Spiroplasmas and Phytoplasmas: Microbes Associated with Plant Hosts
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Biologicals 38 (2010) 193–203 Contents lists available at ScienceDirect Biologicals journal homepage: www.elsevier.com/locate/biologicals Review Spiroplasmas and phytoplasmas: Microbes associated with plant hosts Gail E. Gasparich* Department of Biological Sciences, Towson University, 8000 York Road, Towson, MD 21252, USA article info abstract Article history: This review will focus on two distinct genera, Spiroplasma and ‘Candidatus Phytoplasma,’ within the class Received 4 November 2009 Mollicutes (which also includes the genus Mycoplasma, a concern for animal-based cell culture). As Accepted 12 November 2009 members of the Mollicutes, both are cell wall-less microbes which have a characteristic small size (1–2 mM in diameter) and small genome size (530 Kb–2220 Kb). These two genera contain microbes Keywords: which have a dual host cycle in which they can replicate in their leafhopper or psyllid insect vectors as Spiroplasmas well as in the sieve tubes of their plant hosts. Major distinctions between the two genera are that most Phytoplasmas spiroplasmas are cultivable in nutrient rich media, possess a very characteristic helical morphology, and Phytopathogenic Plant host are motile, while the phytoplasmas remain recalcitrant to cultivation attempts to date and exhibit Microoganism a pleiomorphic or filamentous shape. This review article will provide a historical over view of their discovery, a brief review of taxonomical characteristics, diversity, host interactions (with a focus on plant hosts), phylogeny, and current detection and elimination techniques. Ó 2009 The International Association for Biologicals. Published by Elsevier Ltd. All rights reserved. 1. Overview and rationale host interactions (with a focus on plant hosts), phylogeny, and current detection and elimination techniques. Spiroplasmas and phytoplasmas belong to two genera of under- studied microbes whose presence in plants, from which extracts are 2. Historical perspective on discovery made to supplement the growth of animal cells in vitro,isofgreat interest to the pharmaceutical industry. The industry is moving away 2.1. Spiroplasmas from animal serum as a supplement because of the risk of animal virus contamination. Plant extracts seem to be an acceptable alter- The term spiroplasma was first used in 1973 [1] to describe the native, but microbial contamination remains a concern. uncultivated helical bacteria found to be associated with corn stunt This review will focus on two distinct genera, Spiroplasma and disease [2,3]. In France and California, similar microbes were ‘Candidatus Phytoplasma’, within the class Mollicutes (which determined to be the causative agent for citrus stubborn disease, includes the genus Mycoplasma, a concern for animal-based cell but originally described as mycoplasmalike organisms [4,5]. This culture). As members of the Mollicutes, both are cell wall-less organism was the first spiroplasma of plant origin to be cultivated microbes which have a characteristic small size (1–2 mM in diam- and Spiroplasma was elevated to the generic name for this group of eter) and small genome size (530 Kb–2220 Kb). These two genera microbes [6] and added to the Approved List of Bacterial Names in contain microbes which have a dual host cycle in which they can 1983 [7]. replicate in their leafhopper or psyllid insect vectors as well as in Several previously observed microorganisms were later deter- the sieve tubes of their plant hosts. Major distinctions between the mined to belong to the genus Spiroplasma. These included the two genera are that most spiroplasmas are cultivable in nutrient causative agent for the sex-ratio disease in Drosophila [8,9]; Spi- rich media, possess a very characteristic helical morphology, and roplasma mirum, a rabbit tick isolate, [10,11] originally thought to be are motile, while the phytoplasmas remain recalcitrant to cultiva- a virus due to its filterability; and Spiroplasma strain 277F, tion attempts to date and exhibit a pleiomorphic or filamentous cultivated in 1968, which was thought to be a spirochete due to its shape. This review article will provide a historical over view of their helical morphology [12]. The corn stunt agent, Spiroplasma kunkelii, discovery, a brief review of taxonomical characteristics, diversity, was finally cultivated in 1975 [13,14]. Within ten years of their discovery, T. B. Clark [15] showed that spiroplasmas were widely found associated with arthropod hosts. * Tel.: þ1 410 704 4515; fax: þ1 410 704 2405. While exploring potential reservoirs for these organisms, several E-mail address: [email protected] Spiroplasma strains were isolated from plant surfaces, including 1045-1056/$36.00 Ó 2009 The International Association for Biologicals. