Sulfur status and acquisition in response to different sulfate concentration and time exposure in maize

Two cultivars of maize (Zea mays L.) KW2 and KW7, previously identified as being productive under low-input conditions, possess different sulphate uptake characteristics when grown in S-deficient conditions but not under S-replete conditions. These characteristics did not match the transcript abundance of the sulphate transporter ZmST1 in roots suggesting different transporters or differing regulation aspects are responsible for the measured uptake differences. To further characterise the high-affinity uptake characteristics, both cultivars were grown in 1 mM sulphate for seven days. Afterwards they were transferred to SO42-- deplete conditions for a further seven days, during which the uptake system is gradually derepressed. Both cultivars had identical uptake rates in SO42- replete conditions. However, as soon as four hours after exposure to –S conditions, the SO42- uptake rate in radiolabelled 100 μM SO42- of KW2 was higher than that of KW7 despite no difference in their growth rates, and remained so until the end of the experimental period. Maintaining the pH of the root surface by buffering with 10 mM MES/Tris did not change the measured difference in uptake rates between the cultivars. This indicates that the difference in uptake rates was not due to differences in external pH affecting the thermodynamics of H+-coupled transport, but rather in the inherent activity or regulation of the SO42- transporter itself.