Taxonomy: Group Tenericutes Class Mollicutes Order Acholeplasmatales Family Acholeplasmataceae Genus Candidatus Phytoplasma Species Onion yellows Strain OY-M

Structure: Virions filamentous; not enveloped; usually flexuous; with no clear modal length; of 772 and 823 nm (Schmidt and Schmelzer, 1978)). Axial canal obscure. Basic helix obscure.

Habitat: from minus 4 to 60-65 °C, life expectation: 2-3 days, infectivity of sap not changed by treatment with di-ethyl ether, leaf sap contains few virions. Probably distributed worldwide.

Phytoplasmas are a group of microscopic organisms that cause over 700 diseases in plants. Phytoplasmas grow and multiply within host plants and insect vectors. In host plants, phytoplasmas are found only in the phloem tissue of leaves, stems, and roots. When the concentration of phytoplasmas within the plant reaches a certain level, it is believed to cause hormonal imbalance, resulting in the development of symptoms such as chlorotic leaves, stunting, flower petals changing to a green color (phyllody or virescence), and witches-broom (shoot proliferation). (Source)

Onion yellows phytoplasma causes a yellowing disease in onions
Phytoplasmas associated with yellows diseases were firstly described in 1926 by Kunkle in American aster plants showing virescence and flower malformations. Since then the etiological agent of this and of similar diseases was believed to be a virus;until in 1967 Doi et al. detected under electron microscope phytoplasmas in aster and other plants with "yellows" symptoms opening phytoplasmology history.
Like other mollicutes, phytoplasmas lack cell walls, are pleomorphic and reside endocellularly in insects and plant phloem. Onion yellows phytoplasma is a parasite that lives inside leafhopper insects and spreads to plants through the saliva of feeding insects.
The metabolic pathways and host interactions of Onion yellows phytoplasma is of interest in agricultural and basic sciences. The inability to culture phytoplasmas in vitro has hindered their characterization at the molecular level, the determination of factors involved in their pathogenicity and the discovery of effective ways to control or cure phytoplasma diseases.
Analysis of the phytoplasma genome suggests reductive evolution as a consequence of its life as an intracellular parasite in a nutrient-rich environment and shows that phytoplasma lacks several genes previously considered to be essential for autonomously replicating cells. Instead, it possesses transporter genes involved in the uptake of substances from the cytoplasm of surrounding host cells. Although it is difficult to define the minimal genome concept for pathogens and symbionts that rely on their hosts, phytoplasmas are unique bacteria that contain a new minimal gene set.
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