Protein Turnover in Isolated Barley Leaf Segments and the Effects of Stress

Abstract

Leaf segments prepared from the first leaves of barley (Hordeum vulgare) exhibit a rapid loss of protein when given a matric stress with polyethylene glycol. Protein synthesis was reduced by the stress but a greater effect of stress was seen on protein degradation. Growing leaves were exposed to ³H₂O for 4 d or more to label total protein, and the half-life of protein 2-³H, in the isolated segments prepared from such leaves, was shown to be c. 140 h in the absence of stress. Stress reduced this to c. 62 h. A short pulse with ³H₂O preferentially labels rapidly turning-over protein and a 24 h pulse given to isolated leaf segments labelled proteins with a half-life of c. 64 h in the presence or absence of stress. Degradation of the 24 h pulse-labelled proteins was inhibited by cycloheximide. Proline accumulation occurred in the stressed segments and was inhibited by cycloheximide. The results are discussed in the light of current views concerning protein degradation and possible relationships between proteolysis and proline accumulation.