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

• Published in: Journal of experimental botany Vol. 71; no. 19; pp. 6043 - 6056
• Main Authors: Jiménez-Guerrero, Irene, Acosta-Jurado, Sebastián, Medina, Carlos, Ollero, Francisco Javier, Alias-Villegas, Cynthia, Vinardell, José María, Pérez-Montaño, Francisco, López-Baena, Francisco Javier
• Format: Journal Article
• Language: English
• Published: UK Oxford University Press 07-10-2020
• Subjects: Bacterial Proteins - genetics; Lotus; Plant Root Nodulation; Sinorhizobium; Sinorhizobium fredii - genetics; Symbiosis; Type III Secretion Systems; nodulation; infection thread; plant defence; type III secretion system (T3SS); legume symbiosis; Effector; Rhizobium–legume symbiosis; Lotus japonicus; Sinorhizobium fredii
• ISSN: 0022-0957; 1460-2431
• DOI: 10.1093/jxb/eraa297

The Sinorhizobium fredii HH103 RifR T3SS effector NopC blocks nodulation with Lotus japonicus Gifu. The infection-blocking event occurs at early stages of the infection process.
Abstract
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 (T3Es) into the host cell cytosol to alter host signaling and/or suppress host defence responses to promote infection. However, some of these T3Es are recognized by specific plant receptors and hence trigger a strong defence response to block infection. In rhizobia, T3Es are involved in nodulation efficiency and 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 T3Es, gain nodulation with L. japonicus Gifu through infection threads, 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 that affect secretion of each T3E identified in HH103 RifR so far. The nopC mutant could infect L. japonicus Gifu by infection thread invasion and switch the infection mechanism in Lotus burttii from intercellular infection to infection thread formation. Lotus japonicus gene expression analysis indicated that the infection-blocking event occurs at early stages of the symbiosis.