The Lotus japonicus ROP3 Is Involved in the Establishment of the Nitrogen-Fixing Symbiosis but Not of the Arbuscular Mycorrhizal Symbiosis.

Ivette García-Soto,Mario Serrano,Raphael Boussageon,Daniel Wipf,Pierre-Emmanuel Courty,Victor Bustos-Zagal,Alexandre Tromas,Georgina Hernández,Liz Xochiquetzal Hernández-Cerezo,Damien Formey,Jesus Daniel Castro-Chilpa,Yareni Marlene Cruz-Farfán,Alfonso Leija-Salas,Martha Torres

doi:10.3389/fpls.2021.696450

Abstract

Legumes form root mutualistic symbioses with some soil microbes promoting their growth, rhizobia, and arbuscular mycorrhizal fungi (AMF). A conserved set of plant proteins rules the transduction of symbiotic signals from rhizobia and AMF in a so-called common symbiotic signaling pathway (CSSP). Despite considerable efforts and advances over the past 20 years, there are still key elements to be discovered about the establishment of these root symbioses. Rhizobia and AMF root colonization are possible after a deep cell reorganization. In the interaction between the model legume Lotus japonicus and Mesorhizobium loti, this reorganization has been shown to be dependent on a SCAR/Wave-like signaling module, including Rho-GTPase (ROP in plants). Here, we studied the potential role of ROP3 in the nitrogen-fixing symbiosis (NFS) as well as in the arbuscular mycorrhizal symbiosis (AMS). We performed a detailed phenotypic study on the effects of the loss of a single ROP on the establishment of both root symbioses. Moreover, we evaluated the expression of key genes related to CSSP and to the rhizobial-specific pathway. Under our experimental conditions, rop3 mutant showed less nodule formation at 7- and 21-days post inoculation as well as less microcolonies and a higher frequency of epidermal infection threads. However, AMF root colonization was not affected. These results suggest a role of ROP3 as a positive regulator of infection thread formation and nodulation in L. japonicus. In addition, CSSP gene expression was neither affected in NFS nor in AMS condition in rop3 mutant. whereas the expression level of some genes belonging to the rhizobial-specific pathway, like RACK1, decreased in the NFS. In conclusion, ROP3 appears to be involved in the NFS, but is neither required for intra-radical growth of AMF nor arbuscule formation.

Highlights

Roots of legumes interact with multiple soil beneficial microorganisms, including bacteria and fungi
We found that ROP3 is involved in the establishment of the nitrogen-fixing symbiosis (NFS) and that this gene is not required for the arbuscular-mycorrhizal symbiosis (AMS) in L. japonicus
We observed significantly reduced nodulation in rop3 compared to Gifu wild-type plants, suggesting a strong effect on nodule formation (Figure 1A)

Summary

Introduction

Roots of legumes interact with multiple soil beneficial microorganisms, including bacteria and fungi. In NFS, plants offer an anoxic environment, nutrients and organic acids to the diazotrophic bacteria, which in exchange provide fixed atmospheric nitrogen to the host plant This process occurs in the nodule, a root structure derived from inner or outer cortical cells, depending on whether the organ belongs to the indeterminate or determinate type, respectively (Roy et al, 2020). In AMS, hyphae of arbuscular mycorrhizal fungi (AMF) penetrate root epidermis, colonize cortical cells and form highly branched structures called arbuscules This ancestral interaction allows plants to improve the use of natural soil resources including macronutrients (e.g., phosphorus and nitrogen) and water, and to better respond to abiotic and biotic stresses (Gianinazzi et al, 2010; Wipf et al, 2019)

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Conclusion