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biologicals.2009.11.007 194 G.E. Gasparich / Biologicals 38 (2010) 193–203 flowers. The first floral spiroplasma, Spiroplasma floricola,was characterization of a novel species involves numerous phenotypic, described in 1981 [16]. genotypic, serological and biochemical tests [64]. Several hundred new Spiroplasma species have been identified and many have been fully described. Isolates have been made from 3.2. Phytoplasmas a wide range of arthropods (including crustaceans) and plant hosts from a wide geographic range. Several of these spiroplasmas have Phytoplasmas are cell-wall-less, non-helical, uncultivable been shown to use both plant phloem and insect host habitats [17], prokaryotes associated with diseases in more than a thousand plant however insects are the major reservoir for spiroplasmas [15,18] species [23,26,71–80]. The phytoplasmas are spread by leafhopper and plant surfaces are important in horizontal transmission among and planthopper insect vectors to host plants, requiring an ability insect hosts [19–21]. to adapt to both insect and plant habitats. During their transmission cycle, phytoplasmas cross the insect midgut lining, circulate and 2.2. Phytoplasmas reproduce in the hemolymph, and invade and multiply in insect tissues including salivary glands, where phytoplasmas are inte- Since the early 1900s, Yellows diseases have been described for grated into saliva and injected into the plant phloem during feeding many plant species [22], but were originally assumed to be caused [75,81]. by viruses because the disease agent was filterable, unable to be The inability to cultivate phytoplasmas has limited the use of cultivated, and symptomology was similar to those known to be traditional microbial taxonomic methods. Early taxonomic classi- caused by viruses. In 1967 Japanese scientists were able to observe fication of phytoplasmas was based on biological properties, such microbes in the phloem of plants with Yellows diseases that were as the symptoms induced in plants, the plant host range, and similar in morphology to mycoplasmas known to cause animal and identification of the insect vector(s) if known [82–85]. However, human diseases [23]. Due to the resemblance, these plant- problems with the use of these biological properties are that two or associated microbes were referred to as mycoplasmalike organisms more phytoplasmas can cause similar symptomology in the same (or MLOs). The term MLO was used until 1994 when it was replaced plant in different geographical regions and the same phytoplasma with the trivial name ‘‘phytoplasma’’ [24,25]. Although they remain can cause different diseases in different plant species. A molecular- uncultivable, phytoplasmas have been identified as the causative based approach was developed in the 1980s [86,87] and is now agent for a variety of diseases in over a thousand plant species [26]. used to detect various phytoplasmas associated with plants and Phytoplasma infection can, in some cases, lead to desirable insects and study their genetic interrelationships. ornamental flora as in the case of poinsettias, but in most cases they cause millions of dollars of economic loss globally every year in 4. Diversity and host interactions important crop plants (e.g. rice, corn, potato, soybean), fruit trees (e.g. peach, pear, cherry, apple, citrus, coconut), ornamental plants One commonality between all phytoplasmas and plant-infect- (e.g. hydrangeas, coneflowers), and other trees and bushes (e.g. ing Spiroplasma strains is that they have a dual host transmission elm, mulberry) [27–43]. cycle. They are acquired by insect vectors (leafhoppers or psyllids) that feed on infected plants. To be transmitted to a plant, the 3. Taxonomic characteristics mollicutes need to multiply in the insect midgut, cross the midgut lining, multiply in the hemolymph, and subsequently infect the 3.1. Spiroplasmas salivary glands where they mix with saliva and are injected into a plant as the insect feeds on the phloem. Such a cycle takes 15–20 Taxonomically, the genus Spiroplasma is characterized as days. a group of motile, helical, wall-less procaryotes with genomes ranging in size from 780 to 2220 kbp [44,45] that are associated 4.1. Spiroplasmas primarily with insects, and less frequently, with ticks, plants and crustaceans [46–48]. The helical forms are usually 100–200 nm in Spiroplasmas display a broad range of diversity based on host diameter and 3–5 mm in length. Colonies on solid media are microhabitats with some spiroplasmas exhibiting strict host and/or 0.1–4.0 mm and are frequently diffuse at the borders due to their geographic ranges, while others are generalists. They are able to motility. Fimbriae and pili on the cell surfaces of insect- and plant- invade a diverse range