Development and Environmental Stress Employ Different Mechanisms in the Expression of a Plant Gene Family.

Ribulose bisphosphate carboxylase small subunit (RbcS) genes in the common 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 Crassulacean acid metabolism (CAM), which this plant undergoes during development and under environmental stress. Four RbcS genes are differentially expressed in leaves but are regulated in a coordinate fashion. A developmentally engrained, sharp decline in the steady state mRNA levels, which is observed during the juvenile-to-adult growth phase transition, coincides with the time interval when the C3-to-CAM switch occurs. Developmental down regulation of RbcS is due to down regulation of transcription. In contrast, NaCl stress specifically affected RbcS transcript accumulation post-transcriptionally, resulting in decreased RbcS mRNA levels. Antagonistic regulatory programs are apparent in stress/stress relief experiments. The results indicate complex controls, affecting both transcriptional and post-transcriptional processes, that act differentially during plant development, stress, and recovery from stress.