Satoru Mita

Negative Regulation in the Expression of a Sugar-Inducible Gene in Arabidopsis thaliana (A Recessive Mutation Causing Enhanced Expression of a Gene for [beta]-Amylase)

Expression of a [beta]-amylase gene of Arabidopsis thaliana (AT[beta]-Amy)is regulated by sugars. We identified a mutant, hba1, in which the level of expression of AT[beta]-Amy in leaves of plants that had been grown in a medium with 2% sucrose was significantly higher than that in wild-type plants. Higher than wild-type levels of [beta]-amylase in hba1 plants depended on the presence of 1 to 2% sucrose or 1% glucose in the medium, whereas leaves of mutant plants grown with higher levels of sugars had [beta]-amylase activities similar to those in leaves of wild-type plants. The hba1 phenotype was recessive and did not affect levels of sugars and starch in leaves. It is proposed that expression of AT[beta]-Amy is regulated by a combination of both positive and negative factors, dependent on the level of sugars, and that HBA1 might function to maintain low-level expression of AT[beta]-Amy until the level of sugars reaches some high level. Results of crosses of hba1 plants with transgenic plants that harbored an AT[beta]-Amy:GUS transgene with 1587 bp of the 5[prime]-upstream region suggested that HBA1 affects expression of AT[beta]-Amy in trans. The hba1 plants also had growth defects and elevated levels of anthocyanin in their petioles. However, sugar-regulated changes in levels of several mRNAs other than [beta]-amylase mRNA were unaffected in hba1 plants, suggesting that only a subset of sugar-regulated genes is under the control of HBA1.

The Key Role of Phloroglucinol O-Methyltransferase in the Biosynthesis of Rosa chinensis Volatile 1,3,5-Trimethoxybenzene

1,3,5-Trimethoxybenzene is a key component of the Chinese rose odor. This compound is synthesized in three successive methylation steps from phloroglucinol, the initial precursor. A novel, to our knowledge, phloroglucinol O-methyltransferase (POMT) characterized here methylates the first step to produce the intermediate 3,5-dihydroxyanisole, while two previously described orcinol O-methyltransferases catalyze the subsequent steps. We isolated POMT from rose petals and determined partial amino acid sequences of the purified enzyme. The full-length POMT cDNA was isolated and expressed in Escherichia coli. Both the native and recombinant POMT exhibited substrate specificity for phloroglucinol. POMT was expressed specifically in floral organs, in accordance with its role as a key enzyme in the synthesis of rose floral scent compounds.

Two O-Methyltransferases isolated from flower petals of Rosa chinensis var. spontanea involved in scent biosynthesis

Rosa chinensis var. spontanea predominantly emits 1,3,5-trimethoxybenzene together with methyleugenol and isomethyleugenol as minor floral scent compounds. Two O-methyltransferases (OMTs), designated as RcOMT1 and RcOMT2, were isolated from rose flower petals using homology-based screening strategies. RcOMT1 efficiently methylated eugenol and isoeugenol to yield volatile methyleugenol and isomethyleugenol, respectively. Furthermore, the mRNA transcripts of RcOMT1 were highly expressed in floral organs and the expression pattern coincided with intracellular content changes of methyleugenol and isomethyleugenol in rose flowers. In contrast, RcOMT2, which shows 94% similarity with caffeic acid O-methyltransferase (COMT) of Prunus amygdalus, was expressed in all tissues tested and had the highest activity with caffeic acid, a typical substrate for COMT. However, this COMT-like OMT also showed some degrees of activity with all three putative precursors of 1,3,5-trimethoxybenzene.