A salinity-induced gene from the halophyte M. crystallinum encodes a glycolytic enzyme, cofactor-independent phosphoglyceromutase

Abstract

In the facultative halophyte Mesembryanthemum crystallinum (ice plant), salinity stress triggers significant changes in gene expression, including increased expression of mRNAs encoding enzymes involved with osmotic 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 cDNA library from stressed plants and identified mRNAs that increase in expression upon salt stress. One full-length cDNA clone was found to encode cofactor-independent phosphoglyceromutase (PGM), an enzyme involved in glycolysis and gluconeogenesis. Pgm1 expression increased in leaves of plants exposed to either saline or drought conditions, whereas levels of the mRNA remained unchanged in roots of hydroponically grown plants. Pgm1 mRNA was also induced in response to treatment with either abscisic acid or cytokinin. Transcription run-on experiments confirmed that Pgm1 mRNA accumulation in leaves was due primarily to increased transcription rates. Immunoblot analysis indicated that Pgm1 mRNA accumulation was accompanied by a modest but reproductible increase in the level of PGM protein. The isolation of a salinity-induced gene encoding a basic enzyme of glycolysis and gluconeogenesis indicates that adaptation to salt stress in the ice plant involves adjustments in fundamental pathways of carbon metabolism and that these adjustments are controlled at the level of gene expression. We propose that the leaf-specific expression of Pgm1 contributes to the maintenance of efficient carbon flux through glycolysis/gluconeogenesis in conjunction with the stress-induced shift to CAM photosynthesis.