New insights into the regulation of greening and carbon-nitrogen balance by sugar metabolism through a plastidic invertase.

Since the photosynthetic apparatus of plants contains a massive amount of nitrogen, the regulation of its development by sugar signals is important to the maintenance of the carbon-nitrogen balance. Recently, we isolated a new Arabidopsis mutant, sicy (sugar-inducible cotyledon yellow)-192, whose cotyledons were prevented from greening by treatment with sucrose. On treatment with sucrose, the expression of photosynthesis- and nitrogen assimilation-related genes was respectively weaker and stronger in the mutant seedlings than the wild-type seedlings. In the mutants, the gene encoding plastidic alkaline/neutral (A/N) invertase (INV-E) was point-mutated at codon 294, with Tyr substituted for Cys (C294Y). These findings provide new insights into the regulation of greening and carbon-nitrogen balance by sugar metabolism through INV-E in plastids. In this addendum, we describe the phenotypes of sicy-192 on treatment with sucrose in more detail, and propose a possible relationship among sugar metabolism through INV-E, plastid-to-nucleus retrograde signaling, and ethylene, a plant hormone, in the regulation of plant development and metabolism.

Point mutation of a plastidic invertase inhibits development of the photosynthetic apparatus and enhances nitrate assimilation in sugar-treated Arabidopsis seedlings.

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 carbon-nitrogen 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 wild-type 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.

Posttranscriptional regulation by the upstream open reading frame of the phosphoethanolamine N-methyltransferase gene.

Phosphoethanolamine N-methyltransferase (PEAMT) is involved in choline biosynthesis in plants. The 5' untranslated region (UTR) of several PEAMT genes was found to contain an upstream open reading frame (uORF). We generated transgenic Arabidopsis calli that expressed a chimeric gene constructed by fusing the 5' UTR of the Arabidopsis PEAMT gene (AtNMT1) upstream of the beta-glucuronidase gene. The AtNMT1 uORF was found to be involved in declining levels of the chimeric gene mRNA and repression of downstream beta-glucuronidase gene translation in the calli when the cells were treated with choline. Further, we discuss the role of the uORF.

Similar regulation patterns of choline monooxygenase, phosphoethanolamine N-methyltransferase and S-adenosyl-L-methionine synthetase in leaves of the halophyte Atriplex nummularia L.

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.