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The Sinorhizobium fredii HH103 Type III secretion system effector NopC blocks nodulation with Lotus japonicus Gifu.

The broad host-range bacterium Sinorhizobium fredii HH103 cannot nodulate the model legume Lotus japonicus Gifu. This bacterium possesses a Type III secretion system (T3SS), a specialized secretion apparatus used to deliver effector proteins (T3E) into the host cell cytosol to alter host signaling and/or suppress host defence responses to promote infection. However, some of these T3E are recognized by specific plant receptors and hence trigger a strong defence response to block infection. In rhizobia, T3E are involved in nodulation efficiency, host-range determination and, in some cases, directly activate host symbiosis signalling in a Nod factor-independent manner. In this work, we show that HH103 RifR T3SS mutants, unable to secrete T3E, gain nodulation with L. japonicus Gifu through infection threads (IT), suggesting that plant recognition of a T3E could block the infection process. To identify the T3E involved, we performed nodulation assays with a collection of mutants affected in secretion of each T3E identified in HH103 RifR so far. The nopC mutant could infect L. japonicus Gifu by IT invasion and switch the infection mechanism in L. burttii from intercellular infection to IT formation. L. japonicus gene expression analysis indicated that the infection-blocking event occurs at early stages of the symbiosis.