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ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Molecular Plant Pathology Laboratory » Research » Publications at this Location » Publication #374250

Title: Candidatus Phytoplasma

item GASPARICH, GAIL - Salem State University
item BERTACCINI, ASSUNTA - University Of Bologna, Italy
item Zhao, Yan

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 8/2/2020
Publication Date: 12/31/2020
Citation: Gasparich, G.E., Bertaccini, A., Zhao, Y. 2020. Candidatus Phytoplasma. Book Chapter.

Interpretive Summary: Phytoplasmas are small, plant pathogenic bacteria without a cell wall. They invade nutrition-conducting tissues of affected plants and are spread by a special group of insects, causing diseases in many agriculturally and economically important plants worldwide. Genetically, phytoplasmas are closely related to animal- and human-infecting mycoplasmas. Despite decades of efforts, scientists are still unable to establish pure phytoplasma culture in cell-free media. This book chapter summaries the current status of some important aspects of phytoplasma research including taxonomy, classification, the diseases phytoplasmas cause, the interactions the pathogens have with their hosts, and the molecular makers for phytoplasma disease diagnosis. This article will be of interest to research scientists, plant disease diagnosticians, and extension personnel who are concerned with plant disease management and pathogen genetic diversity. This information is also important to regulatory agencies for enhancing border control to prevent spread of phytoplasmas of international importance.

Technical Abstract: Phytoplasmas are wall-less, nutritionally fastidious, phytopathogenic prokaryotes 0.2-0.8 µm in diameter that morphologically resemble non-helical members of the class Mollicutes. Sequencing of nearly full-length PCR-amplified 16S rRNA genes, combined with earlier studies, provided the first comprehensive phylogeny of the organisms and showed that they constitute a unique, monophyletic clade within the Mollicutes. These organisms are most closely related to members of the genus Acholeplasma within the Anaeroplasma clade. Despite decades of efforts and recent progress, sustained pure culture in cell-free media has not yet been demonstrated for any phytoplasma. Their genome sizes have been estimated to range from 530 to 1,350 kb, and the G+C content is about 23-30 mol%. The presence of a characteristic oligonucleotide sequence in the 16S rRNA gene, CAA GAY BAT KAT GTK TAG CYG GDC T, and standard codon usage indicate that phytoplasmas represent a distinct genus level taxon for which the name 'Candidatus Phytoplasma' has been adopted. At present, for any subtaxa within the provisional genus, the designation 'Candidatus' must still be used.