Atmospheric H 2S as sulfur source for Brassica oleracea: kinetics of H 2S uptake and activity of O-acetylserine (thiol)lyase as affected by sulfur nutrition

Abstract

The uptake of hydrogen sulfide (H 2S) by shoots of curly kale ( Brassica oleracea) showed saturation kinetics with respect to the atmospheric concentration. The kinetics are largely determined by the rate of metabolism of the absorbed H 2S into cysteine, catalyzed by O-acetylserine (thiol)lyase, and can be described by the Michaelis–Menten equation. When B. oleracea was grown under sulfate (SO 4 2−)-deprived conditions, plants developed sulfur (S) deficiency symptoms and H 2S uptake kinetics were substantially altered. Shoots of SO 4 2−-deprived plants had a lower affinity to H 2S uptake, whereas the maximal H 2S uptake rate was higher. When SO 4 2−-deprived plants were simultaneously exposed to 0.2 μl l −1 H 2S all S deficiency symptoms disappeared and H 2S uptake kinetics returned rapidly to values observed for S-sufficient shoots. The activity of the H 2S-fixating enzyme O-acetylserine (thiol)lyase was hardly affected upon either prolonged H 2S exposure or SO 4 2− deprivation. Evidently, the activity of O-acetylserine (thiol)lyase was not the rate-limiting step in the H 2S uptake by shoots. The significance of the in situ availability and rate of synthesis of the substrate O-acetylserine for O-acetylserine (thiol)lyase as determining factor in the uptake kinetics of H 2S needs further evaluation.