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Research topics

Outline

Based on experimental approaches of molecular biology, we are focusing on two topics as follows:

  1. Dynamic regulation of nuclear tRNA genes in plants
  2. Role of γ-aminobutyric acid (GABA), non-protein amino acid, that is rich in plant cells

Arabidopsis and rice plants that we are mainly using in our projects are known as model plants. Because their genome analyses are most progressed among higher plants, genome information, EST clones, various genetic mutants, efficient transformation system are all available, which are allowing us to do effective approaches to our topics.

Since 2008 Genomics of Agricultural Innovation (Ministry of Agriculture, Forestry and Fisheries of Japan) has been started as a new rice genome project. Our research plan (i.e., production of GABA-enriched rice), was selected in the subproject of the Development of Health Promoting Transgenic Rice, thus our performing study of molecular breeding on rice that accumulates GABA paid much more attention as healthy substance.


Exploring unknown function of the enzymes for plant precursor tRNA splicing

Two kinds of nuclear genes encoding tRNA-Met and tRNA-Tyr in plants, respectively, are interrupted by introns. Eukaryotic splicing reaction of the precursor tRNAs is unique because enzymatic cleavage and ligation are involved. It is believed that machinery of eukaryotic RNA splicing is occurred in nucleus. In collaboration with Prof. Beier’s lab (Wuerzburg, Germany), we have disclosed that plant tRNA splicing enzymes are localized predominantly in chloroplasts or mitochondria, using green fluorescent protein (GFP) as a marker.

Now we are concentrating on functional analysis of tRNA ligase that is found to be imported to chloroplasts (see below).

GFP

Development of GABA-enriched rice

Recently, lifestyle-related diseases such as diabetes, obesity, hyperlipidemia and hypertension have become a severe problem throughout the world, in terms of both human health and increasing medical expenses. GABA, a ubiquitous non-protein amino acid, is an inhibitory neurotransmitter that has an antihypertensive effect in mammals. GABA is synthesized from L-glutamate by cellular glutamate decarboxylase (GAD).

Our lab has reported that rice GAD2ΔC, lacking a C-terminal coding region, conferred an increase in enzymatic activity and its over-expression resulted in extreme GABA accumulation in the calli. In order to accumulate high levels of GABA in rice grains, we performed seed-specific expression of GAD2ΔC using a rice endosperm-specific promoter (glutelin gene, GluB-1) to produce stable lines. Amino acid analysis showed that milled rice grains from transformed rice contain three- to 30-fold higher GABA levels than those from non-transformed rice.

We are currently evaluating the effect of a combination of GAD2ΔC over-expression and knock-down of GABA-T in seeds in an effort to establish lines that stably produce GABA-rich rice grains.