Sulfur Stable Isotope Discrimination in Rice: A Sulfur Isotope Mass Balance Study.

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 (SO ₄ ^2- ) 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 SO ₄ ^2- availabilities in appropriate closed and open hydroponic-plant systems. The main results indicate that fractionation against ³⁴ S occurred during SO ₄ ^2- uptake. Fractionation was dependent on the amount of residual SO ₄ ^2- in the solution, showing a biphasic behavior related to the relative expression of two SO ₄ ^2- 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 SO ₄ ^2- 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 ³² S/ ³⁴ S analysis for the detailed characterization of the metabolic and molecular processes involved in plant S nutrition and homeostasis.
Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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