Transgenic modification of gai or rgl1 causes dwarfing and alters gibberellins, root growth, and metabolite profiles in Populus

In Arabidopsis and other plants, gibberellin (GA)-regulated responses are mediated by proteins including GAI, RGA and RGL1-3 that contain a functional DELLA domain. Through transgenic modification, we found that DELLA-less versions of GAI (gai) and RGL1 (rgl1) in a Populus tree have profound, dominant effects on phenotype, producing pleiotropic changes in morphology and metabolic profiles. Shoots were dwarfed, likely via constitutive repression of GA-induced elongation, whereas root growth was promoted two- to threefold in vitro. Applied GA₃ inhibited adventitious root production in wild-type poplar, but gai/rgl1 poplars were unaffected by the inhibition. The concentrations of bioactive GA₁ and GA₄ in leaves of gai- and rgl1-expressing plants increased 12- to 64-fold, while the C₁₉ precursors of GA₁ (GA₅₃, GA₄₄ and GA ₁₉) decreased three- to ninefold, consistent with feedback regulation of GA 20-oxidase in the transgenic plants. The transgenic modifications elicited significant metabolic changes. In roots, metabolic profiling suggested increased respiration as a possible mechanism of the increased root growth. In leaves, we found metabolite changes suggesting reduced carbon flux through the lignin biosynthetic pathway and a shift towards allocation of secondary storage and defense metabolites, including various phenols, phenolic glucosides, and phenolic acid conjugates.