Understanding of Molecular Mechanisms and Improvement of Adventitious Root Formation in Apple

Adventitious root formation is essential for vegetative propagation, which is the main method for propagation of many horticultural crops, especially woody species. For propagation of apple rootstocks, adventitious root formation is often a limiting step. The molecular mechanisms underlying adventitious root formation are still largely unknown and need to be extensively investigated. In this thesis approaches towards an increased understating of adventitious root formation were made using both apple and the model plant Arabidopsis. For Arabidopsis, a protocol for efficient rooting of hypocotyls and stem segments using two pulses of auxin induction was developed. This protocol was used for studying the expression of a number of rooting related genes during adventitious root formation in both stems and hypocotyls, and for anatomical studies of the timing of the rooting process. The transcript levels of GH3-3, LBD16, LBD29, LRP1 and ARF17 were clearly affected by the auxin treatment, and some of these genes displayed considerable differences between stem segments and hypocotyls. In apple, the function of the rooting related gene ARRO-1 was investigated through down-regulation of the gene using RNAi. The results indicate a possible role for ARRO-1 in promoting adventitious root initiation or regulating hormone homeostasis. The effect of apple rootstocks transformed with the Agrobacterium rolB gene on non-transgenic scions was also investigated, regarding tree growth, flowering and fruit quality. The results showed that the transgenic rootstock had no clear effect on fruit quality but greatly affected vegetative growth. No translocation of transgene mRNA or DNA from rootstock to scion could be detected. Attempts were also made to produce ROLB specific antibodies to investigate if the ROLB protein is transported from the transgenic rootstock into the non-transgenic scion which could be of concern for the consumer