Nitric Oxide: A Multitask Player in Plant–Microorganism Symbioses

Abstract

Symbiosis is a close and often long-term interaction between two dif- ferent biological organisms, i.e. plants or fungi and microorganisms. Two main types of plant–microorganism interactions, mutualistic and cooperative, have been categorized. Mutualistic interactions, including nitrogen-fixing and mycorrhizal symbioses, refer to mostly obligate relationships between a host plant and a symbiont microorganism. Cooperative interactions correspond to less obligate and specific relationships. They involve microorganisms, referred to as plant growth-promoting rhizobia (PGPR), able to colonize root surface or inner tissues. Lichens are symbiotic associations of host fungi and photosynthetic partners that may be Cyanobacteria or green algae. Increasing evidence has been reporting the