R. J. Neil Emery,1 and
1Biology Department, Trent University, 2140 East Bank Drive, Peterborough K9J 7B8, Canada; 2Department of Genetics and Plant Biotechnology, University of Technology and Life Sciences, 85-789 Bydgoszcz, Poland; 3Institut des Sciences du Végétal (ISV), CNRS, avenue de la Terrasse, 91198 Gif-sur-Yvette cedex, France
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Accepted 26 April 2013.
Bacteria present in the rhizosphere of plants often synthesize phytohormones, and these signals can consequently affect root system development. In legumes, plants adapt to nitrogen starvation by forming lateral roots as well as a new organ, the root nodule, following a symbiotic interaction with bacteria collectively referred to as rhizobia. As cytokinin (CK) phytohormones were shown to be necessary and sufficient to induce root nodule organogenesis, the relevance of CK production by symbiotic rhizobia was questioned. In this study, we analyzed quantitatively, by liquid chromatography-tandem mass spectrometry, the production of 25 forms of CK in nine rhizobia strains belonging to four different species. All bacterial strains were able to synthesize a mix of CK, and bioactive forms of CK, such as iP, were notably found to be secreted in bacterial culture supernatants. Use of a mutant affected in extracellular polysaccharide (EPS) production revealed a negative correlation of EPS production with the ability to secrete CK. In addition, analysis of a nonnodulating Sinorhizobium meliloti strain revealed a similar pattern of CK production and secretion when compared with a related nodulating strain. This indicates that bacterially produced CK are not sufficient to induce symbiotic nodulation.
© 2013 The American Phytopathological Society