A protein complex required for polar growth of rhizobial infection threads

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.

Celebrating 20 Years of Genetic Discoveries in Legume Nodulation and Symbiotic Nitrogen Fixation

“…Recent research points to the existence of an exocyst complex at the tip of the IT, termed the "infectosome," which ensures polar growth of the IT toward the nodule cortex ( Figure 4; Liu et al, 2019a). The infectosome comprises the common symbiotic protein MtVPY (MtVAPYRIN) and its two interacting partners, MtLIN (LUMPY INFECTION) and the exocyst complex subunit MtEXO70 H4 (EXOCYST subunit H4; (Pumplin et al, 2010;Murray, 2011;Liu et al, 2019a). Mutating any of the three components causes defects in IT elongation (Liu et al, 2019c).…”

“…Mutating any of the three components causes defects in IT elongation (Liu et al, 2019c). Both Mtvpy and Mtlin mutants produce few nodules, which are small, white, and uninfected, while Mtexo70 h4 mutants form normal, functional nodules, possibly reflecting genetic redundancy (Murray, 2011;Liu et al, 2019a).…”

A Homeotic Mutation Changes Legume Nodule Ontogeny into Actinorhizal-Type Ontogeny

“…MtLIN encodes a predicted E3 ubiquitin ligase required for infection thread formation. Mtlin mutants only form small nodule primordia with very few cortex-derived cells (Kuppusamy et al, 2004;Kiss et al, 2009;Guan et al, 2013;Liu et al, 2019a).…”

Nodule Inception Is Not Required for Arbuscular Mycorrhizal Colonization of Medicago truncatula

“…A second widespread symbiosis, called nodulation, can be formed by most species belonging to the third largest plant family, the legumes (Fabaceae) with gram-negative soil bacteria collectively called rhizobia [4]. The nodulation of most legume plants involves the intracellular infection of root hairs by rhizobia followed by the colonization of the cortical layers of the root [5]. This symbiosis leads to the formation of special root outgrowths called nodules.…”