Gibberellin levels are determined by genes that encode positive regulators such as GIBBERELLIN INSENSITIVE DWARF 1 (GID1) [27] or negative regulators such as GA-INSENSITIVE (GAI) and REPRESSOR OF GA-1 (RGA) [25]. Hybrid poplar (P. tremula × P. alba) over-expressing gai (mutant gene with a 51-bp in-frame deletion) had narrow compact crowns with shorter main stems and branches than the wild type. By contrast, those that expressed the wild-type form of the gene (GAI) had phenotypes that were not significantly different from the non-transgenic controls, except for early and high frequency of flowering (Table 1) [25]. DELLA proteins, which contain an N-terminal DELLA (asp-glu-leu-leu-ala) domain essential for GA-dependent proteasomal degradation, repress GA responses [28]. The GID1-GA complex down-regulates DELLA repressor proteins, consequently stimulating plant growth and development [29]. Hybrid aspen (P. tremula × P. tremuloides) over-expressing GID1 were about 40% taller than the wild type after nine weeks in the greenhouse [19].
mutant gene
n-frame deletion
51-bp
bp
Hybrid aspen (P. tremula × P. tremuloides) over-expressing GID1
taller
about 40%
%