R gene-controlled host specificity in the legume–rhizobia symbiosis

Abstract

Leguminous plants can enter into root nodule symbioses with nitrogen-fixing soil bacteria known as rhizobia. An intriguing but still poorly understood property of the symbiosis is its host specificity, which is controlled at multiple levels involving both rhizobial and host genes. It is widely believed that the host specificity is determined by specific recognition of bacterially derived Nod factors by the cognate host receptor(s). Here we describe the positional cloning of two soybean genes Rj2 and Rfg1 that restrict nodulation with specific strains of Bradyrhizobium japonicum and Sinorhizobium fredii, respectively. We show that Rj2 and Rfg1 are allelic genes encoding a member of the Toll-interleukin receptor/nucleotide-binding site/leucine-rich repeat (TIR-NBS-LRR) class of plant resistance (R) proteins. The involvement of host R genes in the control of genotype-specific infection and nodulation reveals a common recognition mechanism underlying symbiotic and pathogenic host-bacteria interactions and suggests the existence of their cognate avirulence genes derived from rhizobia. This study suggests that establishment of a root nodule symbiosis requires the evasion of plant immune responses triggered by rhizobial effectors.soybean | nodulation | nitrogen fixation | defense

Experimental evolution of nodule intracellular infection in legume symbionts

“…Rhizobia were shown to use T3SS to promote or impair symbiosis upon secretion of type three effectors that often have homologs in pathogenic bacteria (Deakin and Broughton, 2009). Known negative effects of rhizobial T3SS include alteration of root hair infection and nodule formation (Chatterjee et al, 1990), nodule infection (Hubber et al, 2004) or symbiosome differentiation (Yang et al, 2010). Thanks to the dual regulation of HrpG, our work showed that T3SS can block both root hair and intracellular infection on the same host plant.…”

Dating in the dark: how roots respond to fungal signals to establish arbuscular mycorrhizal symbiosis

“…With this knowledge in hand, it will also be possible to learn more about the commonalities with and differences from other plant-microbe interactions, including why similar signals lead to differential morphogenetic processes, as in the case of MycLCOs and Nod-LCOs, and how AMF have learned to escape the default fungal recognition program of the plant. The recent finding that R genes restrict host specificity in N2-fixing symbiosis [45] suggests that effector proteins are required, as in pathogenic interactions, to evade the plant immune defense response and to establish the nodule symbiosis. This opens the compelling question concerning whether AMFs also make symbiotic use of pathogenic strategies as rhizobia [46].…”

De novo assembly of soybean wild relatives for pan-genome analysis of diversity and agronomic traits

“…TIRonly and TIR-X sequences were not included in this study. For R-genes with CNV, the expression analysis result and related traits reported in previous studies [25][26][27][28][77][78][79] were listed in Supplementary Table 23. …”

“…The G. soja genome contained a broader range of NBS R-gene domain architectures than did G. max, but G. max had more NBS-encoding genes (460 versus 334 in GsojaA to 382 in GsojaD) (Supplementary Table 22). R-genes showed more copy number loss in G. soja accessions (88 lost versus 33 gained and 8 copy number gain/loss, Supplementary Table 23), several of which were known to confer resistance to soybean mosaic virus 25 , Asian soybean rust 26 and Phytophthora root rot 27 , and be involved in legume-rhizobia symbiosis 28 . In addition, five structurally diverged regions (located on chromosomes 03, 06, 07 and 18) within cultivated G. max 29 , had a high frequency of CNV in the pan-genome (Supplementary Fig.…”