Overexpression of the Gibberellin 2-Oxidase Gene from Camellia lipoensis Induces Dwarfism and Smaller Flowers in Nicotiana tabacum

Gibberellins (GAs) are plant hormones that control many aspects of plant growth and development. Gibberellin 2-oxidase plays an important role in determining the level of bioactive GAs. In this study, we isolated three GA2ox genes (ClGA2ox1-3) from Camellia lipoensis Chang et Xu. The results of a quantitative real-time reverse transcription polymerase chain reaction analysis indicated that ClGA2ox1-3 may play a tissue-specific role in plant development. The transcript of ClGA2ox1 was more abundant in the stem and apex, ClGA2ox2 was highly expressed in mature leaves, and ClGA2ox3 was more abundant in roots. We produced transgenic plants of Nicotiana tabacum L. by overexpressing the ClGA2ox1-3 genes. Plants with overexpressed ClGA2ox1 or ClGA2ox3 genes exhibited dwarf phenotypes, including reduced growth, delayed flowering, and smaller, rounder, and darker green leaves. All of the transgenic plants overexpressing the ClGA2ox1 gene bloomed normally, but their flowers were half the size of the control plants. Plants overexpressing ClGA2ox3 could be categorized into two classes: moderately dwarfed and severely dwarfed. The ClGA2ox2 gene had little effect on the morphological characterization of transgenic plants. Quantitative real-time PCR analysis showed that the ClGA2ox3 expression level was generally correlated with the level of dwarfism. The endogenous level of bioactive GA(4) and GA(1) largely decreased in transgenic plants and was generally correlated with the degree of dwarfism in transgenic plants with the ClGA2ox1 or ClGA2ox3 gene. The application of GA(3) rescued the dwarf phenotype of transformants, indicating that the GA signaling pathway might function normally in transgenic plants. Therefore, morphological changes in transgenic plants may result from a decrease in the endogenous level of bioactive GAs. Additionally, the possibility of molecular breeding for plant form alternation in Camellia plants by genetically engineering the GA metabolic pathway is discussed.