Gibberellin-associated cisgenes modify growth, stature and wood properties in Populus

We studied the effects on plant growth from insertion of five cisgenes that encode proteins involved in gibberellin metabolism or signalling. Intact genomic copies of PtGA20ox7, PtGA2ox2,Pt RGL1_1, PtRGL1_2 and PtGAI1 genes from the genome-sequenced Populus trichocarpa clone Nisqually-1 were transformed into Populus tremula × alba (clone INRA 717-1B4), and growth, morphology and xylem cell size characterized in the greenhouse. Each cisgene encompassed 1-2 kb of 5' and 1 kb of 3' flanking DNA, as well as all native exons and introns. Large numbers of independent insertion events per cisgene (19-38), including empty vector controls, were studied. Three of the cisgenic modifications had significant effects on plant growth rate, morphology or wood properties. The PtGA20ox7 cisgene increased rate of shoot regeneration in vitro, accelerated early growth, and variation in growth rate was correlated with PtGA20ox7 gene expression. PtRGL1_1 and PtGA2ox2 caused reduced growth, while PtRGL1_2 gave rise to plants that grew normally but had significantly longer xylem fibres. RT-PCR studies suggested that the lack of growth inhibition observed in PtRGL1_2 cisgenic plants was a result of co-suppression. PtGAI1 slowed regeneration rate and both PtGAI1 and PtGA20ox7 gave rise to increased variance among events for early diameter and volume index, respectively. Our work suggests that cisgenic insertion of additional copies of native genes involved in growth regulation may provide tools to help modify plant architecture, expand the genetic variance in plant architecture available to breeders and accelerate transfer of alleles between difficult-to-cross species.