XVI International Botanical Congess

Rubisco activase catalyzes the activation and maintains the activated state of ribulose bisphosphate carboxylase/oxygenase (Rubisco), in mesophyll cell chloroplasts. Arabidopsis mutants lacking Rubisco activase and transgenic tobacco plants with reduced Rubisco activase levels have shown inadequate levels of Rubisco activity. Evidence from many immunocytochemical studies have indicated that Rubisco is indeed present in guard cells. However, whether the activity of Rubisco in guard cells is comparable to the activity in mesophyll cells is still not clear. In the present study, we have carried out both in vitro and in vivo experiments, using Rubisco activase antibodies to probe for the presence of activase protein. Vicia faba leaf tissue extracts clearly showed the presence of activase protein except in guard cell protoplasts. However, immunogold electron microscopy revealed that activase may be present in negligible amounts compared to Rubisco in guard cells, suggesting that a low Rubisco activity in guard cells may be due to inadequate levels of Rubisco activase in these cells.