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

Abstract

Many molecular signals are exchanged between rhizobia and host legume plants, some of which are crucial for symbiosis to take place, while others are modifiers of the interaction, which have great importance in the competition with the soil microbiota and in the genotype-specific perception of host plants. Here, we review recent findings on strain-specific and host genotype-specific interactions between rhizobia and legumes, discussing the molecular actors (genes, gene products and metabolites) which play a role in the establishment of symbiosis, and highlighting the need for research including the other components of the soil (micro)biota, which could be crucial in developing rational-based strategies for bioinoculants and synthetic communities' assemblage.

Keywords: nitrogen fixation; rhizobium genomics; sustainable agriculture; symbiotic interaction.

Figure 1

Genotype × genotype (G × G) interactions in the nitrogen-fixing root nodule symbiosis. A schematic example of three phases of symbiosis, namely, early interaction with roots, mature nodules and release of bacteria from dehiscent nodules, is shown. The key molecular actors identified along the phases are reported.

Figure 2

Partition and interaction among replicons in host genotype-specific interaction. The example reported refers to the Sinorhizobium meliloti genome, composed of a chromosome, a chromid (pSymB), a megaplasmid (pSymA) and often smaller accessory plasmids. Arrows connecting replicons are based on gene interaction data from [74,130,131].

Figure 3

Cross-talk in the legume rhizosphere. Within- and among-kingdom interactions giving rise to modulation of symbiotic establishment are depicted, spanning from protists’ role as grazers of rhizobia to fungi, soil microbiota and nematodes which are attracted toward roots and may transport rhizobia.