Impact of PGPR inoculation on root morphological traits and root exudation in rapeseed and camelina: interactions with heat stress.

Root exudation is involved in the recruitment of beneficial microorganisms by trophic relationships and/or signalling pathways. Among beneficial microorganisms, Plant Growth-Promoting Rhizobacteria (PGPR) are known to improve plant growth and stress resistance. These interactions are of particular importance for species that do not interact with mycorrhizal fungi, such as rapeseed (Brassica napus L.) and camelina (Camelina sativa (L.) Crantz). However, heat stress is known to have a quantitative and qualitative impact on root exudation and could affect the interactions between plants and PGPR. We aimed to analyse the effects of PGPR inoculation on root morphology and exudation in rapeseed and camelina at the reproductive stage. The modulation of the effects of these interactions under heat stress was also investigated. The plants were inoculated twice at the reproductive stage with two different Pseudomonas species and were exposed to heat stress after the second inoculation. In non-stressing conditions, after bacterial inoculation, rapeseed and camelina exhibited two contrasting behaviours in C root allocation. While rapeseed plants seemed to suffer from the interactions with the bacteria, camelina plants appeared to control the relationship with the PGPR by modifying the composition of their root exudates. Under heat stress, the plant-PGPR interaction was unbalanced for rapeseed, for which the C allocation strategy is mainly driven by the C cost from the bacteria. Alternatively, camelina plants prioritized C allocation for their own above-ground development. This work opens up new perspectives for understanding plant-PGPR interactions, especially in an abiotic stress context.

Using plant traits to explain plant-microbe relationships involved in nitrogen acquisition.

It has long been recognized that plant species and soil microorganisms. are tightly linked, but understanding how different species vary in their effects on soil is currently limited. In this study, we identified those. plant characteristics (identity, specific functional traits, or resource acquisition strategy) that were the best predictors of nitrification and denitrification processes. Ten plant populations representing eight species collected from three European grassland sites were chosen for their contrasting plant trait values and resource acquisition strategies. For each individual plant, leaf and root traits and the associated potential microbial activities (i.e., potential denitrification rate [DEA], maximal nitrification rate [NEA], and NH4+ affinity of the microbial community [NHScom]) were measured at two fertilization levels under controlled growth conditions. Plant traits were powerful predictors of plant-microbe interactions, but relevant plant traits differed in relation to the microbial function studied. Whereas denitrification was linked to the relative growth rate of plants, nitrification was strongly correlated to root trait characteristics (specific root length, root nitrogen concentration, and plant affinity for NH4+) linked to plant N cycling. The leaf economics spectrum (LES) that commonly serves as an indicator of resource acquisition strategies was not correlated to microbial activity. These results suggest that the LES alone is not a good predictor of microbial activity, whereas root traits appeared critical in understanding plant-microbe interactions.

How to improve the hygienic quality of forages for horse feeding.

BACKGROUND: Improving the hygienic quality of forages for horse nutrition seems to be a reasonable target for decreasing the prevalence of pulmonary diseases. The aim of the experiment was to study the effects of different agricultural practices on the main aero-allergens contained in forages, including breathable dust, fungi, mycotoxins and pollens. RESULTS: Results showed that the late harvest of hay, a second crop or a haylage production provides a good alternative to increase hygienic quality by reducing fungi contamination and breathable dust content. Barn drying of hay, while having no effect on breathable dust, similarly reduced fungi contamination. In contrast, when hay was harvested at a lower dry mass content (750 g DM kg⁻¹ versus 850 g DM kg⁻¹), both breathable dust and fungi contaminations were increased, which could at least be reversed by adding propionic acid just before baling. Zearalenone was detected in different hays, and even in one case, in breathable dust. CONCLUSION: Overall, our data suggest that different approaches can be used to increase forage hygienic quality for horse feeding and thus reduce their exposure to factors involved in equine pulmonary disease.

How does sulphur availability modify N acquisition of white clover (Trifolium repens L.)?

The role of S in legume growth, N uptake, and N2 fixation was investigated using white clover (Trifolium repens L.) as a model species. We examined whether the effect of sulphate addition on N fixation resulted from a stimulation of host plant growth, a specific effect of S on nodulation, or a specific effect of S on nodule metabolism. Clones of white clover, inoculated with Rhizobium leguminosarum, were grown for 140 d in a hydroponic system with three levels of sulphate concentration (0 mM, 0.095 mM, and 0.380 mM). Nodule morphological and biochemical traits, such as root length, nodule biomass and volume, nodule protein contents (nitrogenase and leghaemoglobin obtained by an immunological approach), and root amino acid concentrations, were used to analyse the effect of sulphate availability on N2 fixation. The application of sulphate increased whole plant dry mass, root length, and nodule biomass, expressed on a root-length basis. N uptake proved less sensitive than N2 fixation to the effects of S-deficiency, and decreased as a consequence of the lower root length observed in S-deficient plants. N2 fixation was drastically reduced in S-deficient plants as a consequence of a low nodule development, but also due to low nitrogenase and leghaemoglobin production. This effect is likely to be due to down-regulation by a N-feedback mechanism, as, under severe S-deficiency, the high concentration of whole plant N and the accumulation of N-rich amino acids (such as asparagine) indicated that the assimilation of N exceeded the amount required for plant growth.

Experimental evaluation of an efflux-influx model of C exudation by individual apical root segments.

The aim of this study was to evaluate if a model describing the efflux and the influx of C through the root surface could be fitted to experimental short-term kinetics of carbon (C) exudation by individual apical root segments in maize (Zea mays L.). The efflux of C was set constant or modelled by a power function of the distance from the apex to simulate the greater release of C around the root tip commonly reported in the literature. The influx was proportional to the C concentration in the external solution to simulate the active re-uptake of exudates by the root. Plants were exposed to full light or to shade to manipulate C allocation to roots. The model with a constant efflux gave satisfactory fits to the kinetics of exudation (average R(2)=0.66). The average gross efflux was then 2.1 mug C cm(-2) root surface h(-1). The model was improved if exudation was set more intense towards the root apex (average R(2)=0.74). The estimated gross efflux decreased then from 5.2 mug C cm(-2) h(-1) at the apex to 1.8 mug C cm(-2) h(-1) for the region located 5-25 cm from the root tip. The decrease in net exudation of individual roots due to the shading of plants was weak, which may indicate that the import of C by the primary roots studied was not reduced significantly. By describing the exudation of an apical root segment of variable length and diameter, the model is a first step in linking exudation to root system architecture models and to whole plant functioning.