Effect of Rapid Dehydration on the activity of PEPC from the C4 grass Setaria sphacelata

Abstract

Plants of Setaria sphacelata were grown in soil in a phytotron under a PPFD of 400 m mol m-2s-1 25/18 oC (day/night) temperature. Young fully expanded leaves were excised and allowed to be rapidly dehydrated by exposure to the phytotron atmosphere. Relative water content (RWC) was monitored in a parallel sample, and leaf fragments at different RWC were frozen in liquid nitrogen. Leaf samples were later ground for enzyme extraction. PEPC activity was measured spectrophotometrically. Both the maximal and the physiological PEPC activities decreased approximately 80% as the RWC decreased from full-saturation to 60%, when expressed on dry weight, soluble protein and chlorophyll bases. At lower RWC there appears to be a partial recovery of both activities. As maximal activity decreased more than the physiological activity a linear increase of the % of activation was observed under stress, from 40% at full water content to 60% at the more severe stress measured. The decrease of maximal activities suggests a lower amount of PEPC protein in stressed samples, due to enzyme degradation or to the dissociation of the tetrameric complex. The increase of the % of activation with decreasing RWC suggests that in stress conditions the enzyme is more phosphorylated. Possible mechanisms will be discussed.