Masahiro Otani

Overexpression of the gibberellin 2-oxidase gene from Torenia fournieri induces dwarf phenotypes in the liliaceous monocotyledon Tricyrtis sp.

Gibberellins (GAs) are the plant hormones that control many aspects of plant growth and development, including stem elongation. Genes encoding enzymes related to the GA biosynthetic and metabolic pathway have been isolated and characterized in many plant species. Gibberellin 2-oxidase (GA2ox) catalyzes bioactive GAs or their immediate precursors to inactive forms; therefore, playing a direct role in determining the levels of bioactive GAs. In the present study, we produced transgenic plants of the liliaceous monocotyledon Tricyrtis sp. overexpressing the GA2ox gene from the linderniaceous dicotyledon Torenia fournieri (TfGA2ox2). All six transgenic plants exhibited dwarf phenotypes, and they could be classified into two classes according to the degree of dwarfism: three plants were moderately dwarf and three were severely dwarf. All of the transgenic plants had small or no flowers, and smaller, rounder and darker green leaves. Quantitative real-time reverse transcription-polymerase chain reaction (PCR) analysis showed that the TfGA2ox2 expression level generally correlated with the degree of dwarfism. The endogenous levels of bioactive GAs, GA1 and GA4, largely decreased in transgenic plants as shown by liquid chromatography-mass spectrometry (LC-MS) analysis, and the level also correlated with the degree of dwarfism. Exogenous treatment of transgenic plants with gibberellic acid (GA3) resulted in an increased shoot length, indicating that the GA signaling pathway might normally function in transgenic plants. Thus, morphological changes in transgenic plants may result from a decrease in the endogenous levels of bioactive GAs. Finally, a possibility of molecular breeding for plant form alteration in liliaceous ornamental plants by genetically engineering the GA metabolic pathway is discussed.

Flower color alteration in the liliaceous ornamental Tricyrtis sp. by RNA interference-mediated suppression of the chalcone synthase gene

Chalcone synthase (CHS) is the key enzyme in an early stage of the flavonoid biosynthetic pathway. In the present study, a full-length cDNA clone for CHS was isolated from flower tepals of the liliaceous ornamental Tricyrtis sp., in which tepals have many reddish-purple spots resulting from accumulation of cyanidin derivatives. The deduced amino acid sequence of the isolated cDNA clone, designated TrCHS1 (accession number AB478624 in the GenBank/EMBL/DDBJ databases), shows 79.4–91.4% identity with those of previously reported CHS genes. An RNA interference (RNAi) construct targeting TrCHS1 was introduced by Agrobacterium-mediated transformation in order to alter the flower color of Tricyrtis sp. Seven transgenic plants that produced flowers could be classified into three types according to flower color phenotype: one transgenic plant had tepals with as many reddish-purple spots as non-transgenic plants (Type I); one had tepals with reduced numbers of reddish-purple spots (Type II); and five had completely white tepals without any spots (Type III). High-performance liquid chromatography analysis showed that tepals of Type III transgenic plants did not accumulate detectable amounts of anthocyanidins. In addition, TrCHS1 mRNA levels in tepals of Type II and Type III transgenic plants decreased substantially compared with non-transgenic plants, as determined by quantitative real-time reverse transcription–polymerase chain reaction analysis. Our results indicate the validity of RNAi suppression of the flavonoid biosynthetic pathway genes for flower color alteration in Tricyrtis sp. To the best of our knowledge, this is the first report on flower color alteration by genetic transformation in monocotyledonous ornamentals.

Suppression of B function strongly supports the modified ABCE model in Tricyrtis sp. (Liliaceae)

B class MADS-box genes play important roles in petal and stamen development. Some monocotyledonous species, including liliaceous ones, produce flowers with petaloid tepals in whorls 1 and 2. A modified ABCE model has been proposed to explain the molecular mechanism of development of two-layered petaloid tepals. However, direct evidence for this modified ABCE model has not been reported to date. To clarify the molecular mechanism determining the organ identity of two-layered petaloid tepals, we used chimeric repressor gene-silencing technology (CRES-T) to examine the suppression of B function in the liliaceous ornamental Tricyrtis sp. Transgenic plants with suppressed B class genes produced sepaloid tepals in whorls 1 and 2 instead of the petaloid tepals as expected. In addition, the stamens of transgenic plants converted into pistil-like organs with ovule- and stigma-like structures. This report is the first to describe the successful suppression of B function in monocotyledonous species with two-layered petaloid tepals, and the results strongly support the modified ABCE model.

Overexpression of the gibberellin 20-oxidase gene from Torenia fournieri resulted in modified trichome formation and terpenoid metabolities of Artemisia annua L.

Gibberellins (GAs) are diterpenoid hormones, control various physiological developments in plants. The role of gibberellins on morphology and secondary metabolite production was examined in Artemisia annua, a medicinal plant that has been acknowledged as a source of artemisinin, an antimalarial compound. Subsequently, the GA20ox gene from Torenia fournieri (TfGA20ox2) was transferred to A. annua by Agrobacterium-mediated transformation. Compared with wild type plants, all nine transgenic plants showed significantly higher plant heights and artemisinin contents. The highest artemisinin content and yield in TfGA20ox2-overexpressing plants was around two-fold higher than wild type. Moreover, transgenic plants had higher numbers of branches (52.4%) and greater branch lengths (60–203%), but smaller leaf size (77.6%). Interestingly, relative to wild type the number and size of glandular trichomes in transgenic leaves was about 30 and 35% higher, respectively. From GC–MS analysis, the proportion of diterpenes in transgenic plant extracts was 1.5-fold lower than those noticed in wild type, while the proportion of sesquiterpenes was increased about 1.6 times when compared to wild type. However, the content proportion of monoterpenes showed a slightly increase, whereas the level of triterpenes showed no variation. In addition, two monoterpenes (eucalyptol and borneol), four sesquiterpenes (α-caryophyllene, β-guaiene, δ-cadinene and β-cubebene) and one triterpenes (isomultiflorenone) were detected only in transgenic extract, whereas d-α-tocopherol, a diterpenoid compound was found only in wild type but not transgenic plant. These results suggested that gibberellins play a significant role in regards to morphology, trichome formation and terpenoid metabolite production in A. annua.

Corrigendum: Suppression of B function strongly supports the modified ABCE model in Tricyrtis sp. (Liliaceae)

Corrigendum: Suppression of B function strongly supports the modified ABCE model in Tricyrtis sp. (Liliaceae)