Opportunities for Innovation in Genetic Transformation of Forest Trees

Abstract

The incorporation of DNA into plant genomes followed by regeneration of non-chimeric stable plants (transformation) remains a major challenge for most plant species. Forest trees are particularly difficult as a result of their biochemistry, aging, desire for clonal fidelity, delayed reproduction, and high diversity. We review two complementary approaches to transformation that appear to hold promise for forest trees.

Keywords: Agrobacterium; BABY BOOM; Populus; WUSCHEL; embryogenesis; organogenesis; regeneration; transformation.

FIGURE 1

Portion of genetic regulatory network with demonstrated ability to enhance regeneration via overexpression. Green nodes represent phytohormones and green arrows indicate transactivation that results from upstream hormone-triggered cascades (not shown). Auxin leads to PLT upregulation through a relatively well-characterized cascade (Horstman et al., 2014), as do cytokinins for B-type ARRs (Xie et al., 2018). Blue nodes represent genes which have been shown to enhance regeneration when overexpressed, and blue arrows indicate direct targets of their transactivation activity (detailed in Supplementary Table 1).

FIGURE 2

Virtual plant showing the diverse organs/meristems targeted by in planta transformation. (A) Primary meristems in vegetative buds leading to the production of transgenic shoots after agro-inoculation. (B) Primary meristems in germinating seeds or seedlings leading to T0 chimeric plants and T1 transgenic plants. (C) Reproductive cells in floral buds or flowers leading to T0 transgenic plants. (D) Cambium secondary meristems leading to transformed wood sectors.