Phytoplasma Genomes: Evolution Through Mutually Complementary Mechanisms, Gene Loss and Horizontal Acquisition
Phytoplasmas are cell wall-less bacteria that inhabit phloem sieve cells of infected plants and are transmitted by phloem-feeding insect vectors. Having descended from a Gram-positive, low G+C walled bacterium and more recently from an Acholeplasma-like a
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Yan Zhao, Robert E. Davis, Wei Wei, Jonathan Shao, and Rasa Jomantiene
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Introduction
Plants are affected by a large number of diseases that are collectively referred to as ‘yellows diseases.’ For decades, the causal agents of plant yellows diseases were mistakenly presumed to be viruses. In 1967, plant pathologists were startled by the news that yellows-diseased plants were infected by microbes resembling mycoplasmas, cell wall-less bacteria already known in humans and animals (Doi et al. 1967; Ishiie et al. 1967). Following this discovery, scientists around the world reexamined many plant diseases of unsolved cause, giving rise to the finding of mycoplasma-like organisms (MLOs, now known as phytoplasmas) in numerous and diverse diseased plants. It is rather remarkable that, within a few years following the discovery of phytoplasmas, several other new types of plant pathogens, including spiroplasmas and viroids, were also discovered (Davis et al. 1972; Davis and Worley 1973; Diener 1971).
Y. Zhao R. E. Davis (&) W. Wei J. Shao Molecular Plant Pathology Laboratory, Agricultural Research Service, United States Department of Agriculture, Beltsville, MD 20705, USA e-mail: [email protected] R. Jomantiene Phytovirus Laboratory, Nature Research Centre, 08412, Vilnius, Lithuania
Phytoplasmas are cell wall-less bacteria that inhabit sieve cells in the phloem tissue of infected plants and are transmitted from plant-to-plant by phloem-feeding insect vectors, principally leafhoppers. Together with mycoplasmas, spiroplasmas, and acholeplasmas, phytoplasmas are classified in class Mollicutes. Having descended from a Gram-positive, low G+C walled bacterium (Gundersen et al. 1994; Zhao et al. 2005), and more recently from an Acholeplasma-like ancestor (Wei et al. 2008a), the phytoplasma clade underwent substantial evolutionary genome shrinkage (Marcone et al. 1999; Oshima et al. 2004). Yet, phytoplasma genomes contain numerous repeated sequences that appear in genomic islands. These islands, first termed sequence-variable mosaics (SVMs) (Jomantiene and Davis 2006), were formed by recurrent and targeted attacks by mobile genetic elements found to be ancient phages (Wei et al. 2008a). Apparently, the evolutionary shrinkage of phytoplasmal genomes was partially countered by horizontal acquisition of repeated mobile genetic elements. While the loss of genes encoding diverse biosynthetic pathways must have led to increased host dependence, new capabilities were acquired that enabled and enhanced phytoplasma–host interactions. Thus, two mutually complementary, genome-sculpting mechanisms acting in concert played key roles in phytoplasma evolution. To date, the genomes of four phytoplasmas have been completely sequenced. The four phytoplasmas are responsible for diseases that
D. C. Gross et al. (eds.), Genomics of Plant-Associated Bacteria, DOI: 10.1007/978-3-642-55378-3_10, Springer-Verlag Berlin Heidelberg (outside the USA) 2014
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seriously impact affected host plant species. ‘Can
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