The war of the worlds

Authors

  • F. Ian Woodward,

  • Holly Slater


The war of the worlds is a novel of science fiction that was written by H. G. Wells, and published in 1898. It has since been made very widely available to the public through film, radio, television, computer games and a musical. Orson Welles’ radio broadcast was so believable that there was a public outcry. The story is of an Earth invasion by Martians, their subsequent use of human blood as a food source and the impact of Martian red weed on terrestrial ecology. Finally, the Earth recovers and all Martian artefacts are killed by pathogenic bacteria because they have no immunity. This science fiction can be recognized as such and laymen may see no parallel with plant science. Plants do feature as the lead parts in television documentaries, but their appeal has to be matched with animals by speeding up growth processes and circumnutation. In fact, the war of the worlds is a very fair parallel to the highly complex everyday life of plants and their interactions with alien invasions by fungi, bacteria, viruses and animals.

Plants have a multilayered defence against pathogens (Dodds et al., 2009), which starts with external barriers, such as the layer of waxy cuticle and the rigid cell wall. If a pathogen breaches these physical barriers then the plant triggers biochemical defence responses, after recognizing the pathogen by its characteristic chemical signatures, such as chitin and flagellin. This defence response has been recognized in a range of plants, even in the simple moss (Lehtonen et al., 2009), where a peroxidase is produced in response to a fungal cell-wall extract. The peroxidase activity appears to prevent the growth of pathogenic fungi. A wide range of toxic defence compounds may be produced by plants. However, pathogens can evolve rapidly in this arms race to produce altered genotypes that are able to circumvent this level of defence (Niks & Marcel, 2009). A range of methods have been developed, such as detoxification and modifying membrane transporters, to pump the toxic compounds out of the host cells (Saunders & Kohn, 2009). Pathogens typically also produce effector proteins that are delivered into the host cells (Dodds et al., 2009; see also Schornack et al., 2008 and Nguyen et al., 2010) and subsequently suppress plant defence responses and reprogramme the cell to pathogen metabolism. Infection of sunflower with Botrytis cinerea (Dulermo et al., 2009) highjacks the sunflower’s carbohydrate metabolism from hexose production and changes it towards the mannitol pathway, which is required by the pathogen.

The effectors produced by pathogens can be recognized as alien products, leading to the stimulation of a further level of plant defence mechanisms, under the control of resistance genes (Dodds et al., 2009). Responses associated with this type of defence usually include an extracellular oxidative burst and localized cell death, which can limit the spread of the pathogen (McLellan et al., 2009). Some pathogens may break through all levels of plant resistance, a major cause of concern when this occurs in crop plants. Some resistant crop cultivars retain their resistance for much longer time-periods than others, but it is not known how this occurs. Palloix et al. (2009) demonstrated, in a potato–virus system, that polygenic host resistance rather than monogenic host resistance favoured a more durable resistance by the potato.

Plant resistance to alien attack can take what seem to be the most unlikely pathways. When the leaves of Nicotiana attenuata (Schuman et al., 2009) are attacked by herbivores, the plant produces volatile organic compounds (VOCs) that attract predators of the herbivore, producing an effective indirect defence. However, the use of gaseous signalling components is not without problems. Ethylene production by plants is involved in regulating defence to pathogens (Chen et al., 2009). Infection of wheat with Fusarium graminearum leads to the production of mycotoxins. The wheat also produces ethylene as part of the defence network, but this appears to stimulate the spread of the mycotoxin around the plant, leading to death of the host. Reducing the level of ethylene production reduces the spread of disease.

The importance of crop plants for food production means that addressing pathogen infestations is an ‘all or nothing’ response. As some pathogenic fungi produce mycotoxins poisonous to humans, no infestation is acceptable. In the natural environment the war of the worlds can often be long term and continuous. The larch bud moth infests European larch over 15° of longitude in Central Europe (Büntgen et al., 2009). Evidence from tree rings indicates a battle of at least 300 yr between the larch bud moth and the larch, with a cycle of approx. 8–9 yr, in which the plant, the bud moth and its parasitoids all interact within seasonal time frames defined by the annual progress of both chilling and high temperatures.

Unlike animals, plants do not move but they possess a tremendous range of methods for keeping aliens at bay, as a means of ensuring survival while rooted to the spot. The fantasy of science fiction in books and film is rather close to this everyday play among pathogens, parasites and potential plant hosts and, on this note, it is interesting that H. G. Wells was a reviewer for Nature, where he found contemporary science a model for science fiction.

The New Phytologist papers featured in this Editorial highlight the fact that plant defence makes up a core element of the journal, particularly in the Interaction section, but also increasingly as cross-disciplinary studies with Physiology & Development, Evolution and the Environment. We are thus pleased to welcome Ralph Panstruga (Max Planck Institute for Plant Breeding Research, Cologne, Germany) to the New Phytologist Editorial board. Ralph’s expertise in plant defence centres on plant–biotrophic fungal disease interactions (see his Tansley review, O’Connell & Panstruga, 2006; also Göllner et al., 2008). His particular passion involves the novel plant-specific seven transmembrane domain MLO proteins, of which the barley MLO is involved in suppressing defence reactions against the powdery mildew fungus. Unravelling the molecular basis of mlo resistance, in addition to understanding the core biochemical and evolutionary significance of this protein family, is thus central to Ralph’s research.

This year we are also pleased to welcome Hongzhi Kong (Institute of Botany, Chinese Academy of Sciences, Beijing, China) to the Editorial board. The Evolution section, since its launch in 2004, is now well established, and Hongzhi’s appointment chiefly recognizes the growth in plant molecular evolution and evolutionary development (evo-devo) research that is submitted to the journal, and to which his research group has also contributed (e.g. Su et al., 2008). Hongzhi’s expertise in evo-devo research is exemplified by his contributions to our understanding of the genetic controls and molecular mechanisms involved in the development and evolution of flowers. Hongzhi’s bioinformatics skills extend his research interests to evolutionary regulatory genomics, phlyogenomics and molecular biogeography. In addition to helping New Phytologist nurture plant evolution research, Hongzhi is also our first Editor from Asia and adds to the commitment of the New Phytologist Trust to promote plant science worldwide. For example, the organization of two scientific writing workshops for Chinese authors (Beijing, December 2007 and Kunming, November 2009) and the recent New Phytologist Symposium ‘Carbon cycling in tropical ecosystems’ that was held in Gaungzhou, China, November 2009 (http://www.newphytologist.org/carbon/default.htm), follow-up reports and a special feature on carbon cycling, will be published in future issues of the journal. So, from the war of the worlds, through plant defence to the more peaceful plant evo-devo, we look forward to further developing these areas of the journal with the expert help of Ralph and Hongzhi.

Ancillary