Some strains of Rhodococcus fascians
exist only as epiphytes on the plant surface whereas others can become endophytic and cause various abnormalities including the release of multiple buds and reduced root growth. The abnormalities reflect the action of cytokinin. The strains that can become endophytic harbour a linear plasmid that carries cytokinin biosynthesis, activation and destruction genes. However, both epiphytic and endophytic forms can release cytokinin into culture, affect cytokinin metabolism within inoculated plants and enhance the expression of sugar and amino acid transporters and cell wall invertases, but only the endophytic form markedly affects the morphology of the plant. A unique methylated cytokinin, dimethylated N6
-isopentenyl)adenine (2-MeiP), operating in a high sugar environment, is the likely causative factor of the severe morphological abnormalities observed when plants are inoculated with R. fascians
strains carrying the linear plasmid.
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