Phytoplasma diseases of medicinal plants

Abstract

A diversity and incresing economic importance of medicinal plants demand a need to study their diseases. Among them phytopathogenic fungi are of a considerable importance, and in the last decade, phytoplasma diseases. Phytoplasma are important obligate intracellular, insect - transmitted pathogenic agents, found inside the phloem, in particular in the sieve tube cells of many plant species. These plant pathogens are wall-less, non-cultivable prokaryotes belonging to the Mollicutes class. They are the smallest prokaryotic organisms of a varied shape and size (50-1000 nm in diameter) and visible under the electron microscope. Their genome is the shortest known of all prokaryotic organisms (680-1600 kb). The most cited and widely accepted system of identification is based on the similarity in the 16S ribosomal gene sequence as well as their biological characteristics. Phytoplasmas are associated with plant diseases in several hundred plant species, inclu­ding many important vegetable, fruit crops, ornamental and medicinal plants. Disease plants exhibit pathological changes (in plants) which are demonstrated through four ty­pical types of symptoms: (1) phyllody, (2) yellowing and reddening, (3) virescence and (4) proliferation of axillary buds or 'witch's broom'. Typical phytoplasma symptoms are confirmed on the following species of medicinal plants: Ehinacea purpurea, E. Angustifolia, Hypericum perforatum, H. barbatum, Plantago major, Saponaria officinalis, Digitalis purpurea, Origanum vulgare, Levisticum officina­le, Carum carvi, Trigonella foenum greacum, Melisa officinalis, Petroselinum sativum, Apium graveolens, Valeriana officinalis, Rubus fruticosus, Vaccinium myrtillus, Arnica montana, Calendula officinalis, Cichorium intybus, Salix alba and Chamomilla recutita. The presence of phytoplasma was detected using electron microscopy (TEM) in 5 species of medicinal plants in Serbia, and using molecular identification in order to identify phyto­plasma in other species. Two types of phytoplasma were identified on more than 20 species of the medicinal plants in Serbia: Stolbur phytoplasma (16SrXII group) and Clover yellow edge (16SrIII-B). There are very few qualitative and quantitative data on the changes in secondary metabolites af­fected by the phytoplasma and these data should be improved.