Molecular and hormonal characterization of Ri plants

The root-inducing (Ri) technology is a biotechnological tool, gaining attention to tackle modern crop breeding challenges. This approach uses rhizogenic rhizobia to integrate bacterial T-DNA, containing rol (root oncogenic locus) genes, into the plant genome, to generate Ri genotypes with altered traits. The main objectives of this thesis included 1) the investigation of the genetic basis of transformability and hairy root (HR) formation in Rosa hybrida, 2) the optimization of HR formation and shoot regeneration in Rosa hybrida and Malus domestica genotypes, and 3), the morphological, molecular and physiological characterization of Rosa hybrida, Malus domestica and Solanum tuberosum Ri genotypes. To investigate the genetic basis of transformability, a genome-wide association study in a diversity panel of 104 rose genotypes was performed, which revealed HR formation to be a complex, polygenic trait. Five key quantitative trait loci (QTL) were identified, explaining 12.0% to 26.9% of the phenotypic variance, with distinct genetic control between the leaf organs lamina and petiole. In addition, petiole-specific QTL were found to partially overlap with QTL for adventitious root formation. Protocol optimizations such as ultrasonic treatments and the bacterial strain ATCC 15834 showed to be particularly efficient for the formation of HRs in in vitro leaves of apple and rose. While regeneration from HRs proved to be a significant, genotype-dependent bottleneck, successful protocols were established for the apple genotypes 'M26' and 'Gala'. Genotype was the most critical factor for regeneration, whereas parameters such as plant growth regulators proved to be less relevant. Morphological characterization across rose, apple and potato Ri genotypes consistently demonstrated characteristic Ri traits of compact growth and smaller leaves, which were correlated with rol gene expression. However, enhanced in vitro rooting did not translate to increased root biomass under greenhouse conditions, and microscopic observations did not support the hypothesis of increased root hair density in Ri genotypes. Hormonal profiling revealed consistently elevated levels of the cytokinins trans-zeatin riboside and dihydrozeatin riboside, and a constitutive stress response, as indicated by altered concentrations of abscisic acid, salicylic acid, jasmonic acid and leaf coloration. Finally, a greenhouse biotest demonstrated the Ri technology's potential for managing replant disease, as some apple and rose Ri genotypes exhibited significantly reduced susceptibility, although in other genotypes increased susceptibility was also observed. These findings show that Ri technology is a valuable tool for modifying plant architecture and stress responses, with the genotypic background being critical for the applicability. Applications in horticultural crops may include the breeding of new, compact rootstocks and varieties with improved tolerance to biotic and abiotic stress factors.