A Light and Transmission Electron Microscope Study of a Black Locust Tree, Robinia pseudoacacia (Fabaceae), Affected by Witches'-Broom, and Classification of the Associated Phytoplasma

Light and transmission electron microscopy of phloem sieve-tube elements, companion cells, and parenchymal cells in thin and ultrathin sections of small and medium rachises and small, medium and large leaflets of a black locust tree, Robinia pseudoacacia L., affected by witches'-broom disease revealed (in the small and medium rachises and leaflets) structures that were characteristic of phytoplasmas, and crystal-like inclusions in the phloem sieve-tube members. A crystal-like inclusion was also seen in a companion cell. Paracrystalline arrays were seen only (and very rarely) in phloem sieve-tube elements of medium rachises. Some elements contained several crystal-like inclusions and each inclusion had fracture planes. The crystal-like inclusions and paracrystalline arrays apparently have not been previously reported in the black locust. The paracrystalline arrays and crystal-like inclusions may merely be by-products of the plant's metabolic activity. Extensive additional work would be required to establish the precise relationship (if any) of the arrays and inclusions to the black locust, witches'-brooming and/or phytoplasmas. Results from analysis of 16S rRNA gene sequences amplified by the polymerase chain reaction indicated for the first time that the phytoplasma associated with black locust witches'-broom is a member of group 16 SrIII (peach X-disease) phytoplasma group. This raised the question of whether black locust may be a significant source of the phytoplasma for infection of other plants, especially agricultural crops.