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The New Putative Type III Effector SkP48 in Bradyrhizobium sp. DOA9 is Involved in Legume Nodulation
Description
<jats:title>Abstract</jats:title> <jats:p><jats:italic>Bradyrhizobium </jats:italic>sp. DOA9 can nodulate a wide spectrum of legumes; however, unlike other bradyrhizobia, DOA9 carries a symbiotic plasmid harboring type III secretion system (T3SS) and several effector (T3E) genes, one of which encodes a new putative type III effector—SkP48. Here, we demonstrated the pivotal roles of SkP48 from <jats:italic>Bradyrhizobium </jats:italic>sp. DOA9 in inhibiting nodulation of various <jats:italic>Vigna </jats:italic>species and <jats:italic>Crotalaria juncea </jats:italic>and suppressing nodulation efficiency of <jats:italic>Arachis hypogea</jats:italic>. By contrast, the nodulation efficiency of a SkP48 mutant did not differ significantly with the DOA9 wild-type strain on <jats:italic>Macroptilium atropurpureum</jats:italic> and <jats:italic>Stylosanthes hamata</jats:italic>. An evolutionary analysis revealed that the SkP48 effector which contains a shikimate kinase and a SUMO protease (C48 cysteine peptidase) domain is distinct from the others effectors previously identified in others bradyrhizobia and pathogenic bacteria. Our findings suggest that the new putative T3E SkP48 is a key factor suppressing nodulation and nodule organogenesis in several legumes by activation of effector-triggered immunity through salicylic acid biosynthesis induction, which is deleterious to rhizobial infection. In addition, nodulation may be modulated by the function of defensins involved in jasmonic acid signalling in <jats:italic>V. radiata</jats:italic> SUT1.</jats:p>