The rhizobial type III effector ErnA confers the ability to form nodules in legumes

Abstract

Several <i>Bradyrhizobium</i> species nodulate the leguminous plant <i>Aeschynomene indica</i> in a type III secretion system-dependent manner, independently of Nod factors. To date, the underlying molecular determinants involved in this symbiotic process remain unknown. To identify the rhizobial effectors involved in nodulation, we mutated 23 out of the 27 effector genes predicted in <i>Bradyrhizobium</i> strain ORS3257. The mutation of <i>nopAO</i> increased nodulation and nitrogenase activity, whereas mutation of 5 other effector genes led to various symbiotic defects. The <i>nopM1</i> and <i>nopP1</i> mutants induced a reduced number of nodules, some of which displayed large necrotic zones. The <i>nopT</i> and <i>nopAB</i> mutants induced uninfected nodules, and a mutant in a yet-undescribed effector gene lost the capacity for nodule formation. This effector gene, widely conserved among bradyrhizobia, was named <i>ernA</i> for "effector required for nodulation-A." Remarkably, expressing <i>ernA</i> in a strain unable to nodulate <i>A. indica</i> conferred nodulation ability. Upon its delivery by <i>Pseudomonas fluorescens</i> into plant cells, ErnA was specifically targeted to the nucleus, and a fluorescence resonance energy transfer-fluorescence lifetime imaging microscopy approach supports the possibility that ErnA binds nucleic acids in the plant nuclei. Ectopic expression of <i>ernA</i> in <i>A. indica</i> roots activated organogenesis of root- and nodule-like structures. Collectively, this study unravels the symbiotic functions of rhizobial type III effectors playing distinct and complementary roles in suppression of host immune functions, infection, and nodule organogenesis, and suggests that ErnA triggers organ development in plants by a mechanism that remains to be elucidated.