Isoprenoid and aromatic cytokinins occur in poplar as free compounds and constituents of tRNA, poplar isopentenyltransferases are involved in the production of isoprenoid cytokinins, while biosynthesis of their aromatic counterparts remains unsolved. Cytokinins are phytohormones with a fundamental role in the regulation of plant growth and development. They occur naturally either as isoprenoid or aromatic derivatives, but the latter are quite rare and less studied. Here, the spatial expression of all nine isopentenyl transferase genes of Populus × canadensis cv. Robusta (PcIPTs) as analyzed by RT-qPCR revealed a tissue preference and strong differences in expression levels for the different adenylate and tRNA PcIPTs. Together with their phylogeny, this result suggests a functional diversification for the different PcIPT proteins. Additionally, the majority of PcIPT genes were cloned and expressed in Arabidopsis thaliana under an inducible promoter. The cytokinin levels measured in the Arabidopsis-overexpressing lines as well as their phenotype indicate that the studied adenylate and tRNA PcIPT proteins are functional in vivo and thus will contribute to the cytokinin pool in poplar. We screened the cytokinin content in leaves of 12 Populus species by ultra-high performance-tandem mass spectrometry (UHPLC-MS/MS) and discovered that the capacity to produce not only isoprenoid, but also aromatic cytokinins is widespread amongst the Populus accessions studied. Important for future studies is that the levels of aromatic cytokinins transiently increase after daybreak and are much higher in older plants.

• Keywords
• Cytokinin, Expression, Isopentenyltransferase, Poplar, Topolin, tRNA,
• MeSH
• Alkyl and Aryl Transferases genetics   metabolism   MeSH
• Arabidopsis genetics   metabolism   MeSH
• Cytokinins biosynthesis   MeSH
• Phylogeny MeSH
• Plants, Genetically Modified MeSH
• Plant Leaves genetics   metabolism   MeSH
• Populus genetics   metabolism   MeSH
• Plant Growth Regulators metabolism   MeSH
• Plant Proteins genetics   metabolism   MeSH
• Tandem Mass Spectrometry MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• adenylate isopentenyltransferase MeSH Browser
• Alkyl and Aryl Transferases MeSH
• Cytokinins MeSH
• Plant Growth Regulators MeSH
• Plant Proteins MeSH

It has been recognized that cytokinins are plant hormones that influence not only numerous aspects of plant growth, development and physiology, including cell division, chloroplast differentiation and delay of senescence but the interaction with other organisms, including pathogens. Cytokinins are not only produced by plants but are also by other prokaryotic and eukaryotic organism such as bacteria, fungi, microalgae and insects. Notably, cytokinins are produced both by pathogenic and also beneficial microbes and are known to induce resistance in plants against pathogen infections. In this review the contrasting role of cytokinin for the defence and susceptibility of plants against bacterial and fungal pathogen and pest insects is assessed. We also discuss the cross talk of cytokinins with other phytohormones and the underlying mechanism involved in enhancing plant immunity against pathogen infections and explore possible practical applications in crop plant production.

• Keywords
• beneficial microbe, host–pathogen interaction, insect, microalgae, phytohormones, plant growth promoting rhizobacteria,
• Publication type
• Journal Article MeSH
• Review MeSH