TRAPPC13 Is a Novel Target of Mesorhizobium amorphae Type III Secretion System Effector NopP

G-type receptor-like kinase AsNIP43 interacts with rhizobia effector nodulation outer protein P and is required for symbiosis

In the Rhizobium-Legume symbiosis, the nodulation outer protein P (NopP) effector is one of the key regulators for rhizobial infection and nodule organogenesis. However, the molecular mechanism through which host legume plants sense NopP remains largely unknown. Here, we constructed an nopP deletion mutant of Mesorhizobium huakuii and found that nopP negatively regulates nodulation on Chinese milk vetch (Astragalus sinicus). Screening for NopP interacting proteins in host plants using the yeast 2-hybrid system identified NopP interacting protein 43 (AsNIP43), which encodes a G-type receptor-like kinase (LecRLK). The B-lectin domain at the N terminus of AsNIP43 was essential in mediating its interaction with NopP, which was confirmed in vitro and in vivo. Subcellular localization, co-localization, and gene expression analyses showed that AsNIP43 and NopP function tightly associated with earlier infection events. RNA interference (RNAi) knockdown of AsNIP43 expression by hairy root transformation led to decreased nodule formation. AsNIP43 plays a positive role in symbiosis, which was further verified in the model legume Medicago truncatula. Transcriptome analysis indicated that MtRLK (a homolog of AsNIP43 in M. truncatula) may function to affect defense gene expression and thus to regulate early nodulation. Taken together, we show that LecRLK AsNIP43 is a legume host target that interacts with rhizobia effector NopP is essential for rhizobial infection and nodulation.

Type III effector provides a novel symbiotic pathway in legume-rhizobia symbiosis

Rhizobia form nodules on the roots of legumes and fix atmospheric nitrogen into ammonia, thus supplying it to host legumes. In return, plants supply photosynthetic products to maintain rhizobial activities. In most cases, rhizobial Nod factors (NFs) and their leguminous receptors (NFRs) are essential for the establishment of symbiosis. However, recent studies have discovered a novel symbiotic pathway in which rhizobia utilize the type III effectors (T3Es) similar to the pathogenic bacteria to induce nodulation. The T3Es of rhizobia are thought to be evolved from the pathogen, but they have a unique structure distinct from the pathogen, suggesting that it might be customized for symbiotic purposes. This review will focus on the recent findings from the study of rhizobial T3Es, discussing their features on a symbiont and pathogen, and the future perspectives on the role of rhizobial T3Es in symbiosis control technology.

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

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.

Rhizobial HmuSpSym as a heme-binding factor is required for optimal symbiosis between Mesorhizobium amorphae CCNWGS0123 and Robinia pseudoacacia

Nitrogen-fixing root nodules are formed by symbiotic association of legume hosts with rhizobia in nitrogen-deprived soils. Successful symbiosis is regulated by signals from both legume hosts and their rhizobial partners. HmuS is a heme degrading factor widely distributed in bacteria, but little is known about the role of rhizobial hmuS in symbiosis with legumes. Here, we found that inactivation of hmuS_pSym in the symbiotic plasmid of Mesorhizobium amorphae CCNWGS0123 disrupted rhizobial infection, primordium formation, and nitrogen fixation in symbiosis with Robinia pseudoacacia. Although there was no difference in bacteroids differentiation, infected plant cells were shrunken and bacteroids were disintegrated in nodules of plants infected by the ΔhmuS_pSym mutant strain. The balance of defence reaction was also impaired in ΔhmuS_pSym strain-infected root nodules. hmuS_pSym was strongly expressed in the nitrogen-fixation zone of mature nodules. Furthermore, the HmuS_pSym protein could bind to heme but not degrade it. Inactivation of hmuS_pSym led to significantly decreased expression levels of oxygen-sensing related genes in nodules. In summary, hmuS_pSym of M. amorphae CCNWGS0123 plays an essential role in nodule development and maintenance of bacteroid survival within R. pseudoacacia cells, possibly through heme-binding in symbiosis.

The Versatile Roles of Type III Secretion Systems in Rhizobia-Legume Symbioses

To suppress plant immunity and promote the intracellular infection required for fixing nitrogen for the benefit of their legume hosts, many rhizobia use type III secretion systems (T3SSs) that deliver effector proteins (T3Es) inside host cells. As reported for interactions between pathogens and host plants, the immune system of legume hosts and the cocktail of T3Es secreted by rhizobia determine the symbiotic outcome. If they remain undetected, T3Es may reduce plant immunity and thus promote infection of legumes by rhizobia. If one or more of the secreted T3Es are recognized by the cognate plant receptors, defense responses are triggered and rhizobial infection may abort. However, some rhizobial T3Es can also circumvent the need for nodulation (Nod) factors to trigger nodule formation. Here we review the multifaceted roles played by rhizobial T3Es during symbiotic interactions with legumes. Expected final online publication date for the Annual Review of Microbiology, Volume 76 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Use of the rhizobial type III effector gene nopP to improve Agrobacterium rhizogenes-mediated transformation of Lotus japonicus

BACKGROUND

Protocols for Agrobacterium rhizogenes-mediated hairy root transformation of the model legume Lotus japonicus have been established previously. However, little efforts were made in the past to quantify and improve the transformation efficiency. Here, we asked whether effectors (nodulation outer proteins) of the nodule bacterium Sinorhizobium sp. NGR234 can promote hairy root transformation of L. japonicus. The co-expressed red fluorescent protein DsRed1 was used for visualization of transformed roots and for estimation of the transformation efficiency.

RESULTS

Strong induction of hairy root formation was observed when A. rhizogenes strain LBA9402 was used for L. japonicus transformation. Expression of the effector gene nopP in L. japonicus roots resulted in a significantly increased transformation efficiency while nopL, nopM, and nopT did not show such an effect. In nopP expressing plants, more than 65% of the formed hairy roots were transgenic as analyzed by red fluorescence emitted by co-transformed DsRed1. A nodulation experiment indicated that nopP expression did not obviously affect the symbiosis between L. japonicus and Mesorhizobium loti.

CONCLUSION

We have established a novel protocol for hairy root transformation of L. japonicus. The use of A. rhizogenes LBA9402 carrying a binary vector containing DsRed1 and nopP allowed efficient formation and identification of transgenic roots.