Abstract
Semidwarfism has been used extensively in row crops and horticulture to promote yield, reduce lodging, and improve harvest index, and it might have similar benefits for trees for short-rotation forestry or energy plantations, reclamation, phytoremediation, or other applications. We studied the effects of the dominant semidwarfism transgenes GA Insensitive (GAI) and Repressor of GAI-Like, which affect gibberellin (GA) action, and the GA catabolic gene, GA 2-oxidase, in nursery beds and in 2-year-old high-density stands of hybrid poplar (Populus tremula × Populus alba). Twenty-nine traits were analyzed, including measures of growth, morphology, and physiology. Endogenous GA levels were modified in most transgenic events; GA(20) and GA(8), in particular, had strong inverse associations with tree height. Nearly all measured traits varied significantly among genotypes, and several traits interacted with planting density, including aboveground biomass, root-shoot ratio, root fraction, branch angle, and crown depth. Semidwarfism promoted biomass allocation to roots over shoots and substantially increased rooting efficiency with most genes tested. The increased root proportion and increased leaf chlorophyll levels were associated with changes in leaf carbon isotope discrimination, indicating altered water use efficiency. Semidwarf trees had dramatically reduced growth when in direct competition with wild-type trees, supporting the hypothesis that semidwarfism genes could be effective tools to mitigate the spread of exotic, hybrid, and transgenic plants in wild and feral populations.
Highlights
Semidwarfism has been used extensively in row crops and horticulture to promote yield, reduce lodging, and improve harvest index, and it might have similar benefits for trees for short-rotation forestry or energy plantations, reclamation, phytoremediation, or other applications
All of the constructs showing very high expression, including GA2, GA Insensitive (GAI)-M, and Repressor of GAI-Like (RGL)-1, were driven by the 35S promoter; in contrast, events driven by the native Arabidopsis (Arabidopsis thaliana) GAI promoter had much lower expression levels (Fig. 1)
The strongest detected expression from the native GAI promoter was with the wild-type GAI gene (GAI-D); expression was considerably lower with the mutant gai gene (GAI-X)
Summary
Semidwarfism has been used extensively in row crops and horticulture to promote yield, reduce lodging, and improve harvest index, and it might have similar benefits for trees for short-rotation forestry or energy plantations, reclamation, phytoremediation, or other applications. Semidwarfism is a valuable trait in many crop species and agricultural environments In cereal crops, it can result in decreased lodging, increased yield, and improved harvest index (Dalrymple, 1985; Hedden, 2003). Against the current orthodoxy of forest tree breeding, where height growth is emphasized, semidwarfism might have benefits for wood and biomass production (Bradshaw and Strauss, 2001). Such trees could be useful if they were less prone to wind throw due to their shorter, stockier forms and expected greater allocation to roots. GA modification has significant effects on plant biochemistry, changing the amounts and distribution of a wide variety of metabolites in shoots and roots (Rossetto et al, 2003; Chen et al, 2004; Busov et al, 2006)
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