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The venue for the 30th New Phytologist Symposium ‘Immunomodulation by plant-associated organisms’ was the Stanford Sierra Conference Centre located on the Shore of Fallen Leaf Lake over 6300ft up in the Sierra Nevada: an idyllic location for the altitude and science to take your breath away. The meeting kicked off on Sunday evening with an address from Clarence Kado about the history of plant pathology conferences at Fallen Leaf. This was followed by the keynote presentation from Jorge Galan; a tour de force of the bacterial type III secretion system assembly and function. All that was left to do was have dinner and a cheeky glass of wine or two before we got our jet-lagged selves to bed.
The meeting began in earnest early on Monday morning with Session 1 on Pathogenomics. Brian Staskawicz gave a talk on how the advances in next generation sequencing are allowing the identification of core effectors from plant pathogenic bacteria to enable targeted pyramiding of cognate R-genes hopefully leading to durable resistance.
The sheer number of pathogenic strains which can be sequenced cheaply and quickly in parallel now is staggering considering the time and prohibitive cost of such a project only a few years ago. There was also a very interesting talk from Regine Kahmann on the progress of identification and functional analysis of effectors from smut fungi where there are no helpful type III secretion signals or RxLR motifs to delineate the pathogens effector repertoire. Among several effectors she mentioned is Cmu1 a chorismate mutase which leads to metabolic priming of plant cells by lowering SA levels. In addition an inactive mutant elicits an HR providing a phenotypic marker for testing domains involved in translocation of the effector. Closing the session was Dinesh-Kumar who gave a talk about the development of protein microarrays which may emerge as a useful new tool for large scale identification of effector targets and their functional characterisation.
In a stroke of scheduling genius the afternoons were left free, allowing energetic delegates to hike up the surrounding peaks or tour the lake on kayaks obtained at the boat dock, while the rest enjoyed the spectacular scenery or took a leisurely swim in the lake. The science recommenced at the poster session and with over 80 posters to admire there was sure to be something for everyone. I was particularly interested in posters by F Cao and S Popescu describing effector modulation of plant ABA and SA hormone signalling respectively.
The evening session covered the rather controversial area of effector secretion and trafficking into host cells. Peter Dodds started off talking about the Rust fungal effector AvrM uptake from the extra haustorial matrix (EHM) being partly dependent on the N-terminus of the protein. He posed the questions asking if it is possible that effectors are taken up by normal plant endocytosis as they happen to be present in the EHM at high concentration, and if that is the case, surely the questions is how do these effectors subsequently escape the endosome? There was also an excellent talk by Barbara Valent showing the results of detailed cell biological studies demonstrating that in Magnaporthe there are two distinct mechanisms for secretion of apoplastic (brefeldin A inhibited) and cytoplasmic effectors (no effect of brefeldin A). She also showed the cytoplasmic effectors accumulate at a structure called the BIC (biotrophic interfacial complex) outside the fungal cell and that effectors are transferred from plant cell to plant cell ahead of the site of pathogen infection.
However the standout talk of the meeting for me was given by Sheng Yang He who showed how the use of the bacterial non-proteinaceous effector coronatine led to the elucidation of the JA regulatory JAZ genes which appear to have an important role in the control of growth-defence crosstalk via their interaction with the GA responsive Della genes. JAZ proteins seem to be targeted by many different effectors which suggests this may allow the pathogens to interfere with the plants ability to sense the levels of growth versus defence hormones.
The evening session on Tuesday gave a good insight into the structural biology of effectors and their targets. Jeff Dangl started with his inspiring talk about plant immune system functions and its battle with pathogen effectors. He referred to the sophisticated two-tiered receptor-based system that plants use to generate a set of downstream responses to stop microbes from growing and how pathogens react with virulence effectors. The focus is to understand how those effector proteins have evolved to manipulate the host signalling machinery, how they are recognized and how successful the immune response from the plant is.
Finding out about principal and evolutional mechanisms of plant pathogen interactions should lead to a better understanding of pathogen ingress and defence suppression. Mark Banfield gave an excellent talk about real structure-led studies of plant pathogenic proteins using crystal structures, such as the Avr3a_11 from Phytophthora capsici, to define virulence activities, evolutionary relationships and sub cellular localization.
With the last morning session on emerging systems the meeting was closed. This session gave an overview of the role of effectors from various plant pests and pathogens, including ectomycorrhizal and arbuscular mycorrhizal fungi, poplar rust, oomycetes, insects, phytoplasmas, nematodes, and, intriguingly, parasitic plants. Francis Martin kicked off the session with a talk on the MiSSP7 effector from the ectomycchorizal fungus Laccaria bicolor. This effector is a small-secreted protein that is important for the establishment of mycorrhizal mutualism. Interestingly, this effector plays a role in altering host jasmonate signalling during plant colonization.
A very interesting talk by Saskia Hogenhout described the role of phytoplasma effectors in the three-way interaction of leaf hoppers , phytoplasmas and plants. She showed that phytoplasma effectors target plant transcription factors to alter host responses. This, in turn, increases leafhopper performance on host plants, and thus the number of insect vectors that can spread the phytoplasma.
The closing talk of the session, and the meeting, was given by Ken Shirasu. He described the hunt for effectors in parasitic Orobanchaceae plants using large – scale genomics and transcriptomics approaches. Already lists of effector candidates specifically expressed during infection have been identified. I thought this was quite exciting; it will be very interesting to see how these effectors compare in activity to pathogen and pest effectors.
With that the meeting was closed, and after a nice lunch at the lake it was off again for a long trip home. The meeting overall was very exciting, with lots of great talks on effectors, their activities, and host targets. There were good opportunities for networking and taking a stroll in the beautiful surrounding. Finally we would like to thank the New Phytologist Trust for organising and funding the symposium and the BSPP for the Travel Grants enabling us to attend this excellent meeting.
Hazel McLellan and Julietta Jupe The University of Dundee and Jorunn Bos The James Hutton Institute
