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.

BackgroundPlant growth-promoting rhizobacteria are increasingly being seen as a way of complementing conventional inputs in agricultural systems. The effects on their host plants are diverse and include volatile-mediated growth enhancement. This study sought to assess the effects of bacterial volatiles on the biomass production and root system architecture of the model grass Brachypodium distachyon (L.) Beauv.ResultsAn in vitro experiment allowing plant-bacteria interaction throughout the gaseous phase without any physical contact was used to screen 19 bacterial strains for their growth-promotion ability over a 10-day co-cultivation period. Five groups of bacteria were defined and characterised based on their combined influence on biomass production and root system architecture. The observed effects ranged from unchanged to greatly increased biomass production coupled with increased root length and branching. Primary root length was increased only by the volatile compounds emitted by Enterobacter cloacae JM22 and Bacillus pumilus T4. Overall, the most significant results were obtained with Bacillus subtilis GB03, which induced an 81 % increase in total biomass, as well as enhancing total root length, total secondary root length and total adventitious root length by 88.5, 201.5 and 474.5 %, respectively.ConclusionsThis study is the first report on bacterial volatile-mediated growth promotion of a grass plant. Contrasting modulations of biomass production coupled with changes in root system architecture were observed. Most of the strains that increased total plant biomass also modulated adventitious root growth. Under our screening conditions, total biomass production was strongly correlated with the length and branching of the root system components, except for primary root length. An analysis of the emission kinetics of the bacterial volatile compounds is being undertaken and should lead to the identification of the compounds responsible for the observed growth-promotion effects. Within the context of the inherent characteristics of our in vitro system, this paper identifies the next critical experimental steps and discusses them from both a fundamental and an applied perspective.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-015-0585-3) contains supplementary material, which is available to authorized users.

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Varin, S., Lemauviel-Lavenant, S., Cliquet, J. B., Diquelou, S., Michaelson-Yeates, T. P. T. (2009). Functional plasticity of Trifolium repens L. in response to sulphur and nitrogen availability. Plant and Soil, 317, (1-2), 189-200. IMPF: 02.52 RONO: 1330 2406Recent control of atmospheric SO2 pollution is leading to important soil sulphur impoverishment. Plasticity could be a mechanism allowing species to adapt to this rapid global change. Trifolium repens L. is a key grassland species whose performances in community are strongly linked to nitrogen availability. Plasticity of three white clover lines contrasting in their ability to use atmospheric N2 or soil N was assessed by analysing a set of functional traits along a gradient of nitrogen and sulphur fertilisation applied on a poor soil. White clover traits showed high morphological and physiological plasticity. Nitrogen appeared to be the most limiting factor for the VLF (Very Low Fixation) line. S was the element that modulated the most traits for the nitrogen fixing lines NNU (Normal Nitrate Uptake) and LNU (Low Nitrate Uptake). As expected, N fertilisation inhibited white clover fixation, but we also observed that N2 fixation was enhanced when S was added. S fertilisation increased nodule length as well as the proportion of nodules containing leghaemoglobin. S fertilisation, with a direct effect and an indirect effect through N2 fixation, increases white clover performances particularly with regards to photosynthesis and potential vegetative reproduction. The important plasticity in response to S availability should allow it to adapt to a large range of abiotic conditions, but its sensitivity to S nutrition would be a disadvantage for competition in a situation of soil sulphur impoverishment. In contrast, S fertilisation could help maintain this species when nitrogen status is against it.Peer reviewe