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

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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.