Activation Tagging of a Dominant Gibberellin Catabolism Gene (GA 2-oxidase) from Poplar That Regulates Tree Stature

Abstract

We identified a dwarf transgenic hybrid poplar (Populus tremula × Populus alba) after screening of 627 independent activation-tagged transgenic lines in tissue culture, greenhouse, and field environments. The cause of the phenotype was a hyperactivated gene encoding GA 2-oxidase (GA2ox), the major gibberellin (GA) catabolic enzyme in plants. The mutation resulted from insertion of a strong transcriptional enhancer near the transcription start site. Overexpression of the poplar GA2ox gene (PtaGA2ox1) caused hyperaccumulation of mRNA transcripts, quantitative shifts in the spectrum of GAs, and similarity in phenotype to transgenic poplars that overexpress a bean (Phaseolus coccineus) GA2ox gene. The poplar PtaGA2ox1 sequence was most closely related to PsGA2ox2 from pea (Pisum sativum) and two poorly known GA2oxs from Arabidopsis (AtGA2ox4 and AtGA2ox5). The dwarf phenotype was reversible through gibberellic acid application to the shoot apex. Transgenic approaches to producing semidwarf trees for use in arboriculture, horticulture, and forestry could have significant economic and environmental benefits, including altered fiber and fruit production, greater ease of management, and reduced risk of spread in wild populations.

Transformation as a Tool for Genetic Analysis in Populus

“…Results from two pilot studies in Populus have demonstrated the effectiveness of activation tagging (Busov et al, 2003;Harrison et al, 2007). A population of 627 events was used to identify the first tagged gene in a tree (Busov et al, 2003).…”

“…A population of 627 events was used to identify the first tagged gene in a tree (Busov et al, 2003). This population is under further investigation and results to date indicate a high rate (∼7%) of mutant discovery based on (1) morphological inspection; (2) successful positioning of the tag in the genome for several dozen events; (3) activation of proximal genes up to 10 kbp; and (4) numerous successful recapitulations of the original phenotypes when the candidates are re-inserted under the control of a strong promoter.…”

Green Revolution Trees: Semidwarfism Transgenes Modify Gibberellins, Promote Root Growth, Enhance Morphological Diversity, and Reduce Competitiveness in Hybrid Poplar

“…These seven GAs have been characterized from Populus spp. and include 2-hydroxylated GAs that are abundant in shoots and altered by GA2ox (Pearce et al, 2002;Busov et al, 2003Busov et al, , 2006. GA levels were analyzed from two ramets per event from the high-density field studies; for each tree, dormant shoots were taken and rooted in a greenhouse, then after plants were approximately 0.5 m in height and growing well, two to five young leaves were collected just below the shoot tip.…”

“…Transformation was performed using Agrobacterium tumefaciens strain C58/pMP90 (GV3101) essentially as described by Filichkin et al (2007). After regeneration in selection medium, all transgenic events were verified for presence using PCR as described previously (Busov et al, 2003(Busov et al, , 2006.…”

“…Semidwarfism has had substantial benefits for fruit tree production, where it enables earlier fruit bearing, higher yields, and easier harvests in orchards (Battisini and Battisini, 2005). Semidwarf woody species are also extensively used in ornamental horticulture, where they allow more compact forms to be fit into small areas around homes and on streets and reduce the need for pruning to avoid interference with structures and transmission lines (Busov et al, 2003).…”