Development of Alnus japonica root nodules after inoculation with Frankia strain HFPArI3

Abstract

The ontogeny and structure of nodules on an exotic tree species, Alnus japonica (Thunb.) Steud. were studied for up to 3 months after inoculation of seedlings with a North American Frankia strain, HFPArI3. Nodules developed with Frankia growth first restricted to one side of the developing vascular cylinder. Nodules became multilobed, developed a complex network of vascular cylinders, and infected cortical cells organized in a concentric ring around each vascular cylinder. Each nodule lobe showed a zonation from the meristem basipetally. Cortical cells adjacent to the meristem contained hyphae, while those distal to this zone developed hyphae and endophytic vesicles. Staining for suberin induced fluorescence of root hairs, the endodermal Casparian strip, endophytic vesicle clusters, and periderm. Staining for callose resulted in fluorescence of the encapsulation material at one stage of Frankia development. Infected cells had few, small starch grains, while uninfected cells had several, large starch grains. The adjacent pericycle was multilayered. Its cells, viewed with the transmission electron microscope, had lobed nuclei, small vacuoles, many mitochondria, and dense plastids containing small starch grains.