STRESS PERCEPTION AND RESPONSE IN A FACULTATIVE HALOPHYTE - THE REGULATION OF SALINITY-INDUCED GENES IN MESEMBRYANTHEMUM-CRYSTALLINUM

The facultative halophyte Mesembryanthemum crystallinum (ice plant) shifts from C3 to CAM photosynthesis in response to water stress. The switch to CAM can also be induced by high salinity, presumably because osmotic stress causes water loss from leaf and stem tissues. A number of mRNAs (Ppc1, Imt1, B5 and Gpd1) that encode proteins involved in different biochemical pathways accumulate in M. crystallinum leaf tissues when plants are salt-stressed. We hypothesized that environmental challenges that result in water stress invoke a common mechanism that triggers the coordinated induction of mRNAs involved in different aspects of the ice plant's adaptive stress response. Nuclear run-on experiments indicated that Ppc1, Imt1, B5 and Gpd1 are all transcriptionally activated in salt-stressed leaf tissue. However, a comparison of Ppc1, Imt1, and B5 transcript levels after exposure of plants to growth regulators (6-benzylamino purine and abscisic acid) and to different environmental stress treatments that affect plant water status (drought, low temperature and salinity) indicated that, despite the coordinated transcriptional activation of these genes during salt stress, their repsonses to other stimuli that also upset water balance are not the same. Our results are consistent with a model involving multiple control mechanisms governing stress perception and molecular response in the ice plant.