The occurrence of amoeboid plastids in the actinorhizal root nodules of Alnus glutinosa (L.) Gaertn.

The invasion of the actinomycete Frankia into the root cells of Alnus glutinosa with subsequent nodule formation effects a number of ultrastructural changes in the host cell cytoplasm. Among other changes the amyloplasts rapidly lose their starch and acquire an amoeboid or pleomorphic form. Such plastids occur predominantly in the mature vesicle‐containing, nitrogen‐fixing cells of the nodule. They lack starch, have an electron dense stroma and a complex lamellar system. This last would appear to be associated with a distinct membranous reticulum which can be extensive. The flexible form of these plastids is mirrored in their ability to enclose portions of host cytoplasm together with organelles and even other plastids. Their close association with cristate mitochondria suggests an active metabolic role in the nodule symbiosis.