Legume plants host nitrogen-fixing endosymbiotic Rhizobium bacteria in root nodules. In Medicago truncatula, the bacteria undergo an irreversible (terminal) differentiation mediated by hitherto unidentified plant factors. We demonstrated that these factors are nodule-specific cysteine-rich (NCR) peptides that are targeted to the bacteria and enter the bacterial membrane and cytosol. Obstruction of NCR transport in the dnf1-1 signal peptidase mutant correlated with the absence of terminal bacterial differentiation. On the contrary, ectopic expression of NCRs in legumes devoid of NCRs or challenge of cultured rhizobia with peptides provoked symptoms of terminal differentiation. Because NCRs resemble antimicrobial peptides, our findings reveal a previously unknown innovation of the host plant, which adopts effectors of the innate immune system for symbiosis to manipulate the cell fate of endosymbiotic bacteria.

Increasing evidence indicates that plants, like animals, use basal resistance (BR), a component of the innate immune system, to defend themselves against foreign organisms. Contrary to the hypersensitive reaction (HR)-type cell death, recognition in the case of BR is unspecific, as intruders are recognised based on their common molecular patterns. Induction of BR is not associated with visible symptoms, in contrast to the HR-type cell death. To analyse the early events of BR in tobacco plants we have carried out a subtractive hybridisation between leaves treated with the HR-negative mutant strain Pseudomonas syringae pv. syringae 61 hrcC and non-treated control leaves. Random sequencing from the 304 EBR clones yielded 20 unique EST-s. Real-time PCR has proved that 8 out of 10 clones are activated during BR. Six of these EST-s were further analyzed. Gene expression patterns in a time course showed early peaks of most selected genes at 3-12 h after inoculation (hpi), which coincided with the development-time of BR. Upon treatments with different types of bacteria we found that incompatible pathogens, their hrp mutants, as well as non-pathogens induce high levels of expression while virulent pathogens induce only a limited gene-expression. Plant signal molecules like salicylic acid, methyl jasmonate, ethylene and spermine, known to be involved in plant defense were not able to induce the investigated genes, therefore, an unknown signalling mechanism is expected to operate in BR. In summary, we have identified representative genes associated with BR and have established important features of BR by analysing gene-expression patterns.