Role of plant root exudate and Sym plasmid-localized nodulation genes in the synthesis by Rhizobium leguminosarum of Tsr factor, which causes thick and short roots on common vetch.

In a previous paper it was shown that cocultivation of Rhizobium leguminosarum with the plant Vicia sativa subsp. nigra on solid medium causes a changed mode of growth of the plant roots, resulting in thick and short roots (Tsr). The Sym plasmid present in the bacterium appeared to be essential for causing Tsr (A. A. N. van Brussel, T. Tak, A. Wetselaar, E. Pees, and C. A. Wijffelman, Plant Sci. Lett. 27:317-325, 1982). In the present paper, we show that a role in causing Tsr is general for Sym plasmids of R. leguminosarum and Rhizobium trifolii. Moreover, mutants with transposon insertions in the Sym plasmid-localized nodulation genes nodA, B, C, and D are unable to cause Tsr, in contrast to nodulation mutants localized in other parts of the Sym plasmid. The observation that Tsr could also be brought about in liquid medium enabled us to show that Tsr is caused by a soluble factor. Experiments in which plants and bacteria were grown separately in the sterile supernatant fluids of each other resulted in establishing the following sequence of events. (i) The plant produces a factor, designated as factor A. (ii) Factor A causes the Sym plasmid-harboring bacteria to produce Tsr factor. (iii) Growth of young plants in the presence of Tsr factor results in the Tsr phenotype. Models explaining this example of molecular signalling between bacteria and plants are discussed.