at sowing. Rooting density was also promoted by increased mineral N availability, leading to more numerous but finer roots for the fertilised treatments. However, the maximum rooting depth and the distribution of roots with depth were unchanged. This suggested an additional direct promoting effect of mineral N on root proliferation.

Cold acclimation of winter and spring peas: carbon partitioning as affected by light intensity

Like most plants, pea (Pisum sativum L.) becomes tolerant to frost if it is first exposed to low non-freezing temperatures, a process known as cold acclimation. Cold acclimation is a complex process involving many physiological and metabolic changes. Two spring dry peas, two winter dry peas and one winter forage line were exposed to cold temperature in a controlled environment in two experiments, one using low light intensity and the other regular light intensity. Plants were harvested throughout the experiment and dry matter accumulation, water content, soluble and insoluble sugar concentrations were determined from shoot and root samples. Cold acclimation did not occur when temperatures were low if light intensity was low, even in winter peas. In contrast, with regular light intensity, the winter peas acquired more freezing tolerance than spring peas and a close relationship was found between the soluble sugar concentration of leaves just before the frost and the degree of freezing tolerance obtained by the different genotypes. Relationships between freezing tolerance and carbon partitioning between shoot and roots are discussed.

The nodulation process is tightly adjusted to plant growth. An analysis using environmentally and genetically induced variation of nodule number and biomass in pea

While many studies have focussed on nodule organogenesis at the molecular, cellular and organ scales, little information is available concerning the establishment and growth of nodules at the whole plant level. Our aim was to identify specific patterns and quantitative determinants of nodule number and biomass in peas grown under contrasting environmental conditions (variation of light intensity, nitrate supply and inoculation date). Using mutants impaired in the autoregulation of nodulation (AON) together with an integrated analysis of plant N nutrition, we revealed the respective roles of the AON and N signalling pathways in the regulation of nodulation. Nodulation was first repressed by a seed-borne AON independent signal. Then, nodulation was initiated under limiting N conditions through plant-N signalling and AON-dependent pathways. The number of nodules formed during the first nodulation wave was linearly related to plant growth, through both AON dependent and N-signalling pathways. Nodule biomass and number were shown to be co-regulated through an AON-independent mechanism. As a tight adaptation to the environment, our results highlight the finely-tuned regulation of nodule number as the main component of the whole plant regulation of N2 fixation, adapting precisely to environmental conditions and thus meeting plant N needs with minimal C costs.

Termination of seed growth in relation to nitrogen content of vegetative parts in soybean plants

Abstract Final seed weight can be analysed as the product of seed growth rate and duration of seed filling, both of which can vary with environment. In indeterminate soybean (Glycine max L. Merr.), seed filling is often considered to be limited by the translocation of N compounds from vegetative organs. Thus, an estimation of the nitrogen amount in vegetative parts could be useful to analyse termination of seed filling. The aim of this work was to estimate the nitrogen available for seeds during the seed filling period and to study the different conditions which lead to termination of seed filling. ‘Maple Arrow’ plants were sown in field experiments for 2 years. During the seed filling period, different treatments were applied to manipulate the source-sink ratio. Values from the literature were used to establish the amount of nitrogen that could not be remobilized, and thus the amount of nitrogen still available for remobilization in vegetative parts was estimated. In all cases, except for a de-podding treatment, seed filling ended when nitrogen available for remobilization was exhausted. However in the de-podding treatment, physiological maturity occurred when nitrogen was still available: seeds had reached their maximal size, a function of cell number in the seed. Consequently two mechanisms could lead to termination of seed growth, depending on source-sink ratio.

How does temperature affect C and N allocation to the seeds during the seed-filling period in pea? Effect on seed nitrogen concentration

The effect of moderate temperature on seed N concentration during the seed-filling period was evaluated in pea (Pisum sativum L.) kept in growth cabinets and the relation between plant assimilate availability and the variation of seed N concentration with temperature was investigated. Seed N concentration of pea was significantly lowered when temperature during the seed-filling period decreased from a day / night temperature of 25 / 20°C to 15 / 10°C. Our results demonstrate that during the seed-filling period mechanisms linked with assimilate availability can modify seed N accumulation rate and / or seed-filling duration between 25 / 20°C and 15 / 10°C. At the lower temperature (15 / 10°C), an increased C availability resulting from an enhanced carbon fixation per degree-day allowed new competing vegetative sinks to grow as pea is an indeterminate plant. Consequently N availability to filling seeds was reduced. Because the rate of seed N accumulation per degree-day mainly depends on N availability to filling seeds, the rate of seed N accumulation was lower at the low temperature of our study (15 / 10°C) than at 25 / 20°C while seed growth rate per degree-day remains unaffected, consequently seed N concentration was reduced. Concomitantly, the increased C availability at the lower temperature prolonged the duration of the seed-filling period.

Soil Nitrogen Availability and Plant Genotype Modify the Nutrition Strategies of M. truncatula and the Associated Rhizosphere Microbial Communities

Plant and soil types are usually considered as the two main drivers of the rhizosphere microbial communities. The aim of this work was to study the effect of both N availability and plant genotype on the plant associated rhizosphere microbial communities, in relation to the nutritional strategies of the plant-microbe interactions, for six contrasted Medicago truncatula genotypes. The plants were provided with two different nutrient solutions varying in their nitrate concentrations (0 mM and 10 mM). First, the influence of both nitrogen availability and Medicago truncatula genotype on the genetic structure of the soil bacterial and fungal communities was determined by DNA fingerprint using Automated Ribosomal Intergenic Spacer Analysis (ARISA). Secondly, the different nutritional strategies of the plant-microbe interactions were evaluated using an ecophysiological framework. We observed that nitrogen availability affected rhizosphere bacterial communities only in presence of the plant. Furthermore, we showed that the influence of nitrogen availability on rhizosphere bacterial communities was dependent on the different genotypes of Medicago truncatula. Finally, the nutritional strategies of the plant varied greatly in response to a modification of nitrogen availability. A new conceptual framework was thus developed to study plant-microbe interactions. This framework led to the identification of three contrasted structural and functional adaptive responses of plant-microbe interactions to nitrogen availability.

Combining ecophysiological and microbial ecological approaches to study the relationship between Medicago truncatula genotypes and their associated rhizosphere bacterial communities

Background and aimsTo assess how plant genotype and rhizosphere bacterial communities may interact, the genetic structure and diversity of bacterial communities in the rhizosphere soil of different Medicago truncatula genotypes were studied in relation to the plant carbon and nitrogen nutrition at the whole plant level.MethodsThe genetic structure and diversity of plant-associated rhizosphere bacterial communities was analysed by Automated Ribosomal Intergenic Spacer Analysis and 454-pyrosequencing. In parallel, the carbon and nitrogen nutrition of the plants was estimated by a phenotypic description at both structural level (growth) and functional level (using carbon and nitrogen isotope labeling and an ecophysiological framework).ResultsAn early effect of the plant genotype was observed on the rhizosphere bacterial communities, while few significant differences were detected at the plant structural phenotypic level. However, at a functional level, the different Medicago truncatula genotypes could be distinguished by their different nutritional strategies. Moreover, a comparison analysis showed that ecophysiological profiles of the different Medicago truncatula genotypes were correlated to the genetic structure and the diversity of the rhizosphere bacterial communities.ConclusionsThe exploration of the genetic structure and diversity of rhizosphere bacterial communities combined with an ecophysiological approach is an innovative way to progress in our knowledge of plant-microbe interactions in the rhizosphere.