Isolation of a symbiotic cyanobacterium, Nostoc cycadae, and its nitrogen metabolism

Nostoc cycadae isolated from the host Cycas revoluta grew well in medium devoid of combined nitrogen but maximum growth was in medium containing nitrate (4.1 μg chlorophyll a ml(-1)). Aerated coralloid roots in the dark produced more NH3 when treated with L-methionine-DL-sulphoximine (MSO), an inhibitor of glutamine synthetase. With cultured N. cycadae and freshly isolated N. cycadae, NH3 production was enhanced by adding a host-tissue extract in the light or in the dark, whereas it was decreased by adding MSO. Nitrogenase activity was four times higher in coralloid root than in the cultured endophyte N. cycadae. The host-tissue extract may inhibit NH3 assimilatory pathways, thus inducing production of NH3 that can be utilized by the host itself.