Nodulation outer proteins: double-edged swords of symbiotic rhizobia.
Rhizobia are nitrogen-fixing bacteria that establish a nodule symbiosis with legumes. Nodule formation depends on signals and surface determinants produced by both symbiotic partners. Among them, rhizobial Nops (nodulation outer proteins) play a crucial symbiotic role in many strain-host combinations. Nops are defined as proteins secreted via a rhizobial T3SS (type III secretion system). Functional T3SSs have been characterized in many rhizobial strains. Nops have been identified using various genetic, biochemical, proteomic, genomic and experimental approaches. Certain Nops represent extracellular components of the T3SS, which are visible in electron micrographs as bacterial surface appendages called T3 (type III) pili. Other Nops are T3 effector proteins that can be translocated into plant cells. Rhizobial T3 effectors manipulate cellular processes in host cells to suppress plant defence responses against rhizobia and to promote symbiosis-related processes. Accordingly, mutant strains deficient in synthesis or secretion of T3 effectors show reduced symbiotic properties on certain host plants. On the other hand, direct or indirect recognition of T3 effectors by plant cells expressing specific R (resistance) proteins can result in effector triggered defence responses that negatively affect rhizobial infection. Hence Nops are double-edged swords that may promote establishment of symbiosis with one legume (symbiotic factors) and impair symbiotic processes when bacteria are inoculated on another legume species (asymbiotic factors). In the present review, we provide an overview of our current understanding of Nops. We summarize their symbiotic effects, their biochemical properties and their possible modes of action. Finally, we discuss future perspectives in the field of T3 effector research.
State Key Laboratory of Biocontrol and Guangdong Key Laboratory of Plant Resources, School of Life Sciences, Sun Yat-sen University, East Campus, Guangzhou 510006, China Shenzhen Research and Development Center of State Key Laboratory of Biocontrol, School of Life Sciences, Sun Yat-sen University, Baoan, Shenzhen 518057, China cst@mail.sysu.edu.cn.
Christian
C
Staehelin
Plant Genetics Research Unit, United States Department of Agriculture-Agriculture Research Service, University of Missouri, Columbia, MO 65211, U.S.A.
Hari B
HB
Krishnan
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D007887:Fabaceae / Q000378:metabolism / Q000382:microbiology; D010861:Fimbriae, Bacterial / Q000378:metabolism / Q000648:ultrastructure; D005419:Flavonoids / Q000378:metabolism; D005798:Genes, Bacterial; D008954:Models, Biological; D009154:Mutation; D010641:Phenotype; D055170:Plant Root Nodulation; D012231:Rhizobium / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D013559:Symbiosis / Q000235:genetics / Q000502:physiology
D001426:Bacterial Proteins; D005419:Flavonoids
effector; nodulation outer protein; plant defence; root nodule; symbiosis; type III secretion system