Improved compact growth habit of Viola × wittrockiana through Rhizobium rhizogenes transformation

Compactness is an important breeding goal in the improvement of garden pansy varieties (Viola × wittrockiana Gams.). The desired compact stature is primarily obtained through the use of chemical growth retardants. Recently, an alternative strategy for creating compact ornamentals was proposed based on transformation by wild type rhizogenic agrobacteria and the regeneration of transformed hairy roots. Here, we report on the successful transformation and creation of compact garden pansy lines. The most efficient hairy root induction was obtained by Rhizobium rhizogenes strain ATCC15834. Shoot regeneration from hairy roots was obtained using 3 mg L−1 2iP and 0.3 mg L−1 2,4-D. The presence of R. rhizogenes T-DNA genes was confirmed in the regenerated lines. These lines displayed a varying degrees of dwarfed growth. Most notably the shoot length was reduced up to 49%. Other phenotypic traits that were induced include reduced peduncle length and smaller, wrinkled flowers. Several of these Ri lines with promising compactness have been selected for further breeding towards compact garden pansy varieties that are less dependent on chemical growth regulation.