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

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.

Reply to Horacek, M.; Cannavan, A. Comment on “Sinkovič et al. Isotope Fingerprints of Common and Tartary Buckwheat Grains and Milling Fractions: A Preliminary Study. Foods 2022, 11, 1414”

“…Although it is generally accepted that less S stable isotope discrimination occurs during sulfate (SO 4 2− ) uptake, S metabolism and allocation are expected to produce separations of stable S isotopes among the different plant S pools and different tissues. For example, the latest study on rice by Cavallaro et al [ 13 ] shows that S org pools in the rice were 34 S-depleted compared to the S-SO 4 2− source. Our results similarly indicate that S org (in semolina) is 34 S-depleted compared to hulls containing more inorganic S. However, the exact mechanisms need further investigation.…”