Abstract
Overexpression of genes that modify gibberellin (GA) metabolism and signaling have been previously shown to produce trees with improved biomass production but highly disturbed development. To examine if more subtle types of genetic modification of GA could improve growth rate and modify tree architecture, we transformed a model poplar genotype (Populus tremula × P. alba) with eight genes, including two cisgenes (intact copies of native genes), four intragenes (modified copies of native genes), and two transgenes (from sexually incompatible species), and studied their effects under greenhouse and field conditions. In the greenhouse, four out of the eight tested genes produced a significant and often striking improvement of stem volume, and two constructs significantly modified the proportion of root or shoot biomass. Characterization of GA concentrations in the cisgenic population that had an additional copy of a poplar GA20-oxidase gene showed elevated concentrations of 13-hydroxylated GAs compared to wild-type poplars. In the field, we observed growth improvement for three of the six tested constructs, but it was significantly greater for only one of the constructs, a pRGL:GA20-oxidase intragene. The greenhouse and field responses were highly variable, possibly to due to cross-talk among the GA pathway and other stress response pathways, or due to interactions between the cisgenes and intragenes with highly similar endogenes. Our results indicate that extensive field trials, similar to those required for conventional breeding, will be critical to evaluating the value and pleiotropic effects of GA-modifying genes.
Highlights
Gibberellins (GAs) are a large class of phytohormones that play fundamental and diverse roles in the regulation of plant growth and development (Olszewski et al 2002; Achard and Genschik 2009; Hedden and Thomas 2012)
We investigated the effects of several kinds of GA-modifying cisgenes, intragenes, and transgenes on the growth rate and architecture of poplars grown in both greenhouse and field environments (Fig. 2)
We report on a number of overlapping studies, conducted over 6 years, that investigated the extent to which a variety of genetic perturbations of GA metabolism or signaling might be useful tools for modifying tree growth rate and allocation of biomass
Summary
Gibberellins (GAs) are a large class of phytohormones that play fundamental and diverse roles in the regulation of plant growth and development (Olszewski et al 2002; Achard and Genschik 2009; Hedden and Thomas 2012). GA1 from the predominant early 13-hydroxylation pathway, and GA4 from the non-13-hydroxylation pathway, are the probable bioeffectors (Fig. 1; Rood and Hedden 1994; Olszewski et al 2002). Their formation is catalyzed by GA20ox and GA3ox, and they are inactivated by GA2ox, and all of these enzymes are 2-oxoglutarate dependent dioxygenases (2-ODDs) and are subject to feedback regulation (reviewed by Yamaguchi and Kamiya 2000; GallegoGiraldo et al 2008; Hedden and Thomas 2012). The feedback regulations of these 2-ODD encoding genes play a central role for maintaining GA homeostasis
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