CopG 1 , a Novel Transcriptional Regulator Affecting Symbiosis in Bradyrhizobium sp. SUTN9-2.

The symbiotic interaction between leguminous and Bradyrhizobium sp. SUTN9-2 mainly relies on the nodulation process through Nod factors (NFs), while the type IV secretion system (T4SS) acts as an alternative pathway in this symbiosis. Two copies of T4SS (T4SS 1 and T4SS 2 ) are located on the chromosome of SUTN9-2. ΔT4SS 1 reduces both nodule number and nitrogenase activity in all SUTN9-2 nodulating legumes. The functions of three selected genes ( copG 1 , traG 1 , and virD2 1 ) within the region of T4SS 1 were examined. We generated deleted mutants and tested them in Vigna radiata cv. SUT4. Δ traG 1 and Δ virD2 1 exhibited lower invasion efficiency at the early stages of root infection but could be recently restored. In contrast, Δ copG 1 completely hindered nodule organogenesis and nitrogenase activity in all tested legumes. Δ copG 1 showed low expression of the nodulation gene and ttsI but exhibited high expression levels of the T4SS genes, traG 1 and trbE 1 . The secreted proteins from ΔT4SS 1 were down-regulated compared to the wild-type. Although ΔcopG 1 secreted several proteins after flavonoid induction, T3SS ( nopP and nopX ) and the C 4 -dicarboxylate transporter ( dct ) were not detected. These results confirm the crucial role of the copG 1 gene as a novel key regulator in the symbiotic relationship between SUTN9-2 and legumes.