The fine-tuned balance of plant regulators plays a key role in growth and development of plants. Many plant-associated bacteria can influence their hosts either by modulating phytohormone production or by producing phytohormones themselves. The Actinomycete Rhodococcus fascians provokes the formation of differentiated leafy galls consisting of numerous shoot primordia that are inhibited in further outgrowth. Based on the shooty phenotype and the presence of an ipt gene on the linear virulence plasmid of R. fascians D188, the role for cytokinins in the pathology had been anticipated for a long time. Subsequent studies identified and characterized the fas operon as a key genetic determinant of virulence and likely cytokinin biosynthesis. Nevertheless, many aspects concerning regulation of fas gene expression, Fas protein function, and, importantly, the encoded cytokinin biosynthetic pathway and the identity of the produced morphogens remained to be uncovered. Therefore, the main objectives of this research were to identify the bacterial cytokinins responsible for the R. fascians pathology, to unravel how they exerted their function, and to elucidate the role of the fas locus and its expression in their production. In conclusion, our data have largely uncovered the role of cytokinins and the fas locus in the R. fascians pathology: the continuous challenge with defined ratios of synergistically acting cytokinins eventually defeats nearly all plants and transforms them into shooty niches. Many intriguing questions derived from the novel insights obtained during this work remain to be answered. Nevertheless, we feel that the results presented here have shed some light on the remaining secrets of this fascinating pathogen
