Delphine Moreau

Acclimation of Leaf Nitrogen to Vertical Light Gradient at Anthesis in Wheat Is a Whole-Plant Process That Scales with the Size of the Canopy

Vertical leaf nitrogen (N) gradient within a canopy is classically considered as a key adaptation to the local light environment that would tend to maximize canopy photosynthesis. We studied the vertical leaf N gradient with respect to the light gradient for wheat (Triticum aestivum) canopies with the aims of quantifying its modulation by crop N status and genetic variability and analyzing its ecophysiological determinants. The vertical distribution of leaf N and light was analyzed at anthesis for 16 cultivars grown in the field in two consecutive seasons under two levels of N. The N extinction coefficient with respect to light (b) varied with N supply and cultivar. Interestingly, a scaling relationship was observed between b and the size of the canopy for all the cultivars in the different environmental conditions. The scaling coefficient of the b-green area index relationship differed among cultivars, suggesting that cultivars could be more or less adapted to low-productivity environments. We conclude that the acclimation of the leaf N gradient to the light gradient is a whole-plant process that depends on canopy size. This study demonstrates that modeling leaf N distribution and canopy expansion based on the assumption that leaf N distribution parallels that of the light is inappropriate. We provide a robust relationship accounting for vertical leaf N gradient with respect to vertical light gradient as a function of canopy size.

Using a physiological framework for improving the detection of quantitative trait loci related to nitrogen nutrition in Medicago truncatula

Medicago truncatula is used as a model plant for exploring the genetic and molecular determinants of nitrogen (N) nutrition in legumes. In this study, our aim was to detect quantitative trait loci (QTL) controlling plant N nutrition using a simple framework of carbon/N plant functioning stemming from crop physiology. This framework was based on efficiency variables which delineated the plant’s efficiency to take up and process carbon and N resources. A recombinant inbred line population (LR4) was grown in a glasshouse experiment under two contrasting nitrate concentrations. At low nitrate, symbiotic N2 fixation was the main N source for plant growth and a QTL with a large effect located on linkage group (LG) 8 affected all the traits. Significantly, efficiency variables were necessary both to precisely localize a second QTL on LG5 and to detect a third QTL involved in epistatic interactions on LG2. At high nitrate, nitrate assimilation was the main N source and a larger number of QTL with weaker effects were identified compared to low nitrate. Only two QTL were common to both nitrate treatments: a QTL of belowground biomass located at the bottom of LG3 and another one on LG6 related to three different variables (leaf area, specific N uptake and aboveground:belowground biomass ratio). Possible functions of several candidate genes underlying QTL of efficiency variables could be proposed. Altogether, our results provided new insights into the genetic control of N nutrition in M. truncatula. For instance, a novel result for M. truncatula was identification of two epistatic interactions in controlling plant N2 fixation. As such this study showed the value of a simple conceptual framework based on efficiency variables for studying genetic determinants of complex traits and particularly epistatic interactions.

A plant nitrophily index based on plant leaf area response to soil nitrogen availability.

This article reports a new experimental method to measure plant nitrophily. Knowledge of the nitrophily of plant species has many potential applications such as studying the long-term evolution of flora and designing nitrogen management strategies in cropping systems. Plant nitrophily is commonly measured by the Ellenberg N score based on the natural occurrence of species along soil nitrogen gradients. The Ellenberg N score is known for species from restricted geographical areas representing a small proportion of total plant species diversity. In addition, measuring Ellenberg N score is not convenient. We propose a new definition of plant nitrophily referring to plant leaf area response to nitrogen availability. We compared habitat-based and response-based values of nitrophily to design a simple method to estimate a plant nitrophily index (NI). Eleven monocotyledonous and dicotyledonous plant species were grown in a greenhouse experiment at two levels of soil nitrogen. Nine species were weeds covering the range of the Ellenberg N score. Two crop species with unknown nitrophily, wheat and oilseed rape, were studied to illustrate our method. Plant leaf area was measured at one date for each species × nitrogen treatment combination. A NI was calculated as the ratio of leaf area at high nitrogen to leaf area at low nitrogen. Our results show for weeds a high interspecific diversity of the NI, ranging from 1.0 to 4.2. The NI was strongly and positively correlated to Ellenberg N score, with R2 of 0.73. The more nitrophilic a species according to habitat preferences, the more leaf area responded to increasing nitrogen supply. This is the first time that a quantitative relationship is found between Ellenberg N score and a growth variable measured non-destructively for both monocotyledonous and dicotyledonous species. Therefore, calculating the response of leaf area to nitrogen supply provides a new simple and non-destructive method that can be used for the assessment of a NI of any species. As an illustration of the method, a NI was estimated for new species. It was at 2.1 and 4.5 for wheat and oilseed rape, respectively, indicating that they were respectively moderately and highly nitrophilic. This method will help to assess the relative nitrophily of weeds vs. crops. Such knowledge could be used to design nitrogen management strategies promoting crop growth but not weed growth, thus reducing the use of herbicides.

