Host-parasite interaction of phytoplasmas from a molecular biological perspective

Phytoplasmas are plant pathogenic bacteria that are transmitted by insects. We examined several molecular biological findings involved in both plant–phytoplasma interactions and insect–phytoplasma interactions. (i) The complete genomic sequence of the phytoplasma suggested that the genome encodes very few metabolic functions, implying that the consumption of metabolites by phytoplasmas in plants may cause disease symptoms. (ii) The approximately 30-kb region, including glycolytic genes, was tandemly duplicated in the genome of a severe pathogenic phytoplasma. The presence of two glycolytic gene clusters suggested that a higher consumption of the carbon source may affect the growth rate of the phytoplasma and may also directly or indirectly cause more severe symptoms. (iii) Positive selection was recognised on Amp, a surface membrane protein of the phytoplasma. This positive selection may reflect an interaction between the phytoplasma and the host cytoplasm. (iv) An affinity column assay showed that Amp formed a complex with insect microfilament proteins. This interaction was correlated with the phytoplasmatransmitting capability of leafhoppers.