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

Simple Summary: In the process of symbiosis, ΔcopG1 in the type IV secretion system (T4SS) demonstrated the ability to invade root cells but was unable to survive and multiply within root cells. Conversely, traG1 and virD21 were found to be essential in the early stages of nodule formation. Intriguingly, copG1 is required for nod gene expression and acts as a repressor of T4SS genes. Moreover, the absence of copG1 results in certain proteins not being produced, especially T3SS (nopX and nopP) and C4-dicarboxylic acid (dct), which affects the symbiosis between Bradyrhizobium sp. SUTN9-2 and legumes. These findings support the hypothesis that the copG1 gene may serve as a new regulator of the symbiotic process. 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 (T4SS1 and T4SS2) are located on the chromosome of SUTN9-2. ΔT4SS1 reduces both nodule number and nitrogenase activity in all SUTN9-2 nodulating legumes. The functions of three selected genes (copG1, traG1, and virD21) within the region of T4SS1 were examined. We generated deleted mutants and tested them in Vigna radiata cv. SUT4. ΔtraG1 and ΔvirD21 exhibited lower invasion efficiency at the early stages of root infection but could be recently restored. In contrast, ΔcopG1 completely hindered nodule organogenesis and nitrogenase activity in all tested legumes. ΔcopG1 showed low expression of the nodulation gene and ttsI but exhibited high expression levels of the T4SS genes, traG1 and trbE1. The secreted proteins from ΔT4SS1 were down-regulated compared to the wild-type. Although ΔcopG1 secreted several proteins after flavonoid induction, T3SS (nopP and nopX) and the C4-dicarboxylate transporter (dct) were not detected. These results confirm the crucial role of the copG1 gene as a novel key regulator in the symbiotic relationship between SUTN9-2 and legumes. [ABSTRACT FROM AUTHOR]