The Rhizosphere Talk Show: The Rhizobia on Stage

Abstract

From bacterial quorum sensing to the signals of bees, communication is the basis of biotic interactions. Frequently, more than two organisms can take part in the speeches, resulting in a complex network of cross-talks. Recent advances in plant-microbe interactions research have shown that communication, both inter-kingdom and intra-kingdom, is shaped by a broad spectrum of factors. In this context, the rhizosphere (i.e., the soil close to the root surface) provides a specific microhabitat where complex interactions occur. The complex environment that makes up the rhizosphere can select for certain microbial populations, which are adapted to this unique niche. Among them, rhizobia have emerged as an important component of the rhizospheric microbiome. The aim of this review is to explore the components of such a rhizospheric Talk Show in the frame of the rhizobium-legume interactions. This symbiosis is a complex process that involves several signals that can be shaped by plant rhizospheric exudates and microbiome composition. The relationship established by rhizobia with other rhizospheric organisms, together with the influence of the environmental factors, results in their beneficial role on host plant health. Here, we resume research accounting strategies, molecules, and organisms that influence the place of rhizobia in the rhizosphere. The focus is on the most recent approaches for the study and subsequent exploitation of the diversity of the organisms. Indeed, the study of plant-microbes communication and evolution is fundamental to develop highly efficient inoculants able to reduce the use of fertilizers in agriculture.

Rhizosphere Signaling: Insights into Plant–Rhizomicrobiome Interactions for Sustainable Agronomy

“…Quorum sensing (QS) is the cell-to-cell signaling process that takes place through the synthesis, release, and detection of chemical signals [ 44 , 45 ]. These chemical signals are also known as autoinducers (AIs) and they regulate the gene expression of some bacterial functions upon the recognition of the signal by the recipient, i.e., biofilm formation, adhesion, motility [ 46 , 47 , 48 ], propagation, virulence, metabolism [ 49 ], and symbiotic association [ 1 ].…”

“…The rhizosphere is a most captivating environment, which harbors a variety of microorganisms that are deeply involved in plant–microbe communication. This high-density niche allows plants to interact with associated microorganisms through chemical signals that are produced in response to specific stimuli, which in turn activate many regulatory mechanisms [ 1 ]. Rhizospheric regions possess higher bacterial activity than non-rhizospheric regions.…”

Scent of a Symbiont: The Personalized Genetic Relationships of Rhizobium—Plant Interaction

“…For legumes, several clues on the interaction between rhizobia and fungi, especially arbuscular mycorrhizal fungi, are present (for a recent review, see [ 143 ]). Arbuscular mycorrhizal fungi (AMF) penetrate the cortical cells of the roots and ensure nutritional uptake to the plant.…”

“…Further dissection of the rhizospheric cross-talk is therefore crucial if we want to better understand the role of microbial networks in holobiont fitness, in view of agronomic applications [ 143 ]. Here, as mentioned before, a systems biology approach and more realistic experimental setups can help to overcome the symbiont- or host-centric single visions and clarify the effects of such symbioses on all partners [ 141 , 147 ].…”

“…A large array of molecular signals is exchanged between rhizobia and the host legume plant; some of them are crucial for the symbiosis to take place (such as flavonoids and Nod factors), while others are modifiers of the interaction. The latter have great importance in competition in the microbiota, in genotype-specific perception of host plants by the rhizobium and in the ability of the plant to maximize symbiont performances (i.e., nitrogen fixation and metabolic exchange) [ 143 ]. Moreover, the fungal component of the microbiota is stirring increased attention.…”