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The aim of this study was to determine (i) whether plant nutritional strategy affects the composition of primary metabolites exuded into the rhizosphere and (ii) the impact of exuded metabolites on denitrification activity in soil. We answered this question by analysing primary metabolite content extracted from the root-adhering soil (RAS) and the roots of three grasses representing different nutrient management strategies: conservative (Festuca paniculata), intermediate (Bromus erectus) and exploitative (Dactylis glomerata). We also investigated the impact of primary metabolites on soil microbial denitrification enzyme activity without carbon addition, comparing for each plant RAS and bulk soils. Our data show that plant nutritional strategy impacts on primary metabolite composition of root extracts or RAS. Further we show, for the first time, that RAS-extracted primary metabolites are probably better indicators to explain plant nutrient strategy than root-extracted ones. In addition, our results show that some primary metabolites present in the RAS were well correlated with soil microbial denitrification activity with positive relationships found between denitrification and the presence of some organic acids and negative ones with the presence of xylose. We demonstrated that the analysis of primary metabolites extracted from the RAS is probably more pertinent to evaluate the impact of plant on soil microbial community functioning.

Constitutive markers of plant defense are required to provide solutions for plant protection by the control of its environment. While dozens of plant secondary metabolites have been identified as putative markers of defense against necrotrophic fungi, few where robustly related to disease intensity. In this work, we explored the potential of primary metabolites as predictors of disease severity. Plant soluble sugars, as main components of primary metabolism, are thought to be implicated in defense against pathogenic fungi. However, the function of sucrose and hexoses remains unclear. A different role for glucose and fructose in tomato defense against Botrytis cinerea was hypothesized. We examined tissue contents in primary metabolites at the time of inoculation with B. cinerea for tomato plants subjected to a range of abiotic environments created by various nitrogen and water supplies. Sugar contents and defense hormonal markers were then quantified during the infection process. In a first step, we assessed multiple combinations of primary metabolites as possible constitutive markers of disease intensity. In a second step, we checked for the kinetics of the selected markers during the infection process. Ratios of soluble sugars, rather than those of organic or amino-acids where identified during the first step as best correlators of disease intensity. Soluble sugar contents of tissues surrounding infection sites evolved differently after inoculation. The fructose content never decreased after inoculation with B. cinerea, while that of glucose and sucrose showed either positive or negative variation, depending on the abiotic environment. Increase in relative fructose content (defined as the proportion of fructose in the soluble sugar pool), was observed in absence of glucose accumulation, and was associated with lower disease intensity. A lower expression of the salicylic acid marker PR1a, and a lower repression of a jasmonate marker COI1 was associated with higher relative fructose contents. We concluded that fructose relative content was both a constitutive and induced marker of symptoms in a range of environments. Small variations of fructose content among the sugar pool are unlikely to affect pathogen growth by itself. Our data strongly suggest that the adjustment of relative fructose content is required for enhanced defense against B. cinerea.

International audience; Many biotic and abiotic parameters affect the metabolites involved in the organoleptic and health value of fruits. It is therefore important to understand how the growers' decisions for cultivar and orchard management can affect the fruit composition. Practices, cultivars and/or year all might participate to determine fruit composition. To hierarchize these factors, fruit weight, dry matter, soluble solids contents, titratable acidity, individual sugars and organics acids, and phenolics were measured in three apple cultivars ('Ariane', 'Melrose' and 'Smoothee') managed under organic, low-input and conventional management. Apples were harvested at commercial maturity in the orchards of the cropping system experiment BioREco at INRA Gotheron (Drôme, 26) over the course of three years (2011, 2012 and 2013). The main factors affecting primary and secondary metabolites, in both apple skin and flesh, were by far the cultivar and the yearly conditions, while the management system had a very limited effect. When considering the three cultivars and the year 2011 to investigate the effect of the management system per se, only few compounds differed significantly between the three systems and in particular the total phenolic content did not differ significantly between systems. Finally, when considering orchards grown in the same pedoclimatic conditions and of the same age, instead of the usual organic vs. conventional comparison, the effect of the management system on the apple fruit quality (Fruit weight, dry matter, soluble solids content, titratable acidity, individual sugars, organic acids, and phenolics) was very limited to non-significant. The main factors of variation were the cultivar and the year of cropping rather than the cropping system. More generally, as each management system (e.g. conventional, organic…) encompasses a great variability of practices, this highlights the importance of accurately documenting orchard practices and design beside the generic type of management in such studies.