A novel methyl transferase induced by osmotic stress in the facultative halophyte Mesembryanthemum crystallinum.
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.
Vernon
D M
DM
Department of Molecular and Cellular Biology, Arizona, Tucson 85721.
Bohnert
H J
HJ
D000595:Amino Acid Sequence; D001483:Base Sequence; D015139:Blotting, Southern; D003001:Cloning, Molecular; D004247:DNA / Q000235:genetics / Q000302:isolation & purification; D004790:Enzyme Induction; D004926:Escherichia coli / Q000235:genetics; D015723:Gene Library; D017343:Genes, Plant; D007053:Ice; D008780:Methyltransferases / Q000096:biosynthesis* / Q000235:genetics* / Q000302:isolation & purification; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009994:Osmolar Concentration; D010944:Plants / Q000201:enzymology* / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics* / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000302:isolation & purification / Q000378:metabolism; D014158:Transcription, Genetic
D007053:Ice; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D004247:DNA; D008780:Methyltransferases