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

Abstract

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.

A Look at Plant-Growth-Promoting Bacteria

“…Recently, new rhizobial VOCs, such as methylketone 2-tridecanone (2-TDC), showed the ability to regulate the growth of Medicago truncatula and sorghum (Sorghum bicolor) and to protect tomato (Solanum lycopersicum) against infection by P. syringae pv. tomato [138]. Knowing the microbial signals that activate specific responses in plants would help develop bioinoculants that allow for the establishment of crops in nutrient-poor soils or in the presence of phytopathogens.…”

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

“…However, no specific role of plant-emitted VOCs has been assigned to the symbiosis with rhizobia. On the contrary, for rhizobium-emitted VOCs, there is evidence that they may have a role in the promotion of non-host plant growth and in the increase in iron uptake mechanisms, rhizosphere acidification and increased root ferric reductase in host legumes [ 101 ]. However, most of the research and data are on soluble molecules, emitted by roots and microbes.…”

“…On the other hand, their role in the interaction with plants has been known for almost two decades, and their emission improves plant growth by stimulating root development, chlorophyll production and the uptake of essential elements such as iron. They are also involved in plant tolerance to abiotic and biotic stresses, through antibiotic activity and activation of plant immunity [ 101 ].…”