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Plant Signalling

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This item first appeared in 2008

Conference Review: SCI Plant Signalling Meeting

A one day conference on Plant Signalling was held in Belgrave Square, London on 4 March 2008. It was organised by the Society for Chemistry and Industry (SCI) BioResources group and chaired by Dr Toby Bruce of Rothamsted Research (UK). Scientists representing research groups from the UK, the Netherlands, Germany and Switzerland presented the latest information about advances in our understanding of the exquisitely complex ways in which plants alter their metabolism in response to stress stimuli. The meeting was attended by academics specialising in this research area and a small but select group of representatives of companies interested in innovations in crop protection.

Like humans whose immune system plays a part in warding off disease, plants have natural metabolic processes that protect them from attacking organisms. By studying plants’ natural defence mechanisms we can find ways of enhancing them which can be exploited in crop protection. It has been discovered that these natural defence responses can be primed by inducing agents, presenting opportunities for exploitation in terms of plant activator treatments which alter gene expression within treated plants. New information about plant defence genes and metabolites, produced when they are expressed, can also be exploited in breeding resistant plant varieties.

This is very different from traditional agrochemistry which is directed at controlling pathogens and insect pests using chemical products with a toxic mode of action. Plant signalling crosses traditional disciplinary boundaries and for a full understanding of it several areas of science are needed. Chemistry enables identification of the signals involved, genetics explains the changes in plant gene expression that underpin plant signalling events, biology is needed to investigate insect and pathogen performance on exposed plants and ecological field experiments are required to determine real world implications of modifying plant signalling.

The conference comprised seven talks and two open discussion sessions. The first speaker was Prof John Pickett (Rothamsted Research, UK) who explained how plant defence metabolism can be enhanced with a plant activator, cis-jasmone, to repel pests and attract their natural enemies. cis-Jasmone is naturally released from plants damaged by insects, for example cotton leaves damaged by Spodoptera exigua larvae. Insect behavioural effects have been found with cis-jasmone long after treatment of crop plants. 

In wheat, cis-jasmone induction made plants less favourable for the grain aphid, Sitobion avenae, reducing their settlement, growth and development, and attracting a natural enemy, the parasitoid Aphidius ervi. Similar effects were observed in studies with the model plant Arabidopsis. Although cis-jasmone is structurally related to jasmonic acid and its volatile derivative methyl jasmonate, transcriptomic microarray analyses of gene expression in Arabidopsis have shown that a unique and more limited set of genes is up-regulated by cis-jasmone treatment compared with methyl jasmonate treatment.

Prof Harro Bouwmeester (Wageningen University, the Netherlands) then described how metabolic engineering can be used to manipulate plant signalling. Terpenoids are a class of secondary metabolites that play an important role in the communication of plants with their environment. In the past few years rapid progress has been achieved in terpenoid metabolic engineering in plants. Hence it is now becoming possible to create transgenic plants producing terpenoids on demand. With such engineered plants fascinating results have been obtained that show that insect behaviour is strongly influenced by terpenoids.

For the future, Prof Bouwmeester foresees great progress in the engineering of terpenoid production in plants, including volatile formation under control of insect-inducible promoters.

Dr Gustavo Bonaventure (Max Planck Institute for Chemical Ecology, Germany) spoke about fatty acid-amino acid conjugates (FAC) that act as elicitors of herbivore specific responses in tobacco. When plants are attacked by insect herbivores, they induce an extensive metabolic reconfiguration by reprogramming the plant’s transcriptome (the set of genes being expressed). Biochemical and transcriptional analyses suggest the existence of central herbivore-activated regulators in tobacco leaves, which, in turn, are regulated by minute amounts of FACs in the insect’s regurgitant. Disentangling the effect of mechanical tissue damage and FAC elicitation will provide critical information on how plants tune defence responses to more efficiently protect themselves against insect herbivores.

In the afternoon Dr Marco D'Alessandro (University of Neuchâtel, Switzerland) discussed herbivore induced plant odours and how to identify odours relevant for foraging parasitoids. When plants are attacked by insect herbivores they respond by emitting a complex blend of volatile compounds, which have been shown to be highly attractive to natural enemies of these herbivores, such as predators and parasitic wasps. Dr D’Alessandro’s results imply that key attractive compounds for these wasps exist, but that they differ for different parasitoid species.

Dr Jurriaan Ton (Rothamsted Research) explained how priming agents can be used to boost plant resistance to stress. Defence sensitisation is called priming and allows plants to adjust their innate immune system to the potentially hostile conditions in their environment. Recently, it has been demonstrated that induction of priming provides durable protection with minimal reductions in plant growth and seed set. Hence, priming represents an adaptive and cost-efficient defence strategy that increases the plant’s ability to survive in hostile environments.

Recent insights in the mechanisms behind systemic acquired resistance (SAR), β-amino-butyric acid-induced resistance (BABA-IR), rhizobacteria-mediated induced systemic resistance (ISR), and volatile-induced resistance against insects have revealed various priming mechanisms. Whereas SAR and BABA-IR are associated with priming for salicylate-dependent defences against biotrophic pathogens, ISR and volatile-induced resistance function through priming for jasmonate-dependent defences against necrotrophic pathogens and herbivorous insects.

Dr Glen Powell (Imperial College London) described how interactions between plants and aphids are influenced by BABA treatment which were investigated using the electrical penetration graph (EPG) technique. EPG recordings of aphid stylet activities within bean tissues indicate that sieve element defences may be enhanced on BABA-treated plants, leading to disrupted phloem sap ingestion.

Finally Dr Marco Busch (Bayer CropScience, Germany) reported plant signalling cascades induced by herbicide safeners and how they make plants more tolerant of drought stress. Herbicide safeners are crop protection agents which selectively protect crops from herbicide injury without impairing weed control. Using gene expression profiling Bayer has shown that various safeners act by transiently inducing similar sets of genes comprising genes encoding monooxygenases, gluthathione S transferases, and glucosyl-transferases involved in degradation of the herbicides by crops. In parallel with the induction of these genes, several safeners also induced genes involved in oxidative stress signalling cascades and tolerance of abiotic stresses.

The morning discussion session was on exploiting new knowledge for improved crop protection: application of plant activator treatments and breeding resistant plant varieties were seen as the main routes. Constraints involved in registering new chemical treatments for crops and public attitudes towards GM were discussed in the context of global food security and increasing demand for agricultural commodities. The afternoon discussion was on underlying themes including the importance of identifying early components in signalling and the elicitors of defence; the generic similarities between plant responses to different stresses, and the need for 1) a holistic view of plant function, 2) testing under real field conditions and 3) metabolomics studies in addition to analysis of plant gene expression.

For further information about plant signalling please contact Toby Bruce, E: toby.bruce@bbsrc.ac.uk, W: http://www.rothamsted.ac.uk/bch/

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