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Title:   Author  item
                                            LIANG, YIN-CHIH - University Of Californiaitem
                                            REID, MICHAEL - University Of Californiaitem
                                            Jiang, Cai-Zhong
  Submitted to: Horticulture Research
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 11/5/2014
                                    Publication  Date: 12/3/2014
                                    Publication URL: http://www.nature.com/articles/hortres201461Citation: Liang, Y., Reid, M.S., Jiang, C. 2014. Controlling plant architecture by manipulation of gibberellic acid signalling in petunia. Horticulture Research. doi: 10.1038/hortres.2014.61.Interpretive Summary: Controlling plants’ growth by deactivating their response to plant hormone gibberellic acid (GA) may facilitate development of compact ornamentals. GA regulates multiple processes including stem elongation. To prevent undesirable height, called leggy plants, ornamental plants are often sprayed with inhibitors of GA synthesis. Given the high cost, potential environmental impact and possible phytotoxicity of these chemicals, we investigated interfering with GA signalling by using molecular techniques as an alternative approach. We isolated three putative GID1 genes (PhGID1A, PhGID1B, and PhGID1C) encoding GA receptors from petunia. Virus-induced gene silencing (VIGS) of these genes results in stunted growth, dark-green leaves and late-flowering.  We also isolated the gai mutant gene (gai-1) from Arabidopsis. We have generated transgenic petunia plants in which the gai mutant protein is over-expressed under the control of a dexamethasone-inducible promoter. This system permits induction of the dominant Arabidopsis gai mutant gene at a desired stage of plant development in petunia plants by the application of dexamethasone (Dex). The induction of gai in Dex-treated T1 petunia seedlings caused dramatic growth retardation with short internodes. Our molecular approaches show potential for modifying ornamental plant architecture with precise timing and minimal environmental impact.
                                Technical Abstract: Gibberellic acid (GA), a plant hormone, regulates many crucial growth and developmental processes, including seed germination, leaf expansion, induction of flowering and stem elongation. A common problem in the production of ornamental potted plants is undesirably tall growth, so inhibitors of gibberellic acid (GA) biosynthesis including A-rest (ancymidol), B-nine (daminozide), Bonzi (paclobutrazol), Cycocel (chlormequat chloride), and Sumagic (uniconazole), are commonly used to control plant height. However, the ornamentals industry is faced with strict regulatory measures guiding chemical usage and the effects of such chemicals on human health and the environment. To provide an alternative strategy for managing plant architecture and preventing postharvest ‘stretching’, we propose to investigate genetic manipulation of the GA response pathway.
In the current model of GA signalling, GA binds to a soluble GID1 receptor, which in turn binds to the DELLA repressor protein. The bound DELLA protein is then targeted for degradation by the 26S proteasome, thus relieving DELLA-mediated repression of GA-dependent growth processes.  Previous researchers showed that heterologous expression of the Arabidopsis gai (GA-insensitive, the DELLA protein) mutant gene reduced plant height and altered GA response in transgenic rice, tobacco, chrysanthemum and apple. However, the native or constitutive promoters used in these studies resulted in permanent inhibition of GA responses, which resulted in severe dwarfing and other undesirable phenotypes. To use this approach in practice would require that expression of the mutant gene be coupled to an inducible system, such as the dexamethazone-inducible promoter or the alcohol-inducible promoter which permits the expression of transgenes to be turned on or off at desired stages of development of an organism or tissue. 
This study tested the hypothesis that interfering with GA signalling by silencing GID1-like receptor genes using the virus-induced gene silencing technology (VIGS), with GA signal transduction by over-expression of the Arabidopsis gai mutant gene under the control of the Dex-inducible promoter, would modulate plant growth and architecture in petunia. We isolated three putative GID1 genes (PhGID1A, PhGID1B, and PhGID1C) encoding GA receptors from petunia. Silencing of these genes results in stunted growth, dark-green leaves and late-flowering.  We also isolated the gai mutant gene (gai-1) from Arabidopsis. We have generated transgenic petunia plants in which the gai mutant protein is over-expressed under the control of a dexamethasone-inducible promoter. The induction of gai in dexamethasone (Dex)-treated T1 petunia seedlings caused dramatic growth retardation with short internodes. This system permits induction of the dominant Arabidopsis gai mutant gene at a desired stage of plant development in petunia plants and provides an alternative approach to control ornamental plant architecture using molecular techniques.


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