Find in Library

The symbiotic interaction between leguminous and <i>Bradyrhizobium</i> 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 (<i>copG₁</i>, <i>traG₁</i>, and <i>virD2₁</i>) within the region of T4SS₁ were examined. We generated deleted mutants and tested them in <i>Vigna radiata</i> cv. SUT4. Δ<i>traG₁</i> and Δ<i>virD2₁</i> exhibited lower invasion efficiency at the early stages of root infection but could be recently restored. In contrast, Δ<i>copG₁</i> completely hindered nodule organogenesis and nitrogenase activity in all tested legumes. Δ<i>copG₁</i> showed low expression of the nodulation gene and <i>ttsI</i> but exhibited high expression levels of the T4SS genes, <i>traG₁</i> and <i>trbE₁</i>. The secreted proteins from ΔT4SS₁ were down-regulated compared to the wild-type. Although <i>ΔcopG₁</i> secreted several proteins after flavonoid induction, T3SS (<i>nopP</i> and <i>nopX</i>) and the C₄-dicarboxylate transporter (<i>dct</i>) were not detected. These results confirm the crucial role of the <i>copG₁</i> gene as a novel key regulator in the symbiotic relationship between SUTN9-2 and legumes.