Identification, taxonomy and biologic behaviours of phytoplasmas

Phytoplasmas were discovered almost 50 years ago, and initially named mycoplasma-like organisms (DOI et al., 1967). The evidence that numerous yellows-type diseases of plants, believed to be caused by viruses, were associated with phloem colonization by prokaryotes was shown by indirect biological proof, such as electron microscopy observation, phytoplasma and symptoms elimination after tetracycline treatments (ISHIIE et al., 1967), and insect and dodder transmission to experimental host plants. These wall lacking bacteria, members of the class Mollicutes inhabit plant phloem sieve elements and are transmitted and spread primarily by leafhoppers, plant hoppers and psyllids feeding on infected plants phloem. Phytoplasmas interact with their hosts in a strong manner manipulating the morphological features of plants and in several cases also the biology of their insect vectors (HOGENHOUT et al., 2008). Final proofs of their aetiological roles in the majority of plant diseases is still missing due to the lack of Koch postulates satisfaction, therefore phytoplasmas must still be referred to as organisms-related to the disease symptoms they are supporting. A common symptom in plants is phyllody, a condition in which leaf like structures are produced instead of flowers by the deregulation of genes involved in flower formation (PRACROS et al., 2006). Phytoplasma-infected plants may also show virescence, the development of green flowers due to the loss of pigment in the petal cells that is very often accompanied with flower sterility (LEE et al., 2000). Leaf yellowing is also commonly reported and it is due to modification in carbohydrate synthesis and transportation and photosynthesis. The induced expression of sucrose synthase and alcohol dehydrogenase I genes in phytoplasma-infected grapevine plants grown in the field has also been demonstrated (HREN et al., 2009). Many phytoplasma-infected plants acquire a bushy or witches’ broom appearance due to changes in their normal growth patterns mainly related to the loss of apical dominance causing proliferation of axillary shoots and decreased internode length. Phytoplasmas may induce many other non specific symptoms resulting from stresses to which the host plants are subjected.