The Rhizobial Type 3 Secretion System: The Dr. Jekyll and Mr. Hyde in the Rhizobium–Legume Symbiosis

Abstract

Rhizobia are soil bacteria that can establish a symbiotic association with legumes. As a result, plant nodules are formed on the roots of the host plants where rhizobia differentiate to bacteroids capable of fixing atmospheric nitrogen into ammonia. This ammonia is transferred to the plant in exchange of a carbon source and an appropriate environment for bacterial survival. This process is subjected to a tight regulation with several checkpoints to allow the progression of the infection or its restriction. The type 3 secretion system (T3SS) is a secretory system that injects proteins, called effectors (T3E), directly into the cytoplasm of the host cell, altering host pathways or suppressing host defense responses. This secretion system is not present in all rhizobia but its role in symbiosis is crucial for some symbiotic associations, showing two possible faces as Dr. Jekyll and Mr. Hyde: it can be completely necessary for the formation of nodules, or it can block nodulation in different legume species/cultivars. In this review, we compile all the information currently available about the effects of different rhizobial effectors on plant symbiotic phenotypes. These phenotypes are diverse and highlight the importance of the T3SS in certain rhizobium–legume symbioses.

GmTNRP1, associated with rhizobial type‐III effector NopT, regulates nitrogenase activity in the nodules of soybean (Glycine max)

“…The establishment of symbiosis between soybean and rhizobia involves the direct and complex signal communication between the host and bacteria, in which the immune system of soybean and mixture of type‐III effectors secreted by rhizobia determine the symbiotic outcome (Jiménez‐Guerrero et al, 2022; Teulet et al, 2022). Similar to other gram‐negative bacteria, rhizobia contain various secretion systems, among which, the type‐III secretion system (T3SS) is widely present in rhizobia (Deakin & Broughton, 2009; Yang, Zhao, et al, 2010).…”

Comparative genome analysis of Sesbania cannabina-nodulating Rhizobium spp. revealing the symbiotic and transferrable characteristics of symbiosis plasmids

“…The T3SS widely distributed in rhizobial genomes determines the symbiotic efficiency, it can be completely necessary for nodule formation or can block nodulation during the rhizobium–legume symbiosis process [57]. It is a protein complex and secretes effector proteins from the cytosol of the bacteria into the host cell through a tube spanning the bacterial and host membranes, and it consists of needle structure proteins and effector proteins secreted by the needle [58].…”

A novel intergenic dSH3-irr insertion mutation in Bradyrhizobium diazoefficiens causes its symbiotic incompatibility with soybean

“…However, immune response, plant stress response transcriptional factors, ethylene response factor, the PTI and ETI were all upregulated in the aberrant nodules. To repress the immunity from the host, the mutant may secret more Nops protein to the host plant through increased T3SS and T4SS [59][60][61][62][63] . In brief, the antagonistic relationship between dSH3_IP mutant and the host soybean increased and damage the effective SNF.…”