Cytokinin synthesis and export from symbiotic root nodules coordinates shoot growth with nitrogen fixation

Development of symbiotic root nodules is a cytokinin-dependent process that is critical to nitrogen acquisition in legumes. The extent and manner in which root nodules contribute to whole-plant cytokinin and nitrogen supply signalling is unknown. Using a combination of genetic, biochemical and physiological approaches, we characterised the role of cytokinin synthesis, export and perception in coordination of symbiotic nodule development and shoot growth in the legume Lotus japonicus. LjPup1 encodes a plasma membrane localised cytokinin exporter with isopentenyladenine (iP) and trans-Zeatin (tZ) export capacity. LjPup1 shows a distinct nodule-specific expression pattern with greatest transcript levels detected in mature nodules. Mutants accumulate more isopentenyladenine riboside (iPR) in nodule tissues and demonstrate hallmarks of reduced cytokinin signalling. Despite normal nodule numbers and function, shoot growth is markedly reduced in Ljpup1 mutants, as well as in mutants impaired in tZ biosynthesis. We found symbiotic root nodules contribute to shoot growth via export of active cytokinins. A cytokinin exporter in the purine permease family thus contributes to long-distance cytokinin homeostasis regulating plant development.