Analysis and modeling of the integrative response of Medicago truncatula to nitrogen constraints

An integrative biology approach was conducted in Medicago truncatula for: (i) unraveling the coordinated regulation of NO3-, NH4+ and N(2) acquisition by legumes to fulfill the plant N demand; and (ii) modeling the emerging properties occurring at the whole plant level. Upon localized addition of a high level of mineral N, the three N acquisition pathways displayed similar systemic feedback repression to adjust N acquisition capacities to the plant N status. Genes associated to these responses were in contrast rather specific to the N source. Following an N deficit, NO3- fed plants maintained efficiently their N status through rapid functional and developmental up regulations while N(2) fed plants responded by long term plasticity of nodule development. Regulatory genes associated with various symbiotic stages were further identified. An ecophysiological model simulating relations between leaf area and roots N retrieval was developed and now furnishes an analysis grid to characterize a spontaneous or induced genetic variability for plant N nutrition.

Can differences of nitrogen nutrition level among Medicago truncatula genotypes be assessed non-destructively?: Probing with a recombinant inbred lines population.

The international consensus on Medicago truncatula as a model system has lead to the development of powerful approaches for dissecting the genetic and molecular bases of legume nitrogen nutrition. However, such approaches now come up against a poor knowledge of the phenotypic traits that should be used for the large-scale screening of the genotypic variability associated with nitrogen nutrition. This issue was unravelled in a previous report, in which an ecophysiological approach allowed a better understanding of the relationships between plant nitrogen nutrition and plant growth traits, for the model symbiotic association between M. truncatula cv. Jemalong and Rhizobium meliloti strain 2001. From this analysis, phenotypic traits were identified as potentially relevant for the large-scale screening of the genotypic variability. Here, by the phenotyping of a recombinant inbred lines population, we show that the proposed methodology provides a valuable support for assisting the detection of genetic variants affected for nitrogen uptake. Especially, the relative expansion rate of plant leaf area is identified as a good proxy for ranking genotypes according to their ability to uptake nitrogen in given environmental conditions. As leaf area can be measured non-destructively, such finding should pave the way for a more efficient evaluation of the genetic variability. Addendum to: Moreau D, Voisin AS, Salon C, Munier-Jolain N. The model symbiotic association between Medicago truncatula cv. Jemalong and Rhizobium meliloti strain 2001 leads to N-stressed plants when symbiotic N2 fixation is the main N source for plant growth. J Exp Bot 2008; 59:3509-22; PMID: 18703494; DOI:10.1093/jxb/ern203.

ArchiSimple: A parsimonious model of the root system architecture

Models of the root system architecture are useful tools for studying the plant soil system, and many of these models have been published during the last decades. They capture several specific and interesting characteristics: (i) they simulate both the structure and spatial distribution of the root system; (ii) they allow a straightforward integration of developmental processes at the root level (e.g. elongation, branching) and their interaction with soil properties; (iii) they enable the simulation of root shoot communication via plant resources or signals. Though, few of them have been integrated into larger crop models, probably because they are not simple enough, too specific of given species or young stages, and many of them do not have an explicit connection to the shoot system and the soil.

Trait-based characterisation of soil exploitation strategies of banana, weeds and cover plant species

Cover plants can be introduced in cropping systems to provide agroecosystem services, including weed control via competition for resources. There is currently no consensus on how to identify the best cover plant species, while trait-based approaches are promising for screening plant species due to their agroecosystem service provision potential. This study was carried out to characterize soil exploitation strategies of cover plant species in banana agroecosystems using a trait-based approach, and in turn identify cover plant species with a high weed control potential via competition for soil resources in banana cropping systems. A field experiment was conducted on 17 cover plant species, two weed species and two banana cultivars grown individually. Four functional traits were measured. Two of them (i.e., the size of the zone explored by roots and the root impact density) were used to characterize root system soil exploration patterns. Two other traits (i.e., specific root length and root diameter) were used to characterize resource acquisition within the soil zone explored by the roots. All studied traits exhibited marked variations among species. The findings suggested a trade-off between the abilities of species to develop a limited number of large diameter roots exploring a large soil zone versus many thin roots exploring a smaller soil zone. Three soil-resource exploitation strategies were identified among species: (i) with large diameter roots that explore a large soil zone; (ii) with small diameter roots and a high specific length that explore a smaller soil zone; and (iii) with a high total root-impact density and an intermediate specific root length that explore the uppermost soil layers. Interestingly, in our panel of species, no correlations with regard to belowground and aboveground strategies were noted: species with an acquisitive belowground strategy could display an acquisitive or a conservative aboveground strategy. The findings of this study illustrated that a trait-based approach could be used to identify plant species with potential for competing with weeds, while minimising competition with banana. Six of the 17 studied cover crop species were identified as having this potential. The next step will be to assess them for their weed control performances in banana cropping systems with low reliance on herbicides.

How to hierarchize the main physiological processes responsible for phenotypic differences in large-scale screening studies?

One difficulty when analyzing the determinants at the origin of plant phenotypic differences is that measured plant traits are frequently integrative: they result from the integration of a large number of physiological processes under the control of genetic and environmental factors. In a previous report, we demonstrated that dissecting integrative traits into simpler components using a simple crop physiology model was a valuable method for detecting quantitative trait loci (QTL) related to the nitrogen nutrition for a recombinant inbred lines population of Medicago truncatula.7 Here, using the same data set, we demonstrate the relevance of decomposing integrative traits for understanding biological differences among phenotypes, independently of QTL detection. Two examples are given to demonstrate that the dissection of integrative traits (i.e., plant leaf area and nitrogen nutrition index) into variables representing the efficiency of the plant to extract and valorize (carbon and nitrogen) resources is an effective method to determine the stream of physiological events that leads to the final phenotype.