A novel methyl transferase induced by osmotic stress in the facultative halophyte Mesembryanthemum crystallinum.

Abstract

Molecular mechanisms of osmotic stress tolerance were studied in Mesembryanthemum crystallinum (ice plant), a facultative halophyte capable of adjusting to and surviving in highly saline conditions. We screened a subtracted cDNA library enriched for salt stress‐induced mRNAs to identify transcripts involved in this plant's adaptation to salinity. One mRNA, Imt1, was found to be up‐regulated in leaves and, transiently, in roots. Nuclear run‐on assays indicated that this mRNA is transcriptionally regulated. Imt1 encoded a predicted polypeptide of M(r) 40,250 which exhibited sequence similarity to several hydroxymethyl transferases. Expression of the protein in Escherichia coli and subsequent activity assays identified the protein as a novel myoinositol O‐methyl transferase which catalyzes the first step in the biosynthesis of the cyclic sugar alcohol pinitol. Pinitol accumulates in salt‐stressed M.crystallinum and is abundant in a number of salt‐ and drought‐tolerant plants. The presence of high levels of sugar alcohols correlates with osmotolerance in a diverse range of organisms, including bacteria, fungi and algae, as well as higher plants. The stress‐initiated transcriptional induction of IMT1 expression in a facultative halophyte provides strong support for the importance of sugar alcohols in establishing tolerance to osmotic stress in higher plants.

Regulation of Cell-Specific Inositol Metabolism and Transport in Plant Salinity Tolerance

“…When 5 Ј and 3 Ј end-specific probes derived from the cloned cDNA were used in DNA gel blot analysis with blots containing DNA cut with a variety of restriction enzymes, single bands were observed in each lane, indicating that the saltinducible transcript is encoded by a single gene (see also Vernon and Bohnert, 1992). Screening of a genomic library produced three clones.…”

“…Salinity tolerance is also a multicellular trait, and the remodeling of complex pathways in transgenic plants will require knowledge of the distribution of such pathways throughout the plant. We have analyzed the myo -inositol biosynthetic pathway-leading to methylated inositols-that feeds into a pathway specific to salinity tolerance in a halophyte, the common ice plant ( Mesembryanthemum crystallinum ) (Loewus and Dickinson, 1982;Vernon and Bohnert, 1992).…”

Proteomic Analysis of a Sea-Ice Diatom: Salinity Acclimation Provides New Insight into the Dimethylsulfoniopropionate Production Pathway

“…SAMsyn is responsible for the regeneration of SAM, the required cofactor for the penultimate methyltransferase reaction in DMSP synthesis. Active methyl cycle genes are also involved in the synthesis of other osmolytes such as pinitol, a sugar alcohol produced by succulent plants (Vernon and Bohnert, 1992), as well as in the methyltransferase synthesis pathway for Gly betaine (Nyyssö lä et al, 2001) that also appears to be utilized by F. cylindrus.…”

The inositol oxygenase gene family of Arabidopsis is involved in the biosynthesis of nucleotide sugar precursors for cell-wall matrix polysaccharides

“…Some plant species like legumes synthesize the raffinose oligosaccharide series, which requires galactinol, made from UDP-galactose and inositol, as substrate for elongation. In some plants, inositol is mono-or di-methylated to produce compatible solutes as osmoprotectant compounds under drought-stress conditions (Vernon and Bohnert 1992). Thus, myo-inositol biosynthesis and conversion must be highly controlled and regulated to meet the requirements of the different pathways.…”