Cytopathological evidence for transport of phytoplasma in infected plants

Abstract

Pleomorphic phytoplasmas were observed in sieve tubes, companion cells and in phloem parenchyma of Tagetes patula L., Helichrysum bracteatum Willd. and Gladiolus sp. L. plants with morphological changes typical for phytoplasma infection. In the pores of the sieve plate phytoplasma cells were seen which suggests that the vertical transport of this pathogen goes in the sieve tubes of infected plants throughout the sieve tube pores. The contact of the sieve tube with the neighbouring cells goes through the plasmodesmata, but no changes of the plasmodesmata were observed in the phloem of infected plants. The size and structure of unchanged plasmodesmata does not allow passing through such big structures like phytoplasma. Instead close contact between phytoplasma cells and vertical sieve tube walls takes place. Damages to the cell wall were observed forming cavities in which the phytoplasma cells were present. The damages of parenchyma and companion cells walls also were seen. In cells where the damages of the walls were observed phytoplasmas were present. The phytoplasma cells were sporadically seen also in the intercellular spaces of parenchyma. These data suggest that horizontal transport depends on damages to the infected plant cell walls caused by the phytoplasma itself.