Cytokinin is a key plant growth regulator that controls many developmental processes. Over the past decades, much research has been done on the molecular components and signal transduction system of the cytokinin pathway. Cytokinin signal transduction is based on the two-component phosphorelay mechanism. Peter Marhavý from the group of Eva Benková at the VIB Department of Plant Systems Biology, UGent, recently identified an alternative mode of cytokinin action that uses endocytic trafficking as a means for directing plant organogenesis.An important part of the cytokinin-mediated regulation of development involves an interaction with the auxin pathway. Can you explain?Peter: Auxin and cytokinin are one of the central plant hormones involved in the regulation of different plant developmental processes, including embryogenesis, leaf, flower and root formation, and shoot branching. The fact that both hormones co-regulate the same developmental events implies that their activities must be mutually tightly coordinated, with cross-talk between both signaling pathways being extremely important for the proper developmental response. Although we have identified the molecular principle behind both auxin and cytokinin signaling pathways, our knowledge about the mechanisms of their interaction is still very limited. So far we know that auxin transcriptionally regulates expression of important components of the cytokinin signaling cascade, and thus modulates the activity of the cytokinin signaling pathway.
In this paper you identified a different mode of cytokinin action for controlling plant organogenesis?Eva: Initiation of new plant organs such as flowers, leaves or lateral roots, as well as their development, is dependent on proper auxin distribution. In plants, auxin is transported through polar auxin transport machinery consisting of influx and efflux carriers. Previously, we and other international teams demonstrated that cytokinin modulates expression of auxin efflux carriers of the PIN family on the transcriptional level. We demonstrated that some of the PIN genes are upregulated while others are downregulated by cytokinin, and that therefore cytokinin contributes to the control of auxin distribution.
Our recent findings revealed that cytokinin might modulate auxin efflux not only on the level of transcription, but also post-transcriptionally. We showed that, at increased cytokinin levels, one of the auxin efflux carriers, namely PIN1, is redirected for rapid degradation to lytic vacuoles.
How important are these findings?Peter: This cytokinin activity is very rapid, and independent of transcription or any new protein biosynthesis. This suggests the existence of a novel cytokinin activity mode, conceptually very different from the known signal transduction cascade in which downstream response occurs on the level of transcription. It seems that part of the cytokinin regulation might be executed by a mechanism controlling protein dynamics and trafficking in cells. To identify molecular components and mechanisms mediating this cytokinin activity is certainly one of the challenges for our future research.
You collaborated with several labs? What are the advantages?Eva: Collaboration with several international teams was very important. Approaches and methods that would be difficult or time consuming to establish in our laboratory were performed in collaboration with the others. This enabled us to gather independent evidence from several sources in support of our discovery.
Marhavý
et al.Cytokinin modulates endocytic trafficking of PIN1 auxin efflux carrier to control plant organogenesis
Developmental Cell 2011