Rhizobial Effector NopT Targets Nod Factor Receptors to Regulate Symbiosis in Lotus japonicus

Core Concepts

The rhizobial effector NopT directly targets and cleaves the Nod factor receptors NFR1 and NFR5 to suppress symbiotic signaling, which is then negatively regulated by NFR1-mediated phosphorylation of NopT.

Abstract

The study investigates the role of the rhizobial effector NopT in regulating the legume-rhizobium symbiosis. Key findings: NopT, a YopT-type cysteine protease from Sinorhizobium fredii NGR234, can specifically suppress the cell death response triggered by the co-overexpression of Nod factor receptors NFR1 and NFR5 in Nicotiana benthamiana leaves. NopT interacts with and colocalizes with both NFR1 and NFR5 at the plasma membrane. NopT proteolytically cleaves NFR5 at its juxtamembrane domain, but not NFR1. NFR1 can phosphorylate NopT, which inactivates its protease activity and suppresses the cleavage of NFR5. The truncated version of NopT after autocleavage retains the ability to cleave NFR5 but loses its interaction with NFR1. The protease activity of NopT and its phosphorylation status are important for regulating rhizobial infection in Lotus japonicus. Strains lacking NopT or expressing phosphomimetic versions of NopT show increased rhizobial infection. Some Sinorhizobium species possess only the truncated version of NopT, which may evade suppression by NFR1, suggesting diverse regulatory mechanisms among rhizobial strains. The study unveils an intricate regulatory interplay between legumes and rhizobia, where the rhizobial effector NopT directly targets the Nod factor receptors to suppress symbiotic signaling, which is then negatively regulated by the legume receptor NFR1 through phosphorylation.

Stats

Cysteine-93 of NopT is essential for its protease activity. NopT can specifically suppress cell death induced by co-expression of NFR1 and NFR5, but not cell death triggered by other proteins. NopT colocalizes and interacts with both NFR1 and NFR5 at the plasma membrane. NopT proteolytically cleaves NFR5 at its juxtamembrane domain, but not NFR1. NFR1 can phosphorylate NopT, which inactivates its protease activity. The truncated version of NopT after autocleavage retains the ability to cleave NFR5 but loses its interaction with NFR1. Strains lacking NopT or expressing phosphomimetic versions of NopT show increased rhizobial infection in Lotus japonicus. Some Sinorhizobium species possess only the truncated version of NopT.

Quotes

"NopT, a YopT-type cysteine protease from Sinorhizobium fredii NGR234, can specifically suppress the cell death response triggered by the co-overexpression of Nod factor receptors NFR1 and NFR5 in Nicotiana benthamiana leaves." "NopT proteolytically cleaves NFR5 at its juxtamembrane domain, but not NFR1." "NFR1 can phosphorylate NopT, which inactivates its protease activity and suppresses the cleavage of NFR5."

Key Insights Distilled From

The Rhizobial effector NopT targets Nod factor receptors to regulate symbiosis in Lotus japonicus

by Bao,H., yana... at www.biorxiv.org 03-11-2024

https://www.biorxiv.org/content/10.1101/2024.03.06.583716v2

Deeper Inquiries

How might the cleavage of NFR5 by NopT affect the downstream symbiotic signaling pathways in legumes?

The cleavage of NFR5 by NopT can have significant implications for downstream symbiotic signaling pathways in legumes. NFR5 is a crucial component in the recognition of rhizobial lipo-chitooligosaccharide NFs, initiating the symbiotic signaling transduction cascade (Broghammer et al., 2012; Bozsoki et al., 2017). By cleaving NFR5 at the juxtamembrane domain, NopT releases the kinase domain of NFR5, potentially disrupting the normal functioning of the receptor. This cleavage could lead to the dysregulation of downstream signaling events, affecting the activation of symbiotic genes, the formation of infection threads, and the development of nodules. The release of the kinase domain of NFR5 might interfere with the proper transmission of signals within the plant cell, ultimately impacting the establishment and maintenance of the symbiotic relationship between the legume and the rhizobium. Additionally, the cleavage of NFR5 by NopT could trigger defense-like responses in the plant, leading to the activation of immune pathways that may hinder the successful establishment of symbiosis.

What other cellular processes or plant responses might be impacted by the proteolytic activity of NopT beyond its effects on Nod factor receptors?

Beyond its effects on Nod factor receptors, the proteolytic activity of NopT could impact various cellular processes and plant responses. One potential impact could be the cleavage of other receptor-like kinases (RLKs) in the plant that are involved in different signaling pathways. As demonstrated by the cleavage of AtLYK5 and LjLYS11, NopT has the ability to target and cleave additional receptors, potentially affecting plant defense responses to other pathogens or environmental stimuli. The proteolytic activity of NopT could also disrupt the normal functioning of other signaling components within the plant, leading to the dysregulation of pathways involved in growth, development, and stress responses. Furthermore, the cleavage of key proteins by NopT may trigger cell death pathways or induce defense-like responses in the plant, impacting overall plant health and fitness. The proteolytic activity of NopT could have far-reaching effects on plant physiology beyond its direct interaction with Nod factor receptors, highlighting the complexity of the plant-microbe interaction.

Given the diverse regulatory mechanisms observed among different rhizobial strains, how might the evolution of NopT homologs and their interactions with legume receptors contribute to the adaptation and host specificity of rhizobia?

The evolution of NopT homologs and their interactions with legume receptors play a crucial role in the adaptation and host specificity of rhizobia. The diversity in regulatory mechanisms observed among different rhizobial strains, such as the presence of truncated NopT variants or variations in proteolytic activity, reflects the evolutionary strategies employed by rhizobia to interact with their host plants. The presence of NopT homologs with varying proteolytic activities and substrate specificities suggests that different rhizobial strains have evolved distinct mechanisms to modulate plant signaling pathways and immune responses. This diversity allows rhizobia to adapt to different host environments and overcome host defenses, contributing to their host specificity. The interactions between NopT homologs and legume receptors determine the outcome of the symbiotic relationship, influencing the efficiency of nodulation, the establishment of symbiosis, and the overall success of the interaction. The co-evolution of NopT homologs and legume receptors likely drives the adaptation of rhizobia to specific host plants, shaping the diversity and specificity of rhizobial symbiotic associations.