NodD1 and NodD2 Are Not Required for the Symbiotic Interaction of Bradyrhizobium ORS285 with Nod-Factor-Independent Aeschynomene Legumes

Signaling in Legume-Rhizobia Symbiosis

Legumes represent an important source of food protein for human nutrition and animal feed. Therefore, sustainable production of legume crops is an issue of global importance. It is well-known that legume-rhizobia symbiosis allows an increase in the productivity and resilience of legume crops. The efficiency of this mutualistic association strongly depends on precise regulation of the complex interactions between plant and rhizobia. Their molecular dialogue represents a complex multi-staged process, each step of which is critically important for the overall success of the symbiosis. In particular, understanding the details of the molecular mechanisms behind the nodule formation and functioning might give access to new legume cultivars with improved crop productivity. Therefore, here we provide a comprehensive literature overview on the dynamics of the signaling network underlying the development of the legume-rhizobia symbiosis. Thereby, we pay special attention to the new findings in the field, as well as the principal directions of the current and prospective research. For this, here we comprehensively address the principal signaling events involved in the nodule inception, development, functioning, and senescence.

Nod factor perception: an integrative view of molecular communication during legume symbiosis

Compatible interaction between rhizobial Nod factors and host receptors enables initial recognition and signaling events during legume-rhizobia symbiosis. Molecular communication is a new paradigm of information relay, which uses chemical signals or molecules as dialogues for communication and has been witnessed in prokaryotes, plants as well as in animal kingdom. Understanding this fascinating relay of signals between plants and rhizobia during the establishment of a synergistic relationship for biological nitrogen fixation represents one of the hotspots in plant biology research. Predominantly, their interaction is initiated by flavonoids exuding from plant roots, which provokes changes in the expression profile of rhizobial genes. Compatible interactions promote the secretion of Nod factors (NFs) from rhizobia, which are recognised by cognate host receptors. Perception of NFs by host receptors initiates the symbiosis and ultimately leads to the accommodation of rhizobia within root nodules via a series of mutual exchange of signals. This review elucidates the bacterial and plant perspectives during the early stages of symbiosis, explicitly emphasizing the significance of NFs and their cognate NF receptors.

Coordinated regulation of symbiotic adaptation by NodD proteins and NolA in the type I peanut bradyrhizobial strain Bradyrhizobium zhanjiangense CCBAU51778

Type I peanut bradyrhizobial strains can establish efficient symbiosis in contrast to symbiotic incompatibility induced by type II strains with mung bean. The notable distinction in the two kinds of key symbiosis-related regulators nolA and nodD close to the nodABCSUIJ operon region between these two types of peanut bradyrhizobia was found. Therefore, we determined whether NolA and NodD proteins regulate the symbiotic adaptations of type I strains to different hosts. We found that NodD1-NolA synergistically regulated the symbiosis between the type I strain Bradyrhizobium zhanjiangense CCBAU51778 and mung bean, and NodD1-NodD2 jointly regulated nodulation ability. In contrast, NodD1-NolA coordinately regulated nodulation ability in the CCBAU51778-peanut symbiosis. Meanwhile, NodD1 and NolA collectively contributes to competitive nodule colonization of CCBAU51778 on both hosts. The Fucosylated Nod factors and intact type 3 secretion system (T3SS), rather than extra nodD2 and full-length nolA, were critical for effective symbiosis with mung bean. Unexpectedly, T3SS-related genes were activated by NodD2 but not NodD1. Compared to NodD1 and NodD2, NolA predominantly inhibits exopolysaccharide production by promoting exoR expression. Importantly, this is the first report that NolA regulates rhizobial T3SS-related genes. The coordinated regulation and integration of different gene networks to fine-tune the expression of symbiosis-related genes and other accessory genes by NodD1-NolA might be required for CCBAU51778 to efficiently nodulate peanut. This study shed new light on our understanding of the regulatory roles of NolA and NodD proteins in symbiotic adaptation, highlighting the sophisticated gene networks dominated by NodD1-NolA.

Synthesis of Bradyrhizose from d-Glucose

We describe the synthesis of the unusual bicyclic sugar bradyrhizose in 14 steps and a 6% overall yield from d-glucose. The synthesis involves the elaboration of a trans-fused carbocyclic ring onto the preexisting glucopyranose framework followed by adjustment of the oxidation levels. Key steps include radical extension of the glucopyranose side chain, ring closing metathesis, allylic oxidation, Luche reduction, hydroxy-directed epoxidation, and acid-catalyzed epoxide opening at the more substituted position.

The Modification of the Flavonoid Naringenin by Bradyrhizobium sp. Strain ORS285 Changes the nod Genes Inducer Function to a Growth Stimulator

As inducers of nodulation (nod) genes, flavonoids play an important role in the symbiotic interaction between rhizobia and legumes. However, in addition to the control of expression of nod genes, many other effects of flavonoids on rhizobial cells have been described. Here, we show that the flavonoid naringenin stimulates the growth of the photosynthetic Bradyrhizobium sp. strain ORS285. This growth-stimulating effect was still observed for strain ORS285 with nodD1, nodD2, or the naringenin-degrading fde operon deleted. Phenotypic microarray analysis indicates that in cells grown in the presence of naringenin, the glycerol and fatty acid metabolism is activated. Moreover, electron microscopic and enzymatic analyses show that polyhydroxy alkanoate metabolism is altered in cells grown in the presence of naringenin. Although strain ORS285 was able to degrade naringenin, a fraction was converted into an intensely yellow-colored molecule with an m/z (+) of 363.0716. Further analysis indicates that this molecule is a hydroxylated and O-methylated form of naringenin. In contrast to naringenin, this derivative did not induce nod gene expression, but it did stimulate the growth of strain ORS285. We hypothesize that the growth stimulation and metabolic changes induced by naringenin are part of a mechanism to facilitate the colonization and infection of naringenin-exuding host plants.

