In this study, we investigated genetic elements of the type IV secretion system (T4SS) found in Sinorhizobium and the role they play in symbiosis. Sinorhizobium meliloti and S. medicae each contain a putative T4SS, similar to that used by Agrobacterium tumefaciens during pathogenesis. The Cre reporter assay for translocation (CRAfT) system was used to validate potential effector proteins. Both S. meliloti and S. medicae contained the effector protein TfeA, which was translocated into the host plant. Sequence analysis revealed the presence of a nod box, involved in transcriptional activation of symbiosis-related genes, upstream of the transcriptional regulator (virG) in the Sinorhizobium T4SS. Replicate qRT-PCR analyses indicated that luteolin, released by roots and seeds of Medicago truncatula, upregulated transcription of tfeA and virG. Mutations in the T4SS apparatus, or tfeA alone, resulted in reduced numbers of nodules formed on M. truncatula genotypes. In addition, S. meliloti strain KH46c, which contains a deletion in the T4SS, was less competitive for nodule formation when co-inoculated with an equal number of cells of the wild-type strain. To our knowledge, TfeA is the first T4SS effector protein identified in Sinorhizobium. Our results indicate that Sinorhizobium 1) uses a T4SS during initiation of symbiosis with Medicago, and 2) alters Medicago cells in planta during symbiosis. This study also offers additional bioinformatic evidence that several different rhizobial species may use the T4SS in symbiosis with other legumes.
Nelson MS, Chun CL, Sadowsky MJ (2016).Mol Plant Microbe Interact. 2016 Dec 5. [Epub ahead of print]