Abstract
Effectors secreted by the type III protein secretion system (T3SS) of rhizobia are host-specific determinants of the nodule symbiosis. Here, we have characterized NopD, a putative type III effector of Bradyrhizobium sp. XS1150. NopD was found to possess a functional N-terminal secretion signal sequence that could replace that of the NopL effector secreted by Sinorhizobium sp. NGR234. Recombinant NopD and the C-terminal domain of NopD alone can process small ubiquitin-related modifier (SUMO) proteins and cleave SUMO-conjugated proteins. Activity was abolished in a NopD variant with a cysteine-to-alanine substitution in the catalytic core (NopD-C972A). NopD recognizes specific plant SUMO proteins (AtSUMO1 and AtSUMO2 of Arabidopsis thaliana; GmSUMO of Glycine max; PvSUMO of Phaseolus vulgaris). Subcellular localization analysis with A. thaliana protoplasts showed that NopD accumulates in nuclear bodies. NopD, but not NopD-C972A, induces cell death when expressed in Nicotiana tabacum. Likewise, inoculation tests with constructed mutant strains of XS1150 indicated that nodulation of Tephrosia vogelii is negatively affected by the protease activity of NopD. In conclusion, our findings show that NopD is a symbiosis-related protein that can process specific SUMO proteins and desumoylate SUMO-conjugated proteins.
Highlights
Various bacteria possess protein secretion systems, through which effectors are translocated into host cells
Whole-genome shotgun sequencing revealed that XS1150 is a Bradyrhizobium strain
Sequence homology searches indicated that the XS1150 genome possesses a type III protein secretion system (T3SS) gene cluster that contains the transcriptional regulator gene ttsI and the putative effector genes nopL, nopE1, and nopP
Summary
Various bacteria possess protein secretion systems, through which effectors are translocated into host cells. Type III (T3) effector proteins secreted via a needle-like type III secretion system (T3SS) are important virulence factors of pathogenic bacteria such as the plant pathogens Pseudomonas syringae and Xanthomonas campestris (Büttner, 2016; Deng et al, 2017). Functional T3SS have been identified in various rhizobia, bacteria that establish a symbiotic relationship with legumes (Staehelin and Krishnan, 2015; Nelson and Sadowsky, 2015; López-Baena et al, 2016). Rhizobia, differentiated into bacteroids, reduce atmospheric nitrogen to ammonia in root nodules of host plants. Fixed nitrogen is delivered to the host plant in exchange of carbon assimilates and nutrients. Growth of legume crops does not depend on application of nitrogen
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