The Small GTPase ROP6 Interacts with NFR5 and Is Involved in Nodule Formation inLotus japonicus

Danxia Ke,Zonglie Hong,Tao Chen,Zhongming Zhang,Qing Fang,Chunfen Chen,Lian Ma,Heng Kang,Songli Yuan,Hui Zhu,Xiaojun Chang

doi:10.1104/pp.112.197269

Danxia Ke, Zonglie Hong + Show 9 more

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https://doi.org/10.1104/pp.112.197269

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Abstract

Nod Factor Receptor5 (NFR5) is an atypical receptor-like kinase, having no activation loop in the protein kinase domain. It forms a heterodimer with NFR1 and is required for the early plant responses to Rhizobium infection. A Rho-like small GTPase from Lotus japonicus was identified as an NFR5-interacting protein. The amino acid sequence of this Rho-like GTPase is closest to the Arabidopsis (Arabidopsis thaliana) ROP6 and Medicago truncatula ROP6 and was designated as LjROP6. The interaction between Rop6 and NFR5 occurred both in vitro and in planta. No interaction between Rop6 and NFR1 was observed. Green fluorescent protein-tagged ROP6 was localized at the plasma membrane and cytoplasm. The interaction between ROP6 and NFR5 appeared to take place at the plasma membrane. The expression of the ROP6 gene could be detected in vascular tissues of Lotus roots. After inoculation with Mesorhizobium loti, elevated levels of ROP6 expression were found in the root hairs, root tips, vascular bundles of roots, nodule primordia, and young nodules. In transgenic hairy roots expressing ROP6 RNA interference constructs, Rhizobium entry into the root hairs did not appear to be affected, but infection thread growth through the root cortex were severely inhibited, resulting in the development of fewer nodules per plant. These data demonstrate a role of ROP6 as a positive regulator of infection thread formation and nodulation in L. japonicus.

Highlights

Small GTPases of the Rho family play critical roles in regulating cell motility, cell division and directional growth (Etienne-Manneville and Hall, 2002)
We conclude that ROP6 is present in the cytoplasm and plasma membrane
We noticed that all of the control hairy roots produced nodules, whereas about 30% of the ROP6 RNA interference (RNAi) hairy roots did not develop any nodule at all. These results suggest that down-regulation of ROP6 gene expression by RNAi significantly impairs nodulation by inhibiting infection thread formation and nodule initiation

Summary

Introduction

Small GTPases of the Rho family play critical roles in regulating cell motility, cell division and directional growth (Etienne-Manneville and Hall, 2002). Rho GTPases from different organisms execute their biochemical functions in a similar manner, acting as molecular switches in signaling pathways (Etienne-Manneville and Hall, 2002; Kost, 2008). Rho GTPases are regulated by distinct groups of regulatory proteins including GTPase-activating proteins (RhoGAPs), guanine nucleotide exchange factors (RhoGEFs) and guanine nucleotide dissociation inhibitors (RhoGDIs, Etienne-Manneville and Hall, 2002; Kost, 2008). ROP proteins in plants have been implicated in controlling cellular processes such as polarized cell growth, cell morphogenesis, hormone signaling, defense, and responses to oxygen deprivation (Yang 2002; Agrawal et al, 2003; Yang and Fu, 2007). Members of the RhoGAP, RhoGEF and RhoGDI families and several downstream effectors have been characterized in plants (Borg et al, 1999; Kost, 2008; Yalovsky et al, 2008)

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