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

The facultative halophyte Mesembryanthemum crystallinum (ice plant) sh ifts from C3 to CAM photosynthesis in response to water stress. The sw itch to CAM can also be induced by high salinity, presumably because o smotic stress causes water loss from leaf and stem tissues. A number o f mRNAs (Ppc1, Imt1, B5 and Gpd1) that encode proteins involved in dif ferent biochemical pathways accumulate in M. crystallinum leaf tissues when plants are salt-stressed. We hypothesized that environmental cha llenges that result in water stress invoke a common mechanism that tri ggers the coordinated induction of mRNAs involved in different aspects of the ice plant's adaptive stress response. Nuclear run-on experimen ts indicated that Ppc1, Imt1, B5 and Gpd1 are all transcriptionally ac tivated in salt-stressed leaf tissue. However, a comparison of Ppc1, I mt1, and B5 transcript levels after exposure of plants to growth regul ators (6-benzylamino purine and abscisic acid) and to different enviro nmental stress treatments that affect plant water status (drought, low temperature and salinity) indicated that, despite the coordinated tra nscriptional activation of these genes during salt stress, their repso nses to other stimuli that also upset water balance are not the same. Our results are consistent with a model involving multiple control mec hanisms governing stress perception and molecular response in the ice plant.