Secret messages - Plant root symbiosis and immunity.

Aside from being able to initiate the development of new organs and changes in behaviour, holobiont relationships can have a really interesting effect on the immune system of the host. An area of particular interest recently is how nitrogen fixing bacteria affect the immune system of their plant hosts.

 Almost all terrestrial plants share symbiotic relationships with rhizobial bacteria or mycorrhizal fungi to gain access to otherwise unavailable nutrients (Heijden et al. 2017; Mcnear 2013). The bacteria or fungi will extract nitrogen or other nutrients from the environment and pass it onto the plant roots in exchange for carbon produced through photosynthesis (Mcnear 2013; Heijden et al.2017). While most plants exchange with fungi, legumes have a unique relationship with symbiotic with bacteria that will invade the root system and grow in chambers known as nodules within them (Mcnear 2013).

Symbiotic bacteria colonize nodules in the legume roots and exchange Nitrogen for Carbon from photosynthesis (Nefranus 2019)

But how does the plant know how to let the good bacteria in and stop pathogenic bacteria?

Recent studies have found that mycorrhizal bacteria can actually supress the immune system of their hosts in order to establish themselves on or in the roots (Yu et al. 2019). Rey et al. (2018) has found that symbiotic nitrogen fixing bacteria release compounds known as lipo-chito oligosaccharides (LCOs) which trigger specific protein receptors known as nod factor receptors in the plant roots to suppress immune response. This allows the mutualistic bacteria to infect the plant root system while pathogenic bacteria without the LCOs are stopped. It is believed that the evolution of these nod factor receptors on the plants has allowed the symbiosis to occur (Rey et al. 2018).

Symbiosis and agriculture

Unravelling this mystery has bought scientists one step closer to understanding the unique relationships between plants and their mutualistic microbiota and may help them to determine exactly what genes can allow this symbiosis to occur while still defending against pathogenic bacteria (Rey & Jacquet 2018; Zipfel et al. 2017). Some studies have suggested that specific genes that have a dual purpose which allow symbiotic bacteria and defend against pathogens (Rey & Jacuet 2018). Scientists are hoping that one day they will be able to transfer these genes into agricultural plants to better defend them from disease (Pfeilmeyer et al. 2019; Jacott et al. 2017).

Thankyou for reading! Check back next week for more 😊