Forced expression of rice 45S rRNA gene conferred ca. 2-fold increase of above-ground growth in transgenic Arabidopsis . This growth increase was probably brought by cell proliferation, not by cell enlargement. Recent increase in carbon dioxide emissions is causing global climate change. The use of plant biomass as alternative energy source is one way to reduce these emissions. Therefore, reinforcement of plant biomass production is an urgent key issue to overcome both depletion of fossil energies and emission of carbon dioxide. Here, we created transgenic Arabidopsis with a 2-fold increase in above-ground growth by forced expression of the rice 45S rRNA gene using the maize ubiquitin promoter. Although the size of guard cells and ploidy of leaf-cells were similar between transgenic and control plants, numbers of stomata and pavement cells were much increased in the transgenic leaf. This data suggested that cell number, not cell expansion, was responsible for the growth increase, which might be brought by the forced expression of exogenous and full-length 45S rRNA gene. The expression level of rice 45S rRNA transcripts was very low, possibly triggering unknown machinery to enhance cell proliferation. Although microarray analysis showed enhanced expression of ethylene-responsive transcription factors, these factors might respond to ethylene induced by abiotic/biotic stresses or genomic incompatibility, which might be involved in the expression of species-specific internal transcribed spacer (ITS) sequences within rice 45S rRNA transcripts. Further analysis of the mechanism underlying the growth increase will contribute to understanding the regulation of the cell proliferation and the mechanism of hybrid vigor.
Antifungal peptides are a potential group of defense molecules that have been utilized to develop resistance to various plant pathogens. Wasabi defensin (WD) gene (0.5 kb) consists of cysteine-rich peptides that show potent growth inhibition of pathogenic filamentous fungi, such as Botrytis cinerea. Under regulation by the root-specific LjNRT2 or AtNRT2.1 promoter, WD gene was expressed in the roots of transgenic tobacco and tomato plants by Agrobacteriummediated transformation. The regenerated plants showed stable integration of the transgene, with different insertion sites, and the transgene was expressed in the root tissues but not in the leaf tissues. This result confirmed that WD protein accumulated only in the roots of transgenic plants. In a bioassay for resistance to Fusarium oxysporum, all transgenic plants showed increased resistance to the fungus as compared to non-transformed plants. Protein extracts from root and leaf tissues were assayed for antifungal activity and the activity was express as the number of colonies formed per cm 2 (CFU cm −2 ). The CFU values of the root and leaf extracts of control plants did not show significant differences. In contrast, the CFU values of the root extracts of the transgenic plants were significantly lower than those of the leaf extracts and much lower than those of control. These results suggest that LjNRT2 and AtNRT2.1 promoters triggered the antifungal gene expression in the roots and conferred increased resistance to the root pathogen F. oxysporum. In the view of bio-safety, the root-specific expression of the transgene is desirable because the roots of tomato are not edible.
An increase in plant biomass production is desired to reduce emission of carbon dioxide emissions and arrest global climate change because it will provide a more source of energy production than fossil fuels. Recently, we found that forced expression of the rice 45S rRNA gene increased aboveground growth by ca. 2-fold in the transgenic Arabidopsis plants. Here, we created transgenic tobacco plants harboring the rice 45S rRNA driven by the maize ubiquitin promoter (UbiP::Os45SrRNA) or cauliflower mosaic virus 35S promoter (35SP::Os45SrRNA). In 35SP::Os45SrRNA and UbiP::Os45SrRNA transgenic tobacco plants, the leaf length and size were increased compared with control plants, leading to an increase of aboveground growth (dry weight) up to 2-fold at the early stage of seedling development. Conversely, leaf physiological traits, such as photosynthetic capacity, stomatal characteristics, and chlorophylls and RuBisCO protein contents, were similar between the transgenic and control plants. Flow cytometry analysis indicated that the transgenic plants had enhanced cell-proliferation especially in seedling root and leaf primordia. Microarray analysis revealed that genes encoding transcription factors, such as GIGANTEA-like, were more than 2-fold up-regulated in the transgenic plants. Although the mechanism underlying the increased growth has yet to be elucidated, this strategy could be used to increase biomass production in cereals, vegetables, and bio-energy plants.
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