Coordinate transcriptional induction of myo‐inositol metabolism during environmental stress

Abstract

The pathway from glucose 6-phosphate (G 6-P) to myoinositol 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.

Growth and development of Mesembryanthemum crystallinum (Aizoaceae)

“…As indicated, a number of genes can be transcriptionally induced in juvenile leaves by salt stress, such as the induction of genes for inositol biosynthesis and the ice-plant-specific extension of this pathway which leads to the production of pinitol (Paul & Cockburn, 1989 ;Vernon & Bohnert, 1992 a ;Rammesmayer et al, 1995 ;Ishitani et al, 1996 ;D. E. Nelson, G. Rammesmayer & H. J. Bohnert, unpublished).…”

“…At the physiological and biochemical level, work on the plant has provided insights into salinity tolerance and the regulation of CAM. Salinity and drought affect a suite of sugar-alcohol-compatible solutes (inositol, ononitol, pinitol ;Paul & Cockburn, 1989 ;Vernon & Bohnert, 1992 a ;Ishitani et al, 1996) in addition to the activity of ion transporters and specific water-channel proteins associated with roots, stems and leaves. In contrast to many facultative CAM plants in which induction is reversible (Griffiths, 1988 ;Borland & Griffiths, 1996), the induction of CAM in adult leaves (stage 3, Table 1) of M. crystallinum is constitutive, although induction of PEPC (phosphoenolpyruvate carboxylase) transcripts and protein by salt stress in old leaves of the juvenile stage (stage 2, Table 1) is reversible when salt is removed (Vernon et al, 1988).…”

“…Some genes are controlled transcriptionally by salt stress at all periods of development, such as Imt1, encoding a myo-inositol O-methyltransferase, the key enzyme leading to the production of compatible solutes, i.e. solutes that will not interfere with biochemical reactions even at high concentration (Vernon & Bohnert, 1992 a ;Ishitani et al, 1996). Although this response might occur during the juvenile phase of growth, the limited capacity to transport sodium into vacuoles (Lo$ w et al, 1996 ;Tsiantis, Bartholomew & Smith, 1996) and to sequester salt in specialized epidermal bladder cells (EBC) (Lu$ ttge, Fischer & Steudle, 1978 ;Lu$ ttge, 1993) results in arrested development in juvenile plants which have not attained the capacity to grow side shoots.…”

“…Plants, in hydroponic culture, were 5-wk-old at the beginning of the experiment (5 d would be high enough to compensate for molar amounts of sodium in the vacuole (P. Adams and H. J. Bohnert, unpublished). Presumably an inducible pathway (Vernon & Bohnert, 1992 a, b ;Ishitani et al, 1996 ;D. E. Nelson, G. Rammesmayer & H. J. Bohnert, unpublished), in conjunction with the osmotic pressure generated by sodium in the vacuole, allows osmotic adjustment in growing tissues and leads to salt tolerance.…”

“…Many glycophytic Mip have been detected as water-stress-induced transcripts (Yamaguchi-Shinozaki et al, 1992), but in the ice plant, Mip transcripts decline following stress. The decline is transient, however, and transcripts recover in a time course that is correlated with the synthesis and accumulation of proline and pinitol (Yamada et al, 1995 ;Ishitani et al, 1996). Using MIP-specific (peptide-based) antibodies, we have now shown that Mip transcript decline is paralleled by a decline of the corresponding proteins in some, though not all, cells in different tissues, most dramatically in cells of the root tip (H. H. Kirch & H. J. Bohnert, unpublished).…”