Signaling in Legume-Rhizobia Symbiosis

Abstract

Legumes represent an important source of food protein for human nutrition and animal feed. Therefore, sustainable production of legume crops is an issue of global importance. It is well-known that legume-rhizobia symbiosis allows an increase in the productivity and resilience of legume crops. The efficiency of this mutualistic association strongly depends on precise regulation of the complex interactions between plant and rhizobia. Their molecular dialogue represents a complex multi-staged process, each step of which is critically important for the overall success of the symbiosis. In particular, understanding the details of the molecular mechanisms behind the nodule formation and functioning might give access to new legume cultivars with improved crop productivity. Therefore, here we provide a comprehensive literature overview on the dynamics of the signaling network underlying the development of the legume-rhizobia symbiosis. Thereby, we pay special attention to the new findings in the field, as well as the principal directions of the current and prospective research. For this, here we comprehensively address the principal signaling events involved in the nodule inception, development, functioning, and senescence.

Keywords: determinate and indeterminate nodules; infection; legume–rhizobia symbiosis; nitrogen fixation; nodule senescence; regulation; signaling.

Figure 1

The stages of the legume–rhizobia symbiosis. CEP—C-terminally encoded peptide; NodD—nodulation protein D; NCR—nodule-specific cysteine-rich peptides; N or N₂—nitrogen.

Figure 2

The role of flavonoids in nodulation and establishment of legume–rhizobia symbiosis. NodD—nodulation protein D; GroEL and GroES—chaperonins, which are the homologs of the eukaryotic heat shock proteins 60 and 10 kDa (HSP60 and HSP10), respectively.

Figure 3

Signaling pathways in the root cells activated by Nod factors. MLD—malectin-like domain; SYMRK—LRR-containing receptor kinase; NF—Nod factors; NFR1 and NFR5—Nod factor receptors 1 and 5; NiCK4—NFR5-interacting cytoplasmic kinase 4; CCaMK—calcium/calmodulin-dependent serine/threonine-protein kinase; NIN—nodulation inception transcription factor.

Figure 4

The effects of NIN protein in root cells. NF—Nod factors; NF-Y—nuclear factor-Y; CRE1- cytokinin response kinase 1; CLE—CLAVATA3/Embryo-surrounding region-related peptides.

Figure 5

The role of the C-terminally encoded peptides (CEP) and CLAVATA3/Embryo-surrounding region-related peptides (CLE) in induction of nodulation. CEPR—C-terminally encoded peptide receptor; CLER—nodule autoregulation receptor kinase (GmNARK/LjHAR1/MtSUNN/PsSYM29); CEPD—C-terminally encoded peptide downstream; NTR—nitrogen root transporters; miRNA—microRNAs or non-coding RNA molecules; TML—regulator of transcription.