From tRNA modification to cytokinins - Characterisation of tRNA-mediated cytokinin biosynthesis in the early land plant Physcomitrella

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For the moss Physcomitrella patens we collected evidence suggesting that in this model organism the plant hormones cytokinins (isopentenyl-derivatives of adenine), in contrast to seed plants, originate exclusively from tRNA. We therefore assume that in early steps of land plant evolution the isopentenylation of tRNA, catalysed by tRNA-isopentenyltransfrases (tRNA-IPTs), took over the function of providing precursors for free cytokinins acting as plant hormones. This tRNA-IPT mediated pathway of cytokinin biosynthesis was preserved during the evolution of seed plants, however seed plants additionally developed the de novo synthesis of free cytokinins via isopentenylation of the adenylates ADP/ATP. The proposed project aims (1) to establish the unequivocal proof for the exclusivity of the tRNA-dependent pathway of cytokinin biosynthesis in moss, (2) to provide basic information on the regulation of the tRNAdependent cytokinin biosynthesis on the levels of gene regulation as well as tRNA-IPT properties and substrate availability. The aims will be achieved by the generation of tRNA-IPT gene knockout plants and tRNA-IPT overexpressors. Transcriptional studies will correlate the localisation of tRNA-IPT expression with the topology of cytokinin biosynthesis/activity. A detailed description of transcription and adenine37- isopentenylation of tRNAs will help to understand the mode of function of tRNA-mediated cytokinin biosynthesis, contributing to growth control also in seed plants.