Nucleoli in differentiated germ tubes of wheat rust uredospores

Nucleoli observed by electron microscopy are illustrated in germ tubes of wheat stem rust uredospores which have been induced by heat shock to differentiate infection structures. The presence of nucleoli in these structures suggests that this obligate parasite may possess the capacity of synthesizing ribosomal RNA independently of its host. In addition to nucleoli, heterochromatin is characteristically observed appressed to the inner membrane of the nuclear envelope. This material is associated with distinct cytoplasmic structures which are appressed to the outer membrane of the nuclear envelope and resemble developing centrioles in other fungi.