Transcript profiling of salinity stress responses by large-scale expressed sequence tag analysis in Mesembryanthemum...

The common ice plant, Mesembryanthemum crystallinum, is a halophytic (salt-loving) member of the Aizoaceae, which switches from C3 photosynthesis to Crassulacean acid metabolism (CAM) when exposed to salinity or water-deficit stress. CAM is a metabolic adaptation of photosynthetic carbon fixation that improves water use efficiency by shifting net CO2 uptake to the night, thereby reducing transpirational water loss. To improve our understanding of the molecular genetic underpinnings and control mechanisms for Crassulacean acid metabolism (CAM) and other salinity stress response adaptations, a total of 9733 expressed sequence tags (ESTs) from cDNAs derived from leaf tissues of well-watered and salinity-stressed (0.5 M NaCl for 30 and 48 h) were characterized. Clustering and assembly of these ESTs resulted in the identification of a total of 3676 tentative unique gene sequences (1249 tentative consensus sequences and 2427 singleton ESTs) expressed in leaves of ice plant under unstressed and salinity stressed conditions. The same number (2782) of ESTs from each library (total=8346 ESTs) were randomly selected and analyzed to compare expression profiles among the control and salt stressed leaf tissues. EST frequencies for transcripts encoding CAM-related enzymes, pathogenesis-related, senescence-associated, cell death-related, and stress-related proteins such as heat shock proteins (HSPs), chaperones, early light-inducible proteins, ion homeostasis, antioxidative stress, detoxification, and biosynthetic enzymes for osmoprotectants increased 2-12-fold in cDNA libraries constructed from salt stressed plants. In contrast, the frequency of ESTs encoding light-harvesting and photosystem complexes and C3 photosynthetic enzymes decreased 4-fold overall following salinity stress with transcripts for ribulose bisphosphate carboxylase/oxygenase (RuBisCO) subunits decreasing 7-fold. Moreover, stressed plants contained a higher percentage of ESTs encoding novel and/or functionally unknown proteins. The rapid discovery of both known and unknown genes related to stress responses in M. crystallinum demonstrates the great utility of EST analysis in unraveling the complex set of adaptive mechanisms contributing to water use efficiency (CAM) and salinity tolerance.