Phytoplasmas: colonizing agents of plant phloem and insects

Plant-pathogenic phytoplasmas are wall-less, unculturable bacteria of the class Mollicutes that are associated with diseases, collectively referred to as yellows diseases, in more than a thousand plant species worldwide. Many of these diseases are of great economic importance. Phytoplasmas reside in the phloem sieve tubes and are transmitted from plant to plant by phloem-feeding homopteran insects, mainly leafhoppers (Cicadellidae) and planthoppers (Fulgoromorpha), and less frequently psyllids (Psyllidae). Most of the host plants of phytoplasmas are angiosperms in which a wide range of specific and non-specific symptoms are induced. Phytoplasmas, which are phylogenetically more closely related to acholeplasmas than to other mollicutes, have a complex life cycle that involves colonization of different environments, the plant phloem and various organs of the insect vectors. The dynamic architecture of phytoplasma genomes may account for variation in their genome size and adaptation to the diverse environments of their plant and insect hosts. Newly acquired knowledge has made it possible to identify a considerable number of phytoplasma genes that are likely to play major roles in phytoplasma-host interactions. Among these, there are genes encoding surface membrane proteins and effector proteins. Also, it has been shown that phytoplasmas dramatically alter their gene expression upon switching between plant and insect hosts.