Asuka Nishimura

A protocol for Agrobacterium -mediated transformation in rice

Agrobacterium-mediated transformation of rice is an important method that has been widely adopted by many laboratories. However, because current approaches rely on culture systems, routine protocols have been established only in japonica rice, especially those varieties with higher regeneration potential. Some very efficient methods have been developed for japonica varieties that enable high-throughput functional analysis in rice; however, many elite japonica, and most indica, varieties are difficult to regenerate, leading to low transformation efficiencies. Much effort has been devoted to improving transformation efficiency for all rice genotypes. Here, we describe an Agrobacterium-mediated rice transformation method that is applicable to easily cultured varieties in addition to elite japonica varieties that are more difficult to culture. Using this method, transgenic rice plants can be obtained in about 2–3 months with a transformation frequency of 30–50%, both in easily cultured varieties and recalcitrant elite japonica rice.

The promoter of rbcS in a C3 plant (rice) directs organ-specific, light-dependent expression in a C4 plant (maize), but does not confer bundle sheath cell-specific expression

The small subunit of ribulose-bisphosphate carboxylase (Rubisco), encoded by rbcS, is essential for photosynthesis in both C3 and C4 plants, even though the cell specificity of rbcS expression is different between C3 and C4 plants. The C3 rbcS is specifically expressed in mesophyll cells, while the C4 rbcS is expressed in bundle sheath cells, and not mesophyll cells. Two chimeric genes were constructed consisting of the structural gene encoding β-glucuronidase (GUS) controlled by the two promoters from maize (C4) and rice (C3) rbcS genes. These constructs were introduced into a C4 plant, maize. Both chimeric genes were specifically expressed in photosynthetic organs, such as leaf blade, but not in non-photosynthetic organs. The expressions of the genes were also regulated by light. However, the rice promoter drove the GUS activity mainly in mesophyll cells and relatively low in bundle sheath cells, while the maize rbcS promoter induced the activity specifically in bundle sheath cells. These results suggest that the rice promoter contains some cis-acting elements responding in an organ-pecific and light-inducible regulation manner in maize but does not contain element(s) for bundle sheath cell-specific expression, while the maize promoter does contain such element(s). Based on this result, we discuss the similarities and differences between the rice (C3) and maize (C4) rbcS promoter in terms of the evolution of the C4 photosynthetic gene.

The Conserved KNOX Domain Mediates Specificity of Tobacco KNOTTED1-Type Homeodomain Proteins

Overproduction of the tobacco KNOTTED1-type homeodomain proteins NTH1, NTH15, and NTH23 in transgenic tobacco plants causes mild, severe, and no morphological alterations, respectively. The deduced amino acid sequences of the homeodomains and adjacent ELK domains are highly conserved, and the N-terminal KNOX domains also are moderately conserved. To investigate the contributions of both the conserved and divergent regions to the severity of morphological alterations, we generated chimeric proteins by exchanging different regions of NTH1, NTH15, and NTH23. The severity of the abnormal phenotype was dependent upon the synergistic action of both the N terminus, containing the KNOX domain, and the C terminus, containing the ELK homeodomain. Detailed analysis focusing on the C terminus revealed that the C-terminal half of the ELK domain is more effective in inducing the abnormal phenotypes than are the homeodomains. For the N terminus, severe morphological alterations were induced by exchanging a part of the KNOX domain of NTH1 with the corresponding region of NTH15. This limited region in the KNOX domain of all homeodomain proteins includes a predicted α-helical region, but only that in NTH15 is predicted to form a typical amphipathic structure. We discuss the possibility, based on these results, that the secondary structure of the KNOX domain is important for the induction of abnormal morphology in transgenic tobacco plants.

The homeobox gene NTH23 of tobacco is expressed in the basal region of leaf primordia

We reported isolation and characterization of a homeobox gene from tobacco, NTH23. The homeodomain structure of NTH23 was highly homologous to the same regions of class 2 genes of the KN1-type homeobox (sharing more than 85% amino acid identity), but was less similar to class 1 genes of KN1-type. RNA gel blot analysis revealed that NTH23 was expressed in all organs we tested although the gene is primarily expressed in young leaves. To determine more precisely the spatial expression pattern of NTH23 in tobacco, a chimeric NTH23::GUS fusion gene was introduced into tobacco. The signal of GUS activity was observed at the basal part of leaf blade primordia in the NTH23::GUS transgenic tobacco plants. This observation suggests the possibility that NTH23 may be important for the lateral growth of leaf blades.

Rice Organelle Genomics: Approaches to Genetic Engineering and Breeding

Although organelle (mitochondria and plastid) genomes have less than 1% of the genes in the nucleus, they encode essential genes, such as those involved in energy production, respiration, and photosynthesis, and genes that control agronomically important characteristics such as cytoplasmic male sterility. Organelle genomes have high copy numbers in each cell (one to two orders of magnitude greater than in the nucleus) and are characterized by maternal inheritance. To know functions of genes encoded in the organelle genomes or to develop new plants adapted to various severe environments, genetic engineering of organelle genomes is one of the promising approaches. However, modifying the mitochondrial or plastid genomes in rice is presently impossible or difficult. Here, we discuss the characteristic features of these genomes and recent attempts at plastid transformation.

Isolation of highly thermostable β-xylosidases from a hot spring soil microbial community using a metagenomic approach

Abstract The DNA extracted from a high-temperature environment in which micro-organisms are living will be a good source for the isolation of thermostable enzymes. Using a metagenomic approach, we aimed to isolate thermostable β-xylosidases that will be exploited for biofuel production from lignocellulosic biomass. DNA samples obtained from the soil near a spout of a hot spring (70°C, pH7.2) were subjected to sequencing, which generated a total of 84.2 Gbp with 967,925 contigs of >500 bp in length. Similarity search for β-xylosidase in the contigs revealed the presence of 168 candidate sequences, each of which may have arisen from more than one gene. Individual genes were amplified by PCR using sequence-specific primers. The resultant DNA fragments were cloned and introduced into Escherichia coli BL21 Star(DE3). Consequently, 269 proteins were successfully expressed in the E. coli cells and then examined for β-xylosidase activity. A total of 82 proteins exhibited β-xylosidase activity at 50°C, six of which retained the activity even at 90°C. Out of the six, three proteins were originated from a single candidate sequence, AR19M-311. An amino acid sequence comparison suggested the amino acid residues that appeared to be crucial for thermal stability of the enzymes.