32S/34S isotope fractionation in plant sulphur metabolism

Sulphur is a crucial microelement for plant metabolism, involved in response mechanisms to oxidative stress, C-1 metabolism, electron transfer, secondary metabolism and post-translational peptide modifications. However, relatively little is known about sulphur transport and partitioning in plants. Although key steps of sulphur assimilation have been shown using labelling with radioactive S-35 in past decades, the use of a natural tracer, allowing the examination of metabolic commitments and mass balance, is desirable. Sulphur stable isotopes (S-32 and S-34) have been proven to be useful for the investigation of the origin of sulphur atoms in geochemistry and for the detection of the origin of sulphur atoms. Nevertheless, their use for the study of primary sulphur metabolism has been impeded by our lack of knowledge of S-32/S-34 isotope fractionations and convenient methods for S-34 analyses. Here, we review documented S-32/S-34 isotope fractionations that may apply to sulphur metabolism, and explain how they should yield disparities amongst sulphur-containing plant compounds.