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 g 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.V. Singh, M.R. Goyle and E.R.S. Talpasayi are with the Laboratory of Algal Physiology and Biochemistry and A.K. Srivastava is with the Ecology Research Laboratory, both of the Centre of Advanced Study in Botany, Banaras Hindu University, Varanasi-221005, India.