The Presence of Plant-Associated Bacteria Alters Responses to N-acyl Homoserine Lactone Quorum Sensing Signals that Modulate Nodulation in Medicago Truncatula

Protocol: a simple method for biosensor visualization of bacterial quorum sensing and quorum quenching interaction on Medicago roots

BACKGROUND

Defining interactions of bacteria in the rhizosphere (encompassing the area near and on the plant root) is important to understand how they affect plant health. Some rhizosphere bacteria, including plant growth promoting rhizobacteria (PGPR) engage in the intraspecies communication known as quorum sensing (QS). Many species of Gram-negative bacteria use extracellular autoinducer signal molecules called N-acyl homoserine lactones (AHLs) for QS. Other rhizobacteria species, including PGPRs, can interfere with or disrupt QS through quorum quenching (QQ). Current AHL biosensor assays used for screening and identifying QS and QQ bacteria interactions fail to account for the role of the plant root.

METHODS

Medicago spp. seedlings germinated on Lullien agar were transferred to soft-agar plates containing the broad-range AHL biosensor Agrobacterium tumefaciens KYC55 and X-gal substrate. Cultures of QS and QQ bacteria as well as pure AHLs and a QQ enzyme were applied to the plant roots and incubated for 3 days.

RESULTS

We show that this expanded use of an AHL biosensor successfully allowed for visualization of QS/QQ interactions localized at the plant root. KYC55 detected pure AHLs as well as AHLs from live bacteria cultures grown directly on the media. We also showed clear detection of QQ interactions occurring in the presence of the plant root.

CONCLUSIONS

Our novel tri-trophic system using an AHL biosensor is useful to study QS interspecies interactions in the rhizosphere.

Interaction of Symbiotic Rhizobia and Parasitic Root-Knot Nematodes in Legume Roots: From Molecular Regulation to Field Application

Legumes form two types of root organs in response to signals from microbes, namely, nodules and root galls. In the field, these interactions occur concurrently and often interact with each other. The outcomes of these interactions vary and can depend on natural variation in rhizobia and nematode populations in the soil as well as abiotic conditions. While rhizobia are symbionts that contribute fixed nitrogen to their hosts, parasitic root-knot nematodes (RKN) cause galls as feeding structures that consume plant resources without a contribution to the plant. Yet, the two interactions share similarities, including rhizosphere signaling, repression of host defense responses, activation of host cell division, and differentiation, nutrient exchange, and alteration of root architecture. Rhizobia activate changes in defense and development through Nod factor signaling, with additional functions of effector proteins and exopolysaccharides. RKN inject large numbers of protein effectors into plant cells that directly suppress immune signaling and manipulate developmental pathways. This review examines the molecular control of legume interactions with rhizobia and RKN to elucidate shared and distinct mechanisms of these root-microbe interactions. Many of the molecular pathways targeted by both organisms overlap, yet recent discoveries have singled out differences in the spatial control of expression of developmental regulators that may have enabled activation of cortical cell division during nodulation in legumes. The interaction of legumes with symbionts and parasites highlights the importance of a comprehensive view of root-microbe interactions for future crop management and breeding strategies.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Microbial Community Field Surveys Reveal Abundant Pseudomonas Population in Sorghum Rhizosphere Composed of Many Closely Related Phylotypes

While the root-associated microbiome is typically less diverse than the surrounding soil due to both plant selection and microbial competition for plant derived resources, it typically retains considerable complexity, harboring many hundreds of distinct bacterial species. Here, we report a time-dependent deviation from this trend in the rhizospheres of field grown sorghum. In this study, 16S rRNA amplicon sequencing was used to determine the impact of nitrogen fertilization on the development of the root-associated microbiomes of 10 sorghum genotypes grown in eastern Nebraska. We observed that early rhizosphere samples exhibit a significant reduction in overall diversity due to a high abundance of the bacterial genus Pseudomonas that occurred independent of host genotype in both high and low nitrogen fields and was not observed in the surrounding soil or associated root endosphere samples. When clustered at 97% identity, nearly all the Pseudomonas reads in this dataset were assigned to a single operational taxonomic unit (OTU); however, exact sequence variant (ESV)-level resolution demonstrated that this population comprised a large number of distinct Pseudomonas lineages. Furthermore, single-molecule long-read sequencing enabled high-resolution taxonomic profiling revealing further heterogeneity in the Pseudomonas lineages that was further confirmed using shotgun metagenomic sequencing. Finally, field soil enriched with specific carbon compounds recapitulated the increase in Pseudomonas, suggesting a possible connection between the enrichment of these Pseudomonas species and a plant-driven exudate profile.

Rhizobial Volatiles: Potential New Players in the Complex Interkingdom Signaling With Legumes

Bacteria release a wide range of volatile compounds that play important roles in intermicrobial and interkingdom communication. Volatile metabolites emitted by rhizobacteria can promote plant growth and increase plant resistance to both biotic and abiotic stresses. Rhizobia establish beneficial nitrogen-fixing symbiosis with legume plants in a process starting with a chemical dialog in the rhizosphere involving various diffusible compounds. Despite being one of the most studied plant-interacting microorganisms, very little is known about volatile compounds produced by rhizobia and their biological/ecological role. Evidence indicates that plants can perceive and respond to volatiles emitted by rhizobia. In this perspective, we present recent data that open the possibility that rhizobial volatile compounds have a role in symbiotic interactions with legumes and discuss future directions that could shed light onto this area of investigation.