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Plant stature can be strongly modified via regulation of endogenous levels and signalling of the plant hormone gibberellin (GA). Down-regulation of GA can produce semi-dwarf tree varieties with improved qualities such as reduced susceptibility to wind damage, enhanced root growth and more compact cultivation. However, these modifications may have unintended, non-target consequences for defence against herbivores, via either of two mechanisms: (i) reduced biomass production may cause trade-offs with chemical resistance traits, as predicted by the growth-differentiation balance hypothesis, and (ii) altered biomass allocation to either roots or photosynthetic tissues may affect regrowth potential and thus tolerance to defoliation. We studied GA down-regulated (GE) and non-transgenic wild-type hybrid poplar (Populus alba × P. tremula) in an outdoor, aboveground common garden and defoliated half of all replicate trees to simulate defoliation. We then quantified the independent and interactive effects of genotype and defoliation on growth and chemical resistance-related traits, including phenolic glycosides (PGs), condensed tannin and nitrogen. We also calculated tolerance to defoliation as the differential in relative growth between undefoliated and defoliated trees. Our results indicate that two of the four GA down-regulated genotypes had significantly reduced stem height, basal diameter, volume (d²h), total biomass and increased allocation to leaves relative to the wild type. One of those two genotypes also had reduced allocation to roots. One and sometimes both of these same two genotypes also had at least 20% lower levels of condensed tannins and PGs and similar increases in lignin and nitrogen. Tolerance, as calculated by the differential in relative growth between undefoliated and defoliated trees, was similar among all experimental genotypes. However, two GE genotypes flushed fewer leaves in response to defoliation relative to the wild type. Our results indicate that GA down-regulation strongly alters biomass production and allocation in poplar but does not necessarily compromise the ability of these trees to tolerate damage. However, some of the modifications we observed do have the potential to alter non-target resistance traits over time, and warrant further research, especially under plantation conditions.

Genetic down-regulation of gibberellin results in semi-dwarf poplar but few non-target effects on chemical resistance and tolerance to defoliation