Symbiotic properties of a chimeric Nod-independent photosynthetic Bradyrhizobium strain obtained by conjugative transfer of a symbiotic plasmid

The lateral transfer of symbiotic genes converting a predisposed soil bacteria into a legume symbiont has occurred repeatedly and independently during the evolution of rhizobia. We experimented the transfer of a symbiotic plasmid between Bradyrhizobium strains. The originality of the DOA9 donor is that it harbours a symbiotic mega-plasmid (pDOA9) containing nod, nif and T3SS genes while the ORS278 recipient has the unique property of inducing nodules on some Aeschynomene species in the absence of Nod factors (NFs). We observed that the chimeric strain ORS278-pDOA9* lost its ability to develop a functional symbiosis with Aeschynomene. indica and Aeschynomene evenia. The mutation of rhcN and nodB led to partial restoration of nodule efficiency, indicating that T3SS effectors and NFs block the establishment of the NF-independent symbiosis. Conversely, ORS278-pDOA9* strain acquired the ability to form nodules on Crotalaria juncea and Macroptillium artropurpureum but not on NF-dependent Aeschynomene (A. afraspera and A. americana), suggesting that the ORS278 strain also harbours incompatible factors that block the interaction with these species. These data indicate that the symbiotic properties of a chimeric rhizobia cannot be anticipated due to new combination of symbiotic and non-symbiotic determinants that may interfere during the interaction with the host plant.

Transcriptomic dissection of Bradyrhizobium sp. strain ORS285 in symbiosis with Aeschynomene spp. inducing different bacteroid morphotypes with contrasted symbiotic efficiency

To circumvent the paucity of nitrogen sources in the soil legume plants establish a symbiotic interaction with nitrogen-fixing soil bacteria called rhizobia. During symbiosis, the plants form root organs called nodules, where bacteria are housed intracellularly and become active nitrogen fixers known as bacteroids. Depending on their host plant, bacteroids can adopt different morphotypes, being either unmodified (U), elongated (E) or spherical (S). E- and S-type bacteroids undergo a terminal differentiation leading to irreversible morphological changes and DNA endoreduplication. Previous studies suggest that differentiated bacteroids display an increased symbiotic efficiency (E > U and S > U). In this study, we used a combination of Aeschynomene species inducing E- or S-type bacteroids in symbiosis with Bradyrhizobium sp. ORS285 to show that S-type bacteroids present a better symbiotic efficiency than E-type bacteroids. We performed a transcriptomic analysis on E- and S-type bacteroids formed by Aeschynomene afraspera and Aeschynomene indica nodules and identified the bacterial functions activated in bacteroids and specific to each bacteroid type. Extending the expression analysis in E- and S-type bacteroids in other Aeschynomene species by qRT-PCR on selected genes from the transcriptome analysis narrowed down the set of bacteroid morphotype-specific genes. Functional analysis of a selected subset of 31 bacteroid-induced or morphotype-specific genes revealed no symbiotic phenotypes in the mutants. This highlights the robustness of the symbiotic program but could also indicate that the bacterial response to the plant environment is partially anticipatory or even maladaptive. Our analysis confirms the correlation between differentiation and efficiency of the bacteroids and provides a framework for the identification of bacterial functions that affect the efficiency of bacteroids.© 2018 Society for Applied Microbiology and John Wiley & Sons Ltd.

The role of rhizobial (NifV) and plant (FEN1) homocitrate synthases in Aeschynomene/photosynthetic Bradyrhizobium symbiosis

In the most studied rhizobium-legume interactions, the host plant supplies the symbiont with homocitrate, an essential co-factor of the nitrogenase enzyme complex, via the expression of a nodule-specific homocitrate synthase FEN1. Photosynthetic bradyrhizobia interacting with Nod factor (NF) dependent and NF-independent Aeschynomene legumes are able to synthesize homocitrate themselves as they contain a nifV gene encoding a homocitrate synthase. Here, we show that in the model strain ORS285, nifV is required for free-living and symbiotic dinitrogen fixation with NF-independent Aeschynomene species. In contrast, in symbiosis with NF-dependent Aeschynomene species, the nifV requirement for efficient nitrogen fixation was found to be host plant dependent. Interestingly, orthologs of FEN1 were found in both NF-dependent and NF-independent Aeschynomene species. However, a high nodule specific induction of FEN1 expression was only observed in A. afraspera, a host plant in which nifV is not required for symbiotic dinitrogen fixation. These data indicate that efficient symbiotic nitrogen fixation in many of the tested Aeschynomene species requires rhizobial homocitrate synthesis. Considering that more than 10% of the fully sequenced rhizobium strains do contain a nifV gene, the Aeschynomene/photosynthetic Bradyrhizobium interaction is likely not the only rhizobium/legume symbiosis where rhizobial nifV expression is required.