A protein complex required for polar growth of rhizobial infection threads

Cheng-Wu Liu,Andrew Breakspear,Karunakaran Ramakrishnan,Jin Nakashima,Fernanda De Carvalho-Niebel,Kim Findlay,Michael K Udvardi,Nicola Stacey,Miaoxia Liu,Gabriella Endre,Giles E D Oldroyd,Joëlle Fournier,Jeremy D Murray,Fang Xie

doi:10.1038/s41467-019-10029-y

Abstract

During root nodule symbiosis, intracellular accommodation of rhizobia by legumes is a prerequisite for nitrogen fixation. For many legumes, rhizobial colonization initiates in root hairs through transcellular infection threads. In Medicago truncatula, VAPYRIN (VPY) and a putative E3 ligase LUMPY INFECTIONS (LIN) are required for infection thread development but their cellular and molecular roles are obscure. Here we show that LIN and its homolog LIN-LIKE interact with VPY and VPY-LIKE in a subcellular complex localized to puncta both at the tip of the growing infection thread and at the nuclear periphery in root hairs and that the punctate accumulation of VPY is positively regulated by LIN. We also show that an otherwise nuclear and cytoplasmic exocyst subunit, EXO70H4, systematically co-localizes with VPY and LIN during rhizobial infection. Genetic analysis shows that defective rhizobial infection in exo70h4 is similar to that in vpy and lin. Our results indicate that VPY, LIN and EXO70H4 are part of the symbiosis-specific machinery required for polar growth of infection threads.

Highlights

During root nodule symbiosis, intracellular accommodation of rhizobia by legumes is a prerequisite for nitrogen fixation
To better define how VPY contributes to infection thread development in root hairs, we used live cell imaging to compare the earliest stages of root hair infection in the M. truncatula vpy-1 mutant and in sunn-2 plants after inoculation with the GFP-tagged Sinorhizobium meliloti strain Sm2011 (Sm2011-CFP) (Fig. 1a–h)
While the main signalling pathway is well characterized in nodulation, the molecular and cellular mechanisms controlling rhizobial infection are far from clear[53]

Summary

Introduction

Intracellular accommodation of rhizobia by legumes is a prerequisite for nitrogen fixation. Medicago truncatula and Lotus japonicus, rhizobial infection initiates in root hairs after perception of rhizobia-produced signalling molecules known as Nodulation (Nod) factors by specific receptors of the host legume[6,7]. Downstream of these Nod factor receptors are various cellular components required for nuclearassociated calcium signalling, involving the generation and decoding of sustained intranuclear calcium spiking[8,9,10,11,12]. The rhizobia are released intracellularly from the infection threads to form symbiosomes where they differentiate into nitrogen-fixing bacteroids[31]

Methods

Results

Conclusion