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The International Joint Workshop on ‘PR -proteins’ and ‘Induced Resistance Against Pathogens and Insects’ was held at Rheinisch-Westfälische Technische Hochschule (RWTH) University. The workshop was attended by 120 delegates and covered a range of topics including, defence signalling, hormonal crosstalk, priming, molecular ecology, and induced resistance in practice. The opening address was delivered by Professor Kees van Loon, one of the pioneers of research on pathogenesis related (PR) proteins and induced systemic resistance (ISR). Prof van Loon reflected on progress over the 45 years since publication of his paper describing PR-proteins.
The first session was focused on defence signalling and was opened by Professor Dan Klessig (Boyce Thompson Institute for Plant Research). Prof Klessig described salicylic acid binding proteins (SABPs) that are involved with plant defence and described some functional commonalities that are shared with SABPs in the inflammatory response in humans. Methods developed for the identification of plant SABPs have enabled discovery of a new SA target in humans, the high mobility group box 1 protein (HMGB1). HMGB1 is a major target of SA and of SA derivatives such as aspirin. HMGB proteins are reported to operate as universal sentinels for nucleic acid mediated innate immune responses in mammals and plants. Dr Ursula Pfitzner (University Hohenheim) continued the theme on SA in her talk on the role played by NPR proteins in the detection of SA signalling. NPR proteins in Arabidopsis undergo conformational changes in the presence of SA; NPR3 and NPR4 can bind SA directly whereas NPR1 becomes an SA receptor only in the presence of NIMIN1. Much of our understanding of SA signalling in plants requires destructive analysis of plant tissue in order to determine SA content and associated changes in protein and gene expression. Dr Alan Slusarenko (RWTH Aachen University) presented an interesting study in which he showed evidence that head space analysis of methyl salicylate can offer a non-destructive approach to infer changes in SA pools in plants.
Dr Dawn Luthe (Pennsylvania State University) delivered an entertaining presentation on plant responses to caterpillar frass that was titled ‘What goes in, comes out’. Frass (faeces) contains components derived from the host plant, the insect itself and associated microbes and hence provides a plethora of cues that may alter plant defence responses. Frass deposited by Fall armyworms grazing on maize leaves was found to stimulate the accumulation of PR proteins and to enhance resistance against fungal pathogen over time. Furthermore, there was a concomitant increase in herbivore performance. Research is underway to identify which frass component(s) are responsible for defence elicitation.
Dr Marcel Van Verk (Utrecht University) presented a fascinating talk on the use of high-resolution RNA-Seq time series experiments to gain a detailed understanding of the dynamics of hormone-controlled gene regulatory networks. A comprehensive study comprising of 15 time points over 16 hours was performed to profile the chronology of SA and jasmonic acid (JA) mediated responses in Arabidopsis.
Differences in the expression of most regulator (e.g. transcription factor) and regulated (e.g. enzymes) genes occurred within 2 hours after treatment with some transcriptional factors being upregulated after only 15 minutes. This technology has great potential to advance our understanding of hormonal networks and the role that they play in orchestrating plant defence.
The second day of the workshop was focussed on priming. Prof Uwe Conrath (RWTH Aachen University) prepared the foundation for the day by describing the phenomenon of priming and molecular mechanisms associated with its induction and persistence. He also discussed the use of priming agents in agriculture such as the fungicide pyraclostrobin and research to uncover new candidates. Prof Conrath’s group is extremely active in this area and has developed screening systems to identify novel priming agents; there were several posters from his group on this theme. Talks by Prof Jurriaan Ton (University of Sheffield) and Leonardo Furci (University of Sheffield) described epigenetic mechanisms controlling transgenerational priming. Prof Ton presented results from studies investigating the impact of DNA methylation and demethylation on transgenerational resistance responses to downy mildew in Arabidopsis. This talk was complemented by Leonardo Furci’s presentation on research to map epigenetic loci that are associated with priming resistance to biotrophic fungi in Arabidopsis. Four loci have been identified that correlate with the level of resistance to downy mildew.
Attention shifted to chemical and microbial priming agents during the afternoon session. The non-protein amino acid â-amino butyric acid (BABA) is a well-known priming agent that has been intensively studied for many years in the lab of Dr Brigitte Mauch-Mani (University of Neuchatel). Dr Mauch- Mani presented a brief history of BABA research before turning her attention to current investigations focused on metabolic responses to BABA in Arabidopsis. BABA has been found to stimulate primary metabolism and to cause an accumulation of carboxylic acids such as citrate, fumarate, (S)- malate and 2-oxoglutarate. She proposed that the stimulation of primary metabolites may help to provide the energy or the carbon skeletons to fuel the more rapid mobilisation of defences upon subsequent pathogen attack. The modes of action of a less well understood priming agent, pipecolic acid, was discussed by Dr Jurgen Zeier (Heinrich Heine University). Pipecolic acid was shown to be as a major regulator of priming in Arabidopsis and to play a central role in the coordination of SA and SA-independent defence responses in pathogen challenged plants.
There is increasing appreciation of the relevance of root-zone microorganisms as priming agents and their role in coordinating the systemic plant response to biotic and abiotic stress. Dr Maria Pozo (Granada, Spain) described how arbuscular mycorrhizal (AM) fungi circumvent host defences in order to colonise root tissue and establish a symbiotic relationship. The AM plants have greater root mass, which aids nutrient uptake and exhibits greater tolerance to salt stress and enhanced resistance to pests and disease. Stress responses are mediated by changes in hormonal levels. For example, enhanced drought tolerance in AM plants was associated with elevated accumulation of abscisic acid whilst induced resistance to Botrytis cinerea in tomato shoots was associated with priming of JA dependent defences. Prof Duncan Cameron (University Sheffield) presented an interesting talk describing different phases involved in the development of mycorrhizal-induced resistance in wheat against wheat yellow rust. After colonising host roots, the AM fungi enable the recruitment of plant growth promoting rhizobacteria (PGPR) and the establishment of the ‘mycorrhizosphere’. There is an additive benefit of AM and PGPR colonisation that includes enhanced root and shoot growth and an enhanced capacity to resist pathogen attack via priming of inducible host defences.
Prof Corne Pieterse (University of Utrecht) described some of the underlying mechanisms associated with rhizobacterial mediated ISR. During the onset of ISR there is considerable reprogramming of gene expression in root tissue but not in leaf tissue. A root specific transcription factor MYB72 has emerged as an important early regulator of ISR and this has also been shown to be associated with plant survival under conditions of iron deficiency. Interestingly it has been shown that MYB72 is induced in response to volatiles that are produced by some rhizobacteria. A ƒÀ-glucosidase, BGLU42, which operates downstream of MYB72 in the ISR signalling pathway, has been shown to play an important role in the secretion of iron-mobilising phenolic metabolites in the root vicinity.
Together these findings demonstrate a mechanistic link between the ability of rhizobacteria to stimulate systemic immunity and to promote iron uptake in host plants.
The last day of the workshop focused on the practical implementation of induced resistance. Prof Adrian Newton (James Hutton Institute) presented a comprehensive assessment of the pros and cons of induced resistance based on the results of research spanning over 25 years. Prof Newton presented examples of differential cultivar responses to different elicitors and also described within-cultivar variability to the same elicitor. There is evidence to suggest that combining elicitors with different modes of action and integrating these with other control options may help to resolve some of these issues in the field. Other talks during the day reflected the increasing interest in the development of new inducing agents. Christoph Andreas Braun (Bayer CropScience) outlined the modes of action of a new elicitor, isotianil, on rice whilst Philipp Lemke (Munster University) discussed research to optimise the activity of chitosan based elicitors through the use of nanoparticle technology. Several posters described the use of chitosan as an inducing agent so demonstrating a resurgence in interest in this compound.
Overall there was feeling of optimism and a sense that induced resistance is gaining greater acceptance as a management component for pest and disease control. It will be interesting to observe developments in this area over the next few years and to see if this is the case.
This workshop was very well organised with an extremely stimulating programme. Everything ran smoothly and the catering was excellent. The location was perfect and was just a few minutes away from the centre of Aachen with its rich history and impressive architecture (Aachen Cathedral is pictured opposite). I enjoyed the workshop immensely and I am extremely grateful to BSPP for financial support towards my attendance.
Tony Reglinski
Plant & Food Research, New Zealand
