The NopD effector of Bradyrhizobium XS1150 is phosphorylated by mitogen-activated protein kinases and promotes soybean nodulation

Effectors secreted by the rhizobial type III secretion system (T3SS) are host-specific determinants of nodule symbiosis. We previously showed that NopD from Bradyrhizobium sp. XS1150 possesses protease activity that negatively affects nodulation on Tephrosia vogelii. Using a NopD-calmodulin-dependent adenylate cyclase fusion expressed in Sinorhizobium sp. NGR234, we demonstrate translocation of NopD into nodule cells of Vigna unguiculata. In soybean, a nopD mutant of XS1150 induced fewer nodules than the wild-type strain. We found that NopD is a substrate of mitogen-activated protein kinases (MAPKs) and identified four phosphorylation sites within its tandem repeat (TR) domain. In Arabidopsis protoplasts, NopD localized to nuclei and recruited co-expressed MAPKs to nuclear bodies. In tobacco cells, NopDΔTR and the phospho-dead NopD4A variant lost the ability to trigger NopD-induced cell death. Using a nopL-deficient Sinorhizobium sp. NGR234 strain expressing NopD, NopDΔTR or NopD4A, we found that only NopD suppressed premature senescence of bean (Phaseolus vulgaris) nodules. Further assays using a nopL knockout mutant of Sinorhizobium fredii USDA257 showed that NopD, but not NopD4A, enhanced nodulation in soybean. Together, these data establish NopD as a bona fide T3SS effector and show that its phosphorylation sites are required for effector activity in legume cells.