Rhizobia utilize pathogen‐like effector proteins during symbiosis

Abstract

SummaryA type III protein secretion system (T3SS) is an important host range determinant for the infection of legumes by Rhizobium sp. NGR234. Although a functional T3SS can have either beneficial or detrimental effects on nodule formation, only the rhizobialspecific positively acting effector proteins, NopL and NopP, have been characterized. NGR234 possesses three open reading frames potentially encoding homologues of effector proteins from pathogenic bacteria. NopJ, NopM and NopT are secreted by the T3SS of NGR234. All three can have negative effects on the interaction with legumes, but NopM and NopT also stimulate nodulation on certain plants. NopT belongs to a family of pathogenic effector proteases, typified by the avirulence protein, AvrPphB. The protease domain of NopT is required for its recognition and a subsequent strong inhibition in infection of Crotalaria juncea. In contrast, the negative effects of NopJ are relatively minor when compared with those induced by its Avr homologues. Thus NGR234 uses a mixture of rhizobial-specific and pathogen-derived effector proteins. Whereas some legumes recognize an effector as potentially pathogen-derived, leading to a block in the infection process, others perceive both the negative-and positive-acting effectors concomitantly. It is this equilibrium of effector action that leads to modulation of symbiotic development.

Functional Analysis of the Type 3 Effector Nodulation Outer Protein L (NopL) from Rhizobium sp. NGR234

“…Thus, specific type 3 effectors can function as avirulence factors on certain host plants. Similarly, rhizobial type 3 effectors, such as NopT of NGR234, possess either a positive or negative role in the symbiosis with legume host plants (27,28), and R-proteins control host-specific nodulation in the interaction between soybean and specific rhizobial strains (37). In this study, the nopL mutant NGR⍀nopL was compared with the parent strain NGR234 in nodulation tests with the host plant P. vulgaris (cv.…”

“…and contains ubiquitin ligase motifs that are present in the E3 ubiquitin ligase IpaH 9.8 of Shigella spp. (28). On the other hand, proteins homologous to NopL have not been identified in pathogenic bacteria, indicating that NopL is a Rhizobium-specific effector.…”

“…strain NGR234 (20) possesses a functional type 3 secretion system (21), which can affect nodulation of host plants either positively or negatively. Up to now, four nodulation outer proteins (Nops) of NGR234 have been identified as type 3 effectors, namely NopL (21-24), NopP (25,26), NopT (27,28), and NopM (28). Their functions remain unclear, however.…”

“…and AvrPphB of Pseudomonas syringae pv. phaseolicola (27,28); NopM displays sequence similarities with YopM of Yersinia spp. and contains ubiquitin ligase motifs that are present in the E3 ubiquitin ligase IpaH 9.8 of Shigella spp.…”

Immunosuppression duringRhizobium-legume symbiosis

“…NGR234 secrets a few effectors including NopJ, NopL, NopM, NopP and NopT into host plant cells, where most of them interact with immune signaling pathways to suppress host defense responses. [21][22][23][24] For example, NopL mimics a MAP substrate to impair the MAPK signaling, while NopM is a ubiqutin ligase to reduce the ROS production induced by flg22. 20,21 Therefore, we propose that rhizobial T3SS effectors could trigger ETS to promote Rhizobium infection and survival in host cells.…”