CHARACTERIZATION OF CHLOROPLAST HEAT-SHOCK PROTEINS IN YOUNG LEAVES OF C-4 MONOCOTYLEDONS

In vivo radiolabeling of chloroplast proteins in grain sorghum (Sorghum bicolor L. cv. Texas 610) leaves and their separation by one-dimensional electrophoresis revealed at least 6 heat shock proteins (HSPs) between 24 and 94 kDa, of which the 24 kDa protein was the most prominent. All of these chloroplast heat shock proteins were found exclusively in the stroma. The 24 kDa heat shock protein, upon closer examination using two-dimensional electrophoresis proved to be two similarly-sized heat shock polypeptides with identical molecular masses and level of radiolabel incorporation, but slightly different in isoelectric points, suggesting isomers. Separation of stromal heat shock proteins synthesised in two other C-4 monocotyledons (Panicum, miliaceum L. and Urochloa panicoides L.) revealed similar putative isomers, each of 24 kDa. Several other, previously unidentified, heat shock proteins between 22 and 38 kDa were also observed in all three species. In P. miliaceum, the most prominent HSP was the pair of 24 kDa proteins, whereas in U. panicoides, it was a group of 35 to 38 kDa HSPs that was most abundant. In vivo chlorophyll fluorescence measurements showed that no sustained impairment to photosynthetic efficiency had occurred for each species after the heat stress regime. However, when cytoplasmic protein synthesis was inhibited during the high temperature treatment, a dramatic decrease was observed in photosynthetic efficiency, suggesting a possible protective role far chloroplast heat shock proteins. It was also shown that a single chloroplast HSP complex of around 380 kDa was observed in the stroma of both S. bicolor and P. miliaceum leaves in vivo. This was in contrast to the smaller HSP complex (200-265 kDa) observed in previous studies on chloroplast heat shock proteins in C-3 species.