Glycinebetaine (betaine) highly accumulates as a compatible solute in certain plants and has been considered to play a role in the protection from salt stress. The betaine biosynthesis pathway of betaine-accumulating plants involves choline monooxygenase (CMO) as the key enzyme and phosphoethanolamine N-methyltransferase (PEAMT), which require S-adenosyl-L-methionine (SAM) as a methyl donor. SAM is synthesized by SAM synthetase (SAMS), and is needed not only for betaine synthesis but also for the synthesis of other compounds, especially lignin. We cloned CMO, PEAMT and SAMS isogenes from a halophyte Atriplex nummularia L. (Chenopodiaceous). The transcript and protein levels of CMO were much higher in leaves and stems than in roots, suggesting that betaine is synthesized mainly in the shoot. The regulation patterns of transcripts for SAMS and PEAMT highly resembled that of CMO in the leaves during and after relief from salt stress, and on a diurnal rhythm. In the leaves, the betaine content was increased but the lignin content was not changed by salt stress. These results suggest that the transcript levels of SAMS are co-regulated with those of PEAMT and CMO to supply SAM for betaine synthesis in the leaves.

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Because the photosynthetic apparatus contains a massive amount of nitrogen in plants, the regulation of its development by sugar signals is important to the maintenance of the carbonnitrogen balance. In this study we isolated an Arabidopsis mutant (sicy-192) whose cotyledon greening was inhibited by treatments with sugars such as sucrose, glucose, and fructose. In the mutant, the gene encoding plastidic alkaline/neutral invertase (INV-E) was point-mutated at codon 294, with Tyr substituted for Cys (C294Y). Interestingly, the greening of cotyledons in the knock-out INV-E lines was not inhibited by treatment with the sugars. In addition, the knock-out INV-E lines expressing an INV-E:C294Y or INV-E:C294A gene had the same phenotype as sicy-192 mutants, whereas the lines expressing a wildtype INV-E gene had the same phenotype as wild-type plants. A recombinant INV-E:C294Y protein had the same enzymatic activity as a recombinant INV-E protein, suggesting that the Cys-294 residue of INV-E is important for its functions in the chloroplasts. On treatment with sucrose, the expression of photosynthesis-related genes was weaker in seedlings of mutant plants than wild-type seedlings, whereas the activity of nitrate reductase was stronger in the mutant plants than wild-type plants. These findings suggest that Cys-294 of INV-E is associated with the development of the photosynthetic apparatus and the assimilation of nitrogen in Arabidopsis seedlings to control the ratio of sucrose content to hexose content.

The cDNA clone encoding a putative germin-like protein (GLP) was isolated from the halophyte Atriplex lentiformis (Torr.) Wats. The deduced amino acid sequence of the protein showed high homology to those of various plant germins or germin-like proteins. The protein was designated as AlGLP (A. lentiformis germin-like protein). It was found that AlGLP has superoxide dismutase (SOD; EC 1.15.1.1) activity by in-gel SOD assays after immunoprecipitation, and that it is a glycosylated oligomer in native form. The AlGLP was strongly expressed in calli and weakly in roots, but not in stems or leaves. Interestingly, the transcript level in roots was decreased by salt or abscisic acid (ABA) treatment. We present evidence that the accumulation level of AlGLP mRNA in leaves is increased by methyl jasmonate treatment or wounding, and that the induction is suppressed by ABA treatment. These results suggest that AlGLP expression is increased to fulfil H 2 O 2 supply to modify the structure of cell wall in response to wounding.