Synthesizes nicotianamine, a polyamine that is the first intermediate in the synthesis of the phytosiderophores of the mugineic acid type found in gramineae which serves as a sensor for the physiological iron status within the plant, and/or might be involved in the transport of iron. 3 S-adenosyl-L-methionine = 3 H(+) + nicotianamine + 3 S-methyl-5'-thioadenosine Homotrimer. Fivefold increase after iron deficiency. Belongs to the nicotianamine synthase (NAS)-like family.