Coordinate transcriptional induction of myo-inositol metabolism during environmental stress

The pathway from glucose 6-phosphate (G 6-P) to myo-inositol 1-phosphate (Ins 1-P) and myo-inositol (Ins) is essential for the synthesis of various metabolites. In the halophyte Mesembryanthemum crystallinum (common ice plant), two enzymes, myo-inositol O-methyltransferase (IMT1) and ononitol epimerase (OEP1), extend this pathway and lead to the accumulation of methylated inositols, D-ononitol and D-pinitol, which serve as osmoprotectants. This paper describes transcripts for the enzyme, Inps1, encoding myo-inositol 1-phosphate synthase (INPS1), from the ice plant. Two Inps-like sequences are present in the genome. The deduced amino acid sequences of the cloned transcript are 49.5% and 87-90%, respectively, identical to those of yeast and other higher plant sequences. Inps1 RNA amounts are upregulated at least fivefold and amounts of free Ins accumulate approximately 10-fold during salinity stress. Inps1 induction is by transcription, similar to the induction of Imt1. In contrast, Arabidopsis thaliana does not show upregulation of Inps1 or increased amounts of Ins when salt-stressed. The lack of Inps1 induction in Arabidopsis exemplifies differences in glycophytic and halophytic regulation of gene expression at the point of entry into a pathway that leads to osmoprotection. The stress-induced coordinate upregulation of this pathway and its extension by novel enzymes in the ice plant also highlights biochemical differences.