Invasion of rhizobial infection thread by non-rhizobia for colonization ofVigna radiataroot nodules

Abstract

Legumes develop symbiotic relationships with Rhizobium by a complex exchange of signals. Despite the high specificity between symbiotic partners, the presence of non-rhizobial bacteria in root nodules has been reported. To investigate how these rhizobacteria enter root nodules, fluorescently tagged Pseudomonas fluorescens and Klebsiella pneumoniae were co-inoculated with host-nodulating Ensifer adhaerens to Vigna radiata seedlings and root hair infection was monitored using confocal microscopy at 5 days post inoculation. Pseudomonas fluorescens and K. pneumoniae invaded the root hair only when co-inoculated with E. adhaerens. Recovery of inoculated tagged strains and confirmation through CLSM and 16S rRNA gene sequencing confirmed that the test rhizobacteria occupied nodules. We hereby report with the help of confocal microscopy that rhizobacteria migrate along the length of host-nodulating rhizobial strain and become localized in root nodules. We further report isolation of eight non-rhizobial bacterial genera, predominantly Bacillus spp. and Paenibacillus spp., from nodules of field-grown V. radiata.

The Root Hair “Infectome” of Medicago truncatula Uncovers Changes in Cell Cycle Genes and Reveals a Requirement for Auxin Signaling in Rhizobial Infection

“…This indicates that M. truncatula may produce antimicrobials in during infection and is consistent with the detection of medicarpin during nodulation (Savouré et al, 1997). This could limit opportunistic and/or pathogenic infections (Pandya et al, 2013). S. meliloti is resistant to medicarpin, while several other incompatible rhizobia (Bradyrhizobium and Mesorhizobium spp) are not (Pankhurst and Biggs, 1980), suggesting limitation of infection by inappropriate rhizobia.…”

The Nodule Microbiome: N2-Fixing Rhizobia Do Not Live Alone

“…Klebsiella and Pseudomonas have been reisolated from nodules containing both g-proteobacteria and also Ensifer adhaerens (Pandya et al 2013). In earlier studies, both genera were isolated from root nodules of Arachis hypogaea and then coinoculated with Bradyrhizobium sp.…”

Nonnodulating Bradyrhizobium spp. Modulate the Benefits of Legume-Rhizobium Mutualism

“…Rhizobia are increasingly being understood to have multifarious lifestyles, including root-nodule symbiosis, colonization of plant roots in the rhizosphere or as root endophytes, and independent growth in the soil or other habitats (24)(25)(26)(27)(28)32). Yet, these lifestyles can be transient and are only partially dependent on the presence or absence of symbiosis loci, which have been a major focus of research.…”

“…Nodulating strains with the canonical nodulation loci, for instance, are not capable of forming nodules on all hosts (e.g., host specificity) (14,43,44). Furthermore, nonnodulating strains (lacking key nodulation genes) can coinfect legume nodules, expropriating a symbiotic role (24,25). Regardless of what factors determine lifestyle, strains that do not form nodules comprise the majority of rhizobial populations that have been sampled (10)(11)(12)(13)(14)(15)(16)(17)(18).…”

“…More specifically, the relative frequency of nonnodulating versus nodulating rhizobia also varies, but nonnodulating genotypes typically dominate and can encompass as much as 99% of the total rhizobial population (10)(11)(12)(13)(14)(15)(16)(17)(18). Nonnodulating strains can reduce the number of nodules formed by nodulating strains on legume hosts (22,23) and can invade nodule tissues in the presence of closely related nodulating strains (24,25). This suggests that they may be able to reduce nodulating strain fitness through competitive exclusion at the root surface.…”