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Plant hormones are important signaling molecules that control many developmental processes, including cell division, differentiation, organogenesis. In contrast to their animal counterparts, plant hormones can regulate a multitude of apparently unrelated physiological processes, their roles often overlap and they mutually modulate their effects. This indicates the important role of synergistic and antagonistic interactions between different plant hormones. However, the molecular basis of hormonal interactions is still largely unknown. The physiologically best characterized interaction is between auxin and cytokinin. These two plant hormones exhibit synergistic interaction to promote cell division in cell culture, but antagonistic interaction to regulate lateral root development or lateral bud outgrowth. The main aim of our studies is to reveal the molecular components and mechanisms balancing the output of auxin and cytokinin pathways in order to regulate plant organogenesis. We use lateral root organogenesis in Arabidopsis as an ideally suited model system for these questions, because it encompasses fundamental aspects of plant development, such as dedifferentiation, re-entry into the cell cycle, coordinated cell divisions and differentiation, and is in antagonistic manner regulated by auxin and cytokinin. Recently, we have shown that cytokinin modulation of polar auxin transport represents an important mode of auxin-cytokinin interaction. We further investigate underlying molecular mechanisms with focus on (i) transcriptional and (ii) postranscriptional control of auxin efflux carriers by cytokinin. We identified novel cytokinin response elements in promoters of AtPIN1 and AtPIN7 auxin efflux carriers and using yeast one hybrid we aim to determine cytokinin controlled upstream regulatory pathway. Beside transcriptional regulation, our recent data document impact of cytokinin on cellular trafficking of AtPIN1 protein and its stability. Investigation of cytokinin interaction with cellular endocytotic machinery applying chemical and forward genetic approaches will be our key strategies in the future to determine components of this regulatory pathway. To identify novel components of auxin-cytokinin interaction we have applied two main research strategies, transcriptome profiling and a forward genetic screen. Genes recovered from both transcriptome profiling and a forward genetic screen represent candidates for novel cross-talk components that will help us to reveal mechanisms integrating auxin and cytokinin signaling pathways.

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