Can isotope ratio mass spectrometry provide insight into sulfate distribution within the plant?

Four stable isotopes of sulfur (S) exist (32S, 33S, 34S, 36S) whose natural isotopic percentage abundances are 0.94499, 0.0075, 0.0425 and 0.0001 atom fraction, respectively (Berglung & Wieser, 2011). The most abundant isotopes – 32S and 34S - are now commonly measured using elemental analyzers coupled with isotope ratio mass spectrometers (EA-IRMS). Such an approach is based on the complete transformation of total S to SO2, which is subsequently analyzed by the mass spectrometer with regards to masses 64 (32S16O2) and 66 (35S16O2 or 32S16O18O) atomic mass units (Grassineau et al., 1999). S stable isotopes have been used to trace the movements of the related compounds in plants, in testing S flux models, and in identifying and determining the impact of natural and anthropogenic S sources on the environment. However, the isotope technique applied for S metabolism investigations, as well as for sulfate transport and allocation within the plants, is limited by our current knowledge of the potential 32S/34S isotope discrimination that may occur during both S metabolism and sulfate transport. The relative 34S abundance is traditionally quantified using the  value: 34S = (Rsample - Rstandard)/Rstandard, were R is 34S/32S isotope ratio. The 34S signature of the total biomass produced by a plant generally reflects that of the available sulfate in the soil solution, thus suggesting the fractionation against 34S during sulfate acquisition negligible. However, a careful analysis of the 34S signature of wheat plant organs, revealed that root and stem are depleted in 34S relative to soil sulfate by  2‰, whilst leaves and grains are enriched up to  2‰, indicating that fractionation occurs during sulfate allocation and metabolism (Tcherkez & Tea, 2013). Our research group is now approaching the topic of 32S/34S fractionation in plants by investigating the hypothesis that the 34S signature of sulfate ions within the leaves could be determined by the activities of the sulfate transporters involved in sulfate uptake, as well as in root-to-shoot sulfate translocation. The experimental approach will be mainly based on: i) the preliminary characterization of single plant sulfate transporters – heterologously expressed in yeast strains defective for sulfate uptake – for their potential ability to discriminate between 32S-sulfate and 34S-sulfate; ii) the comparison of the effects of different growing conditions – known to modulate sulfate uptake and/or translocation – on the 34S signature of the sulfate ions in the leaves. Results will be related to the relative transcript levels of the sulfate transporter genes involved in sulfate uptake and translocation, in order to obtain a comprehensive picture of the 32S/34S isotope effects occurring during sulfate distribution within the plant.