Occurrence, Interconversion, and Perception of Topolins in Poplar

The poplar species Populus × canadensis cv. Robusta was the first organism found to contain aromatic cytokinins. Screening of the content of aromatic cytokinins in leaves of 12 Populus species revealed that the capacity to produce aromatic cytokinins is widespread among Populus accessions. The major aromatic metabolites are ortho-topolin and ortho-topolin riboside. Their levels transiently increase after daybreak and are much higher in older plants. Poplar species contain five genes coding for functional CHASE-containing histidine kinases acting as cytokinin receptors. Poplar genome further contains nine isopentenyl transferase genes coding for enzymes responsible for the biosynthesis of isoprenoid cytokinins, two genes coding for adenosine kinase, two genes of nucleoside N-ribohydrolase, and one gene encoding purine nucleoside phosphorylase. These enzymes contribute to interconversion of cytokinin ribosides. Trans-Zeatin is the most abundant cytokinin in poplar and displays the highest variation in abundance. It shows the strongest affinity to all five cytokinin receptors and activates the cytokinin signaling via A-type response regulators. Among aromatic cytokinins, meta-topolin is efficiently bound to all receptors, while ortho-topolin binds only at micromolar concentrations. The origin of topolins in poplar remains unclear, and it is possible that they are not products of poplar metabolism but indeed endophyte-derived products.