Phytoplasmas: Genomes, Plant Hosts and Vectors
Article first published online: 2 NOV 2010
© 2010 The Authors. Plant Pathology © 2010 BSPP
Volume 59, Issue 6, pages 1177–1178, December 2010
How to Cite
Eden-Green, S. (2010), Phytoplasmas: Genomes, Plant Hosts and Vectors. Plant Pathology, 59: 1177–1178. doi: 10.1111/j.1365-3059.2010.02365.x
- Issue published online: 2 NOV 2010
- Article first published online: 2 NOV 2010
Phytoplasmas: Genomes, Plant Hosts and Vectors . P. G.Wientraub & P.Jones , eds . 331 pp. £85 . ISBN 978-1-84593-530-6 . Wallingford, UK : CAB International ( http://www.cabi.org ). 2010 ( hardback ).
Discovery in 1967 of the association with wall-less prokaryotic ‘mycoplasma-like’ organisms, now known as phytoplasmas, heralded a new era for many of us who were then concerned with the management of the ‘yellows’ group of presumed plant virus diseases. However, many of the early hopes of progress were frustrated by the difficulties of handling what proved to be non-cultivable and physiologically-fragile organisms, and it was another two decades before the emergence of recombinant DNA technologies began to produce novel ways of investigating and understanding them. Even now, practical improvements in disease management remain elusive, although a vast body of new knowledge has been gained on which to base rational decisions for disease exclusion and prevention.
This book is a timely state-of-the-art review of all aspects related to functional and molecular biology of phytoplasmas, their plant hosts and insect vectors, with contributions from leading specialists in each field. The first six chapters, which form a large part of the book, deal with techniques for diagnosing, detecting and quantifying phytoplasmas, together with progress in molecular differentiation, classification and taxonomy to which disease diagnoses are intimately related. These are followed by two chapters reviewing phytoplasma movement, disease development and biochemical changes in infected plants; chapters on host plant resistance and diseases of Poaceae, and two chapters that take somewhat overlapping case study approaches to the epidemiology of phytoplasma diseases in grapevines and multiple plant hosts. A chapter on control of phytoplasma diseases and vectors (perforce concerned mainly with the latter) serves to introduce three chapters on aspects of vector biology and management (psyllid vectors; microbial symbionts as a possible resource for control of phytoplasmoses; and transmission specificity/competition of multiple phytoplasmas in vectors). Finally, there is a summary of the distribution of the major phytoplasma groups of economic importance, coupled with a brave attempt to consider future trends, such as the influence of global warming, in their distribution and impact.
Most chapters provide useful conclusions or closing remarks, which summarise recent findings and point towards future developments. Whilst the book provides good accounts of recent developments and future prospects, readers looking for a ready reference to lists of taxonomic groupings, Candidatus species, principal plant hosts and insect vectors will be disappointed. Most of this information is there, but it is scattered between chapters and in different forms: summary tables, perhaps in the form of an appendix, could have made it easier to cross-reference between the different contributions as well as providing a ready reference to the numerous acronyms. Two pages of colour plates relieve this otherwise sparsely illustrated book, but the choice of colour illustrations does seem rather odd - none is really needed to support the accompanying text. Two show small photographs of different leafhopper and psyllid vectors (neither with any indication of scale), one shows spectacularly photogenic witches’ broom symptoms on Spartina junceum (although there is no symptomless plant to compare), and the fourth is a rather fuzzy and unscaled fluorescence micrograph of DAPI-stained alder-yellows phytoplasmas, leaving detail of the surrounding sieve tubes largely to the imagination.
The individual contributions are well-written and have been conscientiously edited. I did find a few minor niggles, about which the authors would probably be glad to set the record straight. Chapters 7 & 8 describe what (rather little) is known about movement of phytoplasmas, disease development, and accompanying biochemical changes in plants. In a rare harking back to the halcyon days when detection of phytoplasmas relied on transmission electron-microscopy, the authors present TEMs described as showing “phytoplasma bodies passing through sieve plates pores” (Chapter 7) and, in Chapter 8, a “bud-like structure” and a “phytoplasma cell dividing by scission” (whatever that means). It is a pity that a study of the three dimensional form of phytoplasmas in serial sections reported over 30 years ago (Waters H, Hunt P, 1980. The in vivo three-dimensional form of a plant mycoplasma-like organism revealed by the analysis of serial ultrathin sections. Journal of General Microbiology116, 111–31) which so ably showed that many of these features are likely to be artefacts, seems to have been forgotten here. Chapter 13 refers to phytoplasmas as “degenerate Gram-positive prokaryotes”. Whilst they may share common ancestry with Gram-positive bacteria, it is surely wrong to describe organisms that lack cell walls (and that are thus incapable of taking up Gram’s stain) as Gram-positive. The same chapter refers to over 200 known vectors of phytoplasma, spiroplasma, virus and Xylella in the Auchenorryncha and Sternorrhyncha, but surprisingly makes no mention of either psyllid vectors of phloem-limited Gram-positive bacteria (Liberibacter) or other bacteria that are transmitted by Homoptera, which share many of the epidemiological features of phytoplasmas. As examples of the rather few known planthopper (fulgoroid) vectors of phytoplasmas, these authors mention that at least one species from each of the planthopper families Cixiidae, Delphacidae and Derbidae transmit phytoplasma in the coconut lethal yellows group. However, so far as I am aware, the only generally-accepted vector is the cixiid Haplaxius (formerly Myndus) crudus. A splendidly oddball reference (on page 148) to an outbreak of lethal yellowing in Turkey in 1999 is erroneous and was apparently intended to refer to an unconfirmed report from the Turks and Caicos Islands. Another entomological error is the statement in Chapter 10 that the majority of phytoplasma vector species are found in the Delphacidae, a planthopper family, when it is presumably intended here to refer to the leafhopper subfamily Deltocephalinae, as correctly stated in Chapter 13.
But these are minor quibbles. Most readers will be fascinated by accounts of the recent advances in knowledge gained from sequencing phytoplasma genomes, and the tantalising indications of answers to key phenotypic properties such as mechanisms of pathogenicity and host specificity. But whilst molecular technologies have forged ahead, key difficulties and bottlenecks surely remain in achieving controlled experimental infections, especially between more distantly-related host plants, which will be needed to relate genotype to phenotypic response. In that regard, I was surprised to note only one, passing, reference to the use of dodder (Cuscuta spp.) as a bridging host, and little or no mention of plant or insect cell cultures as model systems. Maybe these have passed out of fashion because they are no longer considered essential to relate DNA sequence data to function.
Another notable feature of this book is the explosion of new information about the diversity of phytoplasma ‘strains’, based on differences in 16S rRNA, sec A, sec Y and several other genetic loci. Several authors refer to the identification of over 1500 strains and “hundreds of phytoplasma diseases”. However, whilst a great diversity of genetic variation has been reported, many of these ‘strains’ have yet to be related to any meaningful differences in phenotypic behaviour. Much of the ecological and epidemiological significance of phytoplasma genome variability remains largely unexplored. Are there really hundreds of different phytoplasma diseases, or are there rather fewer diseases but in hundreds of different plants?