Genetic and Molecular Mechanisms Underlying Symbiotic Specificity in Legume-Rhizobium Interactions

Abstract

Legumes are able to form a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia. The result of this symbiosis is to form nodules on the plant root, within which the bacteria can convert atmospheric nitrogen into ammonia that can be used by the plant. Establishment of a successful symbiosis requires the two symbiotic partners to be compatible with each other throughout the process of symbiotic development. However, incompatibility frequently occurs, such that a bacterial strain is unable to nodulate a particular host plant or forms nodules that are incapable of fixing nitrogen. Genetic and molecular mechanisms that regulate symbiotic specificity are diverse, involving a wide range of host and bacterial genes/signals with various modes of action. In this review, we will provide an update on our current knowledge of how the recognition specificity has evolved in the context of symbiosis signaling and plant immunity.

Keywords: host specificity; legume; nitrogen fixation; nodulation; rhizobial symbiosis.

FIGURE 1

Symbiosis signaling and plant immunity involved in recognition specificity in the legume-rhizobial interactions (indicated by the red stars). (A) The process of infection and nodule development. A mature indeterminate nodule contains a meristem zone (I), an infection zone (II), an interzone (IZ), a nitrogen fixing zone (III), and a senescent zone (IV). (B) The host secretes flavonoids to induce the expression of bacterial nodulation (nod) gene through the activation of NodD proteins. The enzymes encoded by the nod genes lead to the synthesis of Nod factors (NF) that are recognized by host Nod factor receptors (NFRs). Recognition specificity occurs both between Flavonoids and NodDs and between NF and NFRs. (C) In addition to NF signaling, bacteria also produce extracellular polysaccharides (EPS) and type III effectors to facilitate their infection in compatible interactions, but these molecules may also induce immune responses causing resistance to infection in incompatible interactions. (D) Certain legumes such as Medicago encode antimicrobial nodule-specific cysteine-rich (NCR) peptides to drive their bacterial partners to terminal differentiation that is required for nitrogen fixation. However, some rhizobial strains cannot survive the antibacterial activity of certain peptide isoforms, leading to formation of nodules defective in nitrogen fixation.