A SALINITY-INDUCED GENE FROM THE HALOPHYTE M-CRYSTALLINUM ENCODES A GLYCOLYTIC ENZYME, COFACTOR-INDEPENDENT PHOSPHOGLYCEROMUTASE

In the facultative halophyte Mesembryanthemum crystallinum (ice plant) , salinity stress triggers significant changes in gene expression, inc luding increased expression of mRNAs encoding enzymes involved with os motic adaptation to water stress and the crassulacean acid metabolism (CAM) photosynthetic pathway. To investigate adaptive stress responses in the ice plant at the molecular level, we generated a subtracted cD NA library from stressed plants and identified mRNAs that increase in expression upon salt stress. One full-length cDNA clone was found to e ncode cofactor-independent phosphoglyceromutase (PGM), an enzyme invol ved in glycolysis and gluconeogenesis. Pgm1 expression increased in le aves of plants exposed to either saline or drought conditions, whereas levels of the mRNA remained unchanged in roots of hydroponically grow n plants. Pgm1 mRNA was also induced in response to treatment with eit her abscisic acid or cytokinin. Transcription run-on experiments confi rmed that Pgm1 mRNA accumulation in leaves was due primarily to increa sed transcription rates. Immunoblot analysis indicated that Pgm1 mRNA accumulation was accompanied by a modest but reproducible increase in the level of PGM protein. The isolation of a salinity-induced gene enc oding a basic enzyme of glycolysis and gluconeogenesis indicates that adaptation to salt stress in the ice plant involves adjustments in fun damental pathways of carbon metabolism and that these adjustments are controlled at the level of gene expression. We propose that the leaf-s pecific expression of Pgm1 contributes to the maintenance of efficient carbon flux through glycolysis/gluconeogenesis in conjunction with th e stress-induced shift to CAM photosynthesis.