Abstract
The use of sulfur (S) stable isotopes to study S metabolism in plants is still limited by the relatively small number of studies. It is generally accepted that less S stable isotope discrimination occurs during sulfate (SO42–) uptake. However, S metabolism and allocation are expected to produce separations of S stable isotopes among the different plant S pools and organs. In this study, we measured the S isotope composition of the main S pools of rice plants grown under different SO42– availabilities in appropriate closed and open hydroponic-plant systems. The main results indicate that fractionation against 34S occurred during SO42– uptake. Fractionation was dependent on the amount of residual SO42– in the solution, showing a biphasic behavior related to the relative expression of two SO42– transporter genes (OsSULTR1;1 and OsSULTR1;2) in the roots. S isotope separations among S pools and organs were also observed as the result of substantial S isotope fractionations and mixing effects occurring during SO42– assimilation and plant S partitioning. Since the S stable isotope separations conserve the memory of the physiological and metabolic activities that determined them, we here underline the potential of the 32S/34S analysis for the detailed characterization of the metabolic and molecular processes involved in plant S nutrition and homeostasis.
Highlights
Since 1865, sulfur (S) has been recognized as an essential element for plant growth (Sachs, 1865; Epstein, 2000)
This study presents a detailed study on the dynamics of S stable isotopes occurring in appropriate closed or steady-state hydroponic-plant systems to dissect the 32S/34S isotope effects associated with SO42− uptake, allocation, and metabolism in rice plants
The correct evaluation of the isotope effects due to S acquisition needs a direct comparison between the isotope compositions of the Stot of a whole plant and the S source used by the same plant, since the δ34S_Stot value of a single plant organ may result from fractionations and mixing effects occurring during SO42− uptake, assimilation, and partitioning
Summary
Since 1865, sulfur (S) has been recognized as an essential element for plant growth (Sachs, 1865; Epstein, 2000). S is found in the amino acid cysteine and methionine, short peptides, vitamins and cofactors, and secondary compounds (Takahashi et al, 2011). Plants mainly utilize sulfate (SO42−), an inorganic form of oxidized S present in the soil solution, to support their growth. SO42− is taken up by roots and allocated to various sink tissues, where it is stored in the cell vacuoles or assimilated into S organic (Sorg) compounds (Saito, 2004; Takahashi et al, 2011). To accomplish the assimilation of S into biomolecules, SO42− is first activated by ATP sulfurylase to adenosine-5 -phosphosulfate (APS), which is channeled toward reduction or sulfation (Leustek et al, 2000). Most of the APS enters the reductive pathway along which sulfite and, subsequently, sulfide are produced through two sequential reactions catalyzed by APS reductase and sulfite reductase, respectively.
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