Plant–microbe interactions in Yucatán: hurricanes didn't curb the whirlwind of discovery

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XII International Congress on Molecular Plant–Microbe Interactions, Mérida, Yucatán, Mexico, December 2005

Uxmal, the beautiful Mayan site near Mérida, means three times rebuilt. A similar fate had befallen the XII International Congress on Molecular Plant–Microbe Interactions, said Federico Sanchez, chair of the organization, in his opening address. He and colleagues of the local Scientific Committee and the Board of the International Society for Molecular Plant–Microbe Interactions (IS-MPMI) had needed to re-organise the meeting three times after two major hurricanes had hit Yucatán in the last season. Thanks to their perseverance, the meeting was eventually held in Mérida (http://www.ismpminet.org/meetings), with excellent attendance and a full breakthrough-packed programme from the first lecture to the last.

The aim of IS-MPMI meetings is to present data at the forefront of research, over a wide field of interactions between plants, fungi, bacteria, viruses and nematodes. Their emphasis is on the genetic and molecular analysis of the processes under study. While viruses and pathogens in general represented the leading topics of earlier meetings, this year mutualistic interactions, such as those of legumes with Rhizobium, helped to balance and broaden the meeting's overall orientation.

‘The increasing evidence of crosstalk in signalling pathways suggests that a few key steps could be shared among a range of different interactions’

Symbioses, plant responses and methods

A glance at the scientific program revealed that lectures on symbioses and plant cellular responses to microbes were amongst the major topics, together with an emerging theme of innate immunity being closely associated with nonhost resistance. Besides this global impression, attendees are likely to have left the XII IS-MPMI meeting with their bags full of novel methods, ideas and perspectives, impatient to go back to work and apply them to their own study. Among these ideas was the use of fluorescent proteins to report gene activation and protein localisation within a cell, giving our approach to plant science a very powerful instrument. Many communications in Mérida reflected this, and most major lectures included microscopy data as key evidence (Adrienne Hardham, Australian National University, Canberra, Australia), providing support for the molecular biology approach and unveiling new opportunities from live cell imaging (Takemoto & Hardham, 2005). Another exciting breakthrough was a demonstration of gene transfer from nonAgrobacterium Rhizobiaceae to plants (Broothaerts et al., 2005), which will open an unexpected potential for easier, and more widely applicable, plant transformation (Richard Jefferson, CAMBIA Canberra, Australia). Whether this will mark a return of plant biotechnologies to the arena of field application and food production remains to be seen.

Pathogen–host control and nonhost resistance

The modulation of plant-host defences by pathogens via the secretion of polypeptide effector molecules is receiving increased attention as several new fungal examples have been discovered. For example, examination of Phytophthora infestans secretome identified proteins – essential for pathogenicity – that contain an RXLR amino-acid motif (Birch et al., 2006). The intriguing fact in this case is that RXLR is similar to the RXLK(E/Q) motif required for targeting proteins from malaria parasites (Plasmodium species) into the red blood cells of their mammalian host, and represents an unexpected case of evolutionary convergence or conservation in host–pathogen interactions (Sophien Kamoun, Ohio State University, Wooster, OH, USA). In true fungi the idea that secreted proteins are involved in determining pathogenicity is receiving further support, for example in the corn smut agent Ustilago maydis whole gene clusters encoding secreted proteins have been seen to be required for full pathogenicity (unpublished results, Regine Kahmann, Max Planck Institute, Marburg, Germany). We await with interest the precise functional definition of these gene products to see