Phytoplasmas: Bacteria that Manipulate Plants and Insects

Abstract

Phytoplasmas are mycoplasmas that infect plants and insects. In plants they cause symptoms such as abnormal flower development (virescence, phyllody) and ‘witches’ broom’. Insects acquire phytoplasmas when they feed on infected plants. After ingestion, the phytoplasmas infect the haemolymph and then migrate to the salivary glands ready to be injected into another plant. Different phytoplasmas infect different vectors and different vectors feed on different plants. Infection with a phytoplasma can affect the behaviour of the insect. The phytoplasma genome is about 50% bigger than that of Mycoplasma genitalium but has fewer genes and so phytoplasmas are dependent on their host plants and insects for essential nutrients. Phytoplasmas contain multicopy gene clusters called Potential Mobile Units (PMUs), some of which are on plasmids. The copy number of these PMUs differs in plants and animals showing a host effect on the bacteria. Phytoplasmas have no cell wall and so membrane proteins modulate the interaction with hosts. There are three types of immunomodulatory protein (Imp) found on the membrane. They are non-allelic but most phytoplasmas have just one. The Imp that a phytoplasma has determines which insects can be its vector. In plants, phytoplasmas are restricted to the phloem and so the disease symptoms are produced by small molecule effectors. Some of these effectors are encoded by PMUs, suggesting acquisition by horizontal gene transfer, but their origin is not known.