DEVELOPMENT AND ENVIRONMENTAL-STRESS EMPLOY DIFFERENT MECHANISMS IN THE EXPRESSION OF A PLANT GENE FAMILY

Ribulose bisphosphate carboxylase small subunit (RbcS) genes in the co mmon ice plant, as in all higher plants, constitute a multigene family . We have measured transcription activity and steady state mRNA levels of individual members of the family, six RbcS genes, in the ice plant with emphasis on the transition from C3 photosynthesis to Crassulacea n acid metabolism (CAM), which this plant undergoes during development and under environmental stress. Four RbcS genes are differentially ex pressed in leaves but are regulated in a coordinate fashion. A develop mentally engrained, sharp decline in the steady state mRNA levels, whi ch is observed during the juvenile-to-adult growth phase transition, c oincides with the time interval when the C3-to-CAM switch occurs. Deve lopmental down regulation of RbcS is due to down regulation of transcr iption. In contrast, NaCl stress specifically affected RbcS transcript accumulation post-transcriptionally, resulting in decreased RbcS mRNA levels. Antagonistic regulatory programs are apparent in stress/stres s relief experiments. The results indicate complex controls, affecting both transcriptional and post-transcriptional processes, that act dif ferentially during plant development, stress, and recovery from stress .