Traditionally, new traits have been introduced into ornamental plants through classical breeding. However, genetic engineering now enables specific alterations of single traits in already successful varieties. New or improved varieties of floricultural crops can be obtained by acting on floral traits, such as color, shape or fragrance, on vase life in cut-flower species, and on rooting potential or overall plant morphology. Overexpression of the rol genes of the Ri plasmid of Agrobacterium rhizogenes in plants alters several developmental processes and affects their architecture. Both A. rhizogenes- and rol-transgenic plants display the hairy-root phenotype, although specific differences are found among species and among transgenic lines. In general, these plants show a dwarfed phenotype, reduced apical dominance, smaller, wrinkled leaves, increased rooting, altered flowering and reduced fertility. Among the rol genes, termed rolA, B, C and D, rolC has been the most widely studied because its effects are the most advantageous in terms of improving ornamental and horticultural traits. In addition to the dwarfness and the increase in lateral shoots that lead to a bushy phenotype, rolC-plants generally display more and smaller flowers, and advanced flowering; surprisingly, these plants may have better rooting capacity and they show almost no undesirable traits.rolD, the least studied among the rol genes, offers promising applications due to its promotion of flowering. Although rolA and rolB require further study in this regard, they are not recommended for introduction into ornamental species, since with them, many abnormalities are obtained. In the case of floricultural crops, molecular regulation of flowering and floral traits, as well as alterations in plant shape, via the application of genes such as rol genes, all contribute to the improvement of the ornamental plants. Currently, the number of rol-transgenic plants continues to increase, and in many cases, beneficial traits are obtained. Thus, rol genes, particularly rolC and the promising rolD, should be introduced into more ornamental species, since they may prove highly useful in yielding improved floricultural traits. However, further research is needed to determine the effects of rol proteins and their interaction with other proteins, organs and environmental factors, in order to understand all the different phenotypical and biochemical effects they may produce in transgenic plants. Consequently, increased knowledge about the functions of these genes would greatly benefit their application in both floriculture and horticulture. In the near future, molecular plant breeding will complement classical breeding, with its obvious shortcomings, for the improvement of commercial species, and many rol transformed species may prove useful in breeding programs to generate ornamentals with new and improved traits.
Keywords: Agrobacterium rhizogenes, hairy root, genetic transformation, ornamental plants