The leguminous plants have developed a unique approach to fix nitrogen

by forming root nodules with nitrogen fixing bacteria. It is currently

understood that the start of this symbiosis is trigged by the exchange of

calcium oscillations, termed calcium spiking, in root hair cells of their

host legume. This calcium spiking acts as a secondary messenger in the

Symbiosis signaling pathway and the calcium signal is decoded by a

calcium and calmodulin dependent protein kinase (CCaMK). In order to

assess the importance of calcium oscillations in symbiosis signaling we

have assessed the activation of calcium oscillations in a number of

different symbiotic interactions.

We have shown that mycorrhizal fungi which interacts more than 80% of land plants can form arbuscular

mycorrhizas (AM), activate calcium oscillations in root hair cells of Medicago truncatula and this requires

genetic components in the Symbiosis signaling pathway. Recently, we have shown that the nitrogen fixing

filamentous bacteria Frankia also activates an oscillatory

calcium response in the actinorhizal tree Alnus

glutinosa. The structure and spatial resolution of this

calcium response is similar to Nod factor-induced calcium spiking. Calcium spiking in A.

glutinosa is activated by a diffusible signal from Frankia and unlike Nod factor-induced

calcium spiking the frequency of Frankia-induced calcium spiking is dependent on the

concentration of the signaling molecule. Our work indicates that calcium oscillations are a

John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7UH, UK

 

Jongho Sun