Phytoplasmas are obligate intracellular parasites of plant phloem tissue and of the insect vectors that are involved in their plant-to-plant transmission. Phytoplasmas were discovered in 1967 by Japanese scientists who termed them mycoplasma-like organisms.[3] Since their discovery, phytoplasmas have resisted all attempts at in vitro culture in any cell-free medium; routine cultivation in an artificial medium thus remains a major challenge. Phytoplasmas are characterized by the lack of a cell wall, a pleiomorphic or filamentous shape, a diameter normally less than 1 μm, and a very small genome.
Phytoplasmas are pathogens of agriculturally important plants, including coconut, sugarcane, sandalwood, and cannabis, as well as horticultural crops like sweet cherry, peaches, and nectarines in which they cause a wide variety of symptoms ranging from mild yellowing, small fruit, and reduced sugar content to death. Phytoplasmas are most prevalent in tropical and subtropical regions. They are transmitted from plant to plant by vectors (normally sap-sucking insects such as leafhoppers) in which they both survive and replicate.
References to diseases now known to be caused by phytoplasmas can be found as far back as 1603 (mulberry dwarf disease in Japan).[4] Such diseases were originally thought to be caused by viruses, which, like phytoplasmas, require insect vectors and cannot be cultured. Viral and phytoplasmic infections share some symptoms.[5] In 1967, phytoplasmas were discovered in ultrathin sections of plant phloem tissue and were termed mycoplasma-like organisms due to their physiological resemblance.[3] The organisms were renamed phytoplasmas in 1994 at the 10th Congress of the International Organization for Mycoplasmology.[5]
Phytoplasmas are Mollicutes that are bound by a triple-layered membrane rather than a cell wall.[6] The phytoplasma cell membranes studied to date usually contain a single immunodominant protein of unknown function that constitutes most of the protein in the membrane.[7] A typical phytoplasma is pleiomorphic or filamentous in shape and is less than 1 μm in diameter. Like other prokaryotes, phytoplasmic DNA is distributed throughout the cytoplasm instead of being concentrated in a nucleus.[citation needed]
Phytoplasmas can infect and cause various symptoms in more than 700 plant species. One characteristic symptom is abnormal floral organ development, including phyllody (the production of leaf-like structures in place of flowers), virescence (the development of green flowers attributable to a loss of pigment by petal cells),[8] and fasciation (abnormal change in the apical meristem structure).[9] Phytoplasma-harboring flowering plants may nevertheless be sterile. The expression of genes involved in maintaining the apical meristem or in the development of floral organs is altered in the morphologically affected floral organs of phytoplasma-infected plants.[10][11]
A phytoplasma infection often triggers leaf yellowing, probably due to the presence of phytoplasma cells in the phloem, which can affect phloem function and carbohydrate transport,[12] inhibit chlorophyll biosynthesis, and trigger chlorophyll breakdown.[6] These symptoms may be attributable to stress caused by the infection rather than a specific pathogenetic process.[citation needed]