Long-distance transport of Gibberellic Acid Insensitive mRNA in Nicotiana benthamiana

BACKGROUND: The Gibberellic Acid (GA) signal is governed by the GAI (Gibberellic Acid Insensitive) repressor, which is characterized by a highly conserved N-terminal DELLA domain. Deletion of the DELLA domain results in constitutive suppression of GA signaling. As the GAI transcript is transportable in phloem elements, a Δ-DELLA GAI (gai) transgenic stock plant can reduce the stature of a scion through transport of gai mRNA from the stock. However, little is known about the characteristics of a scion on a gai stock. RESULTS: Arabidopsis Δ-DELLA GAI (gai) was fused with a T7 epitope tag and expressed under the control of a companion cell-specific expression promoter, Commelina yellow mottle virus promoter (CoYMVp), to enhance transport in the phloem. The CoYMVp:Atgai-T7 (CgT) transgenic Nicotiana benthamiana exhibited a dwarf phenotype and lower sensitivity to GA enhancement of shoot stature. A wild-type (WT) scion on a CgT stock contained both Atgai-T7 mRNA and the translated product. Microarray analysis to clarify the effect of the CgT stock on the gene expression pattern in the scion clearly revealed that the WT scions on CgT stocks had fewer genes whose expression was altered in response to GA treatment. An apple rootstock variety, Malus prunifolia, integrating CoYMVp:Atgai moderately reduced the tree height of the apple cultivar scion. CONCLUSIONS: Our results demonstrate that Atgai mRNA can move from companion cells to sieve tubes and that the translated product remains at the sites to which it is transported, resulting in attenuation of GA responses by reducing the expression of many genes. The induction of semi-dwarfism in an apple cultivar on root stock harbouring Atgai suggests that long-distance transport of mRNA from grafts would be applicable to horticulture crops.