Abstract
BackgroundThe 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.ResultsArabidopsis 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.ConclusionsOur 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.
Highlights
The Gibberellic Acid (GA) signal is governed by the Gibberellic acid insensitive (GAI) (Gibberellic Acid Insensitive) repressor, which is characterized by a highly conserved N-terminal DELLA domain
Atgai transgenic tobacco exhibits dwarfism and lower sensitive to GA3 Transgenic plants over expressing Atgai and showing a dwarf phenotype have been demonstrated in tomato, tobacco and apple [16,27,28]
Since Atgai mRNA has been shown to be transportable through phloem, a T-DNA construct harboring a construct expressing Atgai (CgT) driven by a companion cell-specific promoter (Figure 1) was integrated into N. benthamiana by Agrobacterium transformation
Summary
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. The GA-deficient Arabidopsis thaliana ga mutant that lacks ent-kaurene synthetase A, an enzyme in the GA biosynthesis pathway, exhibits a characteristic severe dwarf phenotype [1]. Mutants such as ga are GA-sensitive dwarf mutants that have been observed in a number of different plant species and typically carry recessive mutations that reduce the activity of GA biosynthesis enzymes [2]. The gai open reading frame carries a small in-frame deletion mutation and encodes an altered product, a mutant gai protein that lacks a 17-amino-acid segment, known as the DELLA domain, named after its first five amino acids. Expression of Arabidopsis gai in rice yields a dwarf phenotype, suggesting that GAI is sufficiently conserved between plant families to allow it to function [13]
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