Symbiosis: R You Friend Or Foe?

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SymbioSiS R you friend or foe?

Establishing a productive symbiosis requires that the two interacting species tolerate each other, even in the presence of an immune system. In a recent issue of the Proceedings ica of the National Academy of Sciences, R USA, Yang and colleagues describe outh af outh

how innate immunity proteins in S legumes determine the host range of plant–bacterium interactions.

Legumes form a root nodule — a macmillan compartment that houses symbiotic, nitrogen-fixing rhizobial bacteria protein family. TIR–NBS–LRR pro- lacking the type III secretion system, — with a defined, narrow range of teins are part of the resistance they found that this strain was not bacteria; nodule formation can be (R) protein family that confers restricted by plants that restrict its triggered by a bacterium in a plant resistance to pathogens but that wild-type parent, indicating that a that restricts a closely related bacterial had not previously been implicated secreted effector of the type III secre- species. To resolve how this specificity in the establishment of symbioses. tion system is the likely target of the is determined, the authors mapped Complementation assays confirmed Glyma16g33780 gene product. two loci in soybeans, Rj2 and Rfg1, the role of Glyma16g33780 in The finding that R proteins have that are known to regulate the specifi symbiosis. a role in defining the symbiotic rela- city of soybean–bacterium interactions. Among 21 soybean strains, tionship in the root nodule provides Using crosses of plants that restrict the authors found three different insight into the intricate interaction either Bradyrhizobium japonicum or responses to B. japonicum and S. fredii: between bacteria and plants. Sinorhizobium fredii, they identified the plants restricted one, the other Christiaan van Ooij genetic regions of 47 kb and 69 kb or neither of the two species (no

that contain the Rj2 and Rfg1 loci, strain restricted both). Sequencing of oRiGiNAL RESEARCH PAPER Yang, S. et al. respectively. Interestingly, these Glyma16g33780 revealed sequence R gene-controlled host specificity in the legume– regions overlapped and contained the variations in the NBS and LRR rhizobia symbiosis. Proc. Natl Acad. Sci. USA 107, 18735–18740 (2010) ORF Glyma16g33780, which encodes domains that correlated with the FuRtHER REAdiNG Deakin, W. J. & a member of the Toll/interleukin restriction of B. japonicum or S. fredii. Broughton, W. J. Symbiotic use of pathogenic strategies: rhizobial protein secretion systems. receptor–nucleotide-binding site– Furthermore, when the authors tested Nature Rev. Microbiol. 7, 312–320 (2009) leucine-rich repeat (TIR–NBS–LRR) the restriction of an S. fredii mutant