Explaining coexistence of nitrogen fixing and non-fixing rhizobia in legume-rhizobia mutualism using mathematical modeling

Diversity of common bean rhizobia in blackland of northeastern China and their symbiotic compatibility with two host varieties

The common bean (Phaseolus vulgaris L.) is an important crop in the world that forms root nodules with diverse rhizobia. Aiming to learn the rhizobial communities associated with the common bean in the black soil of Northeast China, 79 rhizobia were isolated from root nodules of two host varieties (Cuican and Jiadouwang) grown in two sites of blackland and were characterized by comparative sequence analyses of 16S rRNA, recA, atpD, nodC, and nifH genes, and whole genome. As a result, Rhizobium indigoferae, R. anhuiense, and R. croatiense as minor groups and three dominant novel Rhizobium species were identified based on their average nucleotide identity and DNA-DNA hybridization values to the type strains of relative species. This community composition of rhizobia associated with the common bean in the tested black soils was unique. Despite their different species affiliations, all of them were identified into the symbiovar phaseoli according to the phylogenies of symbiotic genes, nodC and nifH. While the phylogenetic discrepancies found in nodC, nifH evidenced that the evolutions of nodulation (nod) and nitrogen fixation (nif ) genes were partially independent. In addition, only one dominant rhizobial species was shared by the two common bean varieties grown in the two soil samples, implying that both the plant variety and the soil characteristics affected the compatibility between rhizobia and their hosts. These findings further enlarged the spectrum of common bean-nodulating rhizobia and added more information about the interactions among the soil factors, rhizobial species, and host plants in the symbiosis.

Innovation and appropriation in mycorrhizal and rhizobial Symbioses

Most land plants benefit from endosymbiotic interactions with mycorrhizal fungi, including legumes and some nonlegumes that also interact with endosymbiotic nitrogen (N)-fixing bacteria to form nodules. In addition to these helpful interactions, plants are continuously exposed to would-be pathogenic microbes: discriminating between friends and foes is a major determinant of plant survival. Recent breakthroughs have revealed how some key signals from pathogens and symbionts are distinguished. Once this checkpoint has been passed and a compatible symbiont is recognized, the plant coordinates the sequential development of two types of specialized structures in the host. The first serves to mediate infection, and the second, which appears later, serves as sophisticated intracellular nutrient exchange interfaces. The overlap in both the signaling pathways and downstream infection components of these symbioses reflects their evolutionary relatedness and the common requirements of these two interactions. However, the different outputs of the symbioses, phosphate uptake versus N fixation, require fundamentally different components and physical environments and necessitated the recruitment of different master regulators, NODULE INCEPTION-LIKE PROTEINS, and PHOSPHATE STARVATION RESPONSES, for nodulation and mycorrhization, respectively.

A compartmental model for antibiotic resistant bacterial infections over networks

This paper presents a compartmental model for bacterial infections in a population distributed over a network of individuals. Within each node, individuals interact, bacteria can be transmitted and the disease may be spread; moreover, the acquisition of bacterial antibiotic resistance is considered. In addition, nodes are connected through weighted edges, and consequently individuals from different nodes may interact. As a result, the infection may be propagated over the network. We perform an analysis on this propagation as well as numerical simulations in order to illustrate the validity of the model.

Type III secretion systems impact Mesorhizobium amorphae CCNWGS0123 compatibility with Robinia pseudoacacia

Rhizobia and legume plants are famous mutualistic symbiosis partners who provide nitrogen nutrition to the natural environment. Rhizobial type III secretion systems (T3SSs) deliver effectors that manipulate the metabolism of eukaryotic host cells. Mesorhizobium amorphae CCNWGS0123 (GS0123) contains two T3SS gene clusters, T3SS-I and T3SS-II. T3SS-I contains all the basal components for an integrated T3SS, and the expression of T3SS-I genes is up-regulated in the presence of flavonoids. In contrast, T3SS-II lacks the primary extracellular elements of T3SSs, and the expression of T3SS-II genes is down-regulated in the presence of flavonoids. Inoculation tests on Robinia pseudoacacia displayed considerable differences in gene expression patterns and levels among roots inoculated with GS0123 and T3SS-deficient mutant (GS0123ΔrhcN1 (GS0123ΔT1), GS0123ΔrhcN2 (GS0123ΔT2) and GS0123ΔrhcN1ΔrhcN2 (GS0123ΔS)). Compared with the GS0123-inoculated plants, GS0123ΔT1-inoculated roots formed very few infection threads and effective nodules, while GS0123ΔT2-inoculated roots formed a little fewer infection threads and effective nodules with increased numbers of bacteroids enclosed in one symbiosome. Moreover, almost no infection threads or effective nodules were observed in GS0123ΔS-inoculated roots. In addition to evaluations of plant immunity signals, we observed that the coexistence of T3SS-I and T3SS-II promoted infection by suppressing host defense response in the reactive oxygen species defense response pathway. Future studies should focus on identifying rhizobial T3SS effectors and their host target proteins.