Extreme specificity of NCR gene expression in Medicago truncatula

Loss of the nodule-specific cysteine rich peptide, NCR169, abolishes symbiotic nitrogen fixation in the Medicago truncatula dnf7 mutant

Host compatible rhizobia induce the formation of legume root nodules, symbiotic organs within which intracellular bacteria are present in plant-derived membrane compartments termed symbiosomes. In Medicago truncatula nodules, the Sinorhizobium microsymbionts undergo an irreversible differentiation process leading to the development of elongated polyploid noncultivable nitrogen fixing bacteroids that convert atmospheric dinitrogen into ammonia. This terminal differentiation is directed by the host plant and involves hundreds of nodule specific cysteine-rich peptides (NCRs). Except for certain in vitro activities of cationic peptides, the functional roles of individual NCR peptides in planta are not known. In this study, we demonstrate that the inability of M. truncatula dnf7 mutants to fix nitrogen is due to inactivation of a single NCR peptide, NCR169. In the absence of NCR169, bacterial differentiation was impaired and was associated with early senescence of the symbiotic cells. Introduction of the NCR169 gene into the dnf7-2/NCR169 deletion mutant restored symbiotic nitrogen fixation. Replacement of any of the cysteine residues in the NCR169 peptide with serine rendered it incapable of complementation, demonstrating an absolute requirement for all cysteines in planta. NCR169 was induced in the cell layers in which bacteroid elongation was most pronounced, and high expression persisted throughout the nitrogen-fixing nodule zone. Our results provide evidence for an essential role of NCR169 in the differentiation and persistence of nitrogen fixing bacteroids in M. truncatula.

Small-peptide signals that control root nodule number, development, and symbiosis

Many legumes have the capacity to enter into a symbiotic association with soil bacteria generically called 'rhizobia' that results in the formation of new lateral organs on roots called nodules within which the rhizobia fix atmospheric nitrogen (N). Up to 200 million tonnes of N per annum is fixed by this association. Therefore, this symbiosis plays an integral role in the N cycle and is exploited in agriculture to support the sustainable fixation of N for cropping and animal production in developing and developed nations. Root nodulation is an expendable developmental process and competency for nodulation is coupled to low-N conditions. Both nodule initiation and development is suppressed under high-N conditions. Although root nodule formation enables sufficient N to be fixed for legumes to grow under N-deficient conditions, the carbon cost is high and nodule number is tightly regulated by local and systemic mechanisms. How legumes co-ordinate nodule formation with the other main organs of nutrient acquisition, lateral roots, is not fully understood. Independent mechanisms appear to regulate lateral roots and nodules under low- and high-N regimes. Recently, several signalling peptides have been implicated in the local and systemic regulation of nodule and lateral root formation. Other peptide classes control the symbiotic interaction of rhizobia with the host. This review focuses on the roles played by signalling peptides during the early stages of root nodule formation, in the control of nodule number, and in the establishment of symbiosis. Here, we highlight the latest findings and the gaps in our understanding of these processes.

The Plant Peptidome: An Expanding Repertoire of Structural Features and Biological Functions

Peptides fulfill a plethora of functions in plant growth, development, and stress responses. They act as key components of cell-to-cell communication, interfere with signaling and response pathways, or display antimicrobial activity. Strikingly, both the diversity and amount of plant peptides have been largely underestimated. Most characterized plant peptides to date acting as small signaling peptides or antimicrobial peptides are derived from nonfunctional precursor proteins. However, evidence is emerging on peptides derived from a functional protein, directly translated from small open reading frames (without the involvement of a precursor) or even encoded by primary transcripts of microRNAs. These novel types of peptides further add to the complexity of the plant peptidome, even though their number is still limited and functional characterization as well as translational evidence are often controversial. Here, we provide a comprehensive overview of the reported types of plant peptides, including their described functional and structural properties. We propose a novel, unifying peptide classification system to emphasize the enormous diversity in peptide synthesis and consequent complexity of the still expanding knowledge on the plant peptidome.

Plant cysteine-rich peptides that inhibit pathogen growth and control rhizobial differentiation in legume nodules

Plants must co-exist with both pathogenic and beneficial microbes. Antimicrobial peptides with broad antimicrobial activities represent one of the first lines of defense against pathogens. Many plant cysteine-rich peptides with potential antimicrobial properties have been predicted. Amongst them, defensins and defensin-like peptides are the most abundant and plants can express several hundreds of them. In some rhizobial-legume symbioses special defensin-like peptides, the nodule-specific cysteine-rich (NCR) peptides have evolved in those legumes whose symbiotic partner terminally differentiates. In Medicago truncatula, >700 NCRs exist and collectively act as plant effectors inducing irreversible differentiation of rhizobia to nitrogen-fixing bacteroids. Cationic NCR peptides have a broad range of potent antimicrobial activities but do not kill the endosymbionts.

Identification of nodule-specific cysteine-rich plant peptides in endosymbiotic bacteria

The symbiosis of Medicago truncatula with Sinorhizobium meliloti or Sinorhizobium medicae soil bacteria results in the formation of root nodules where bacteria inside the plant cells are irreversibly converted to polyploid, nondividing nitrogen-fixing bacteroids. Bacteroid differentiation is host-controlled and the plant effectors are symbiosis-specific secreted plant peptides. In the M. truncatula genome there are more than 600 symbiotic peptide genes including 500 small genes coding for nodule-specific cysteine-rich (NCR) peptides. While NCR transcripts represent >5% of the nodule transcriptome, the existence of only eight NCR peptides has been demonstrated so far. The predicted NCRs are secreted peptides targeted to the endosymbionts. Correspondingly, all the eight detected peptides were present in the bacteroids. Here, we report on large-scale detection of NCR peptides from nodules and from isolated, semipurified endosymbionts at various stages of their differentiation. In total 138 NCRs were detected in the bacteroids; 38 were cationic while the majority was anionic. The presence of early NCRs in nitrogen-fixing bacteroids indicates their high stability, and their long-term maintenance suggests persisting biological roles in the bacteroids.