COORDINATE TRANSCRIPTIONAL INDUCTION OF MYOINOSITOL METABOLISM DURINGENVIRONMENTAL-STRESS

The pathway from glucose 6-phosphate (G 6-P) to myo-inositol 1-phospha te (Ins 1-P) and myo-inositol (Ins) is essential for the synthesis of various metabolites. In the halophyte Mesembryanthemum crystallinum (c ommon ice plant), two enzymes, myo-inositol O-methyltransferase (IMT1) and ononitol epimerase (OEP1), extend this pathway and lead to the ac cumulation of methylated inositols, D-ononitol and D-pinitol, which se rve as osmoprotectants. This paper describes transcripts for the enzym e, Inps1, encoding myo-inositol 1-phosphate synthase (INPS1), from the ice plant. Two Inps-like sequences are present in the genome. The ded uced amino acid sequences of the cloned transcript are 49.5% and 87-90 %, respectively, identical to those of yeast and other higher plant se quences. Inps1 RNA amounts are upregulated at least fivefold and amoun ts of free Ins accumulate approximately 10-fold during salinity stress . Inps1 induction is by transcription, similar to the induction of Imt 1. In contrast, Arabidopsis thaliana does not show upregulation of Inp s1 or increased amounts of Ins when salt-stressed. The lack of Inps1 i nduction in Arabidopsis exemplifies differences in glycophytic and hal ophytic regulation of gene expression at the point of entry into a pat hway that leads to osmoprotection. The stress-induced coordinate upreg ulation of this pathway and its extension by novel enzymes in the ice plant also highlights biochemical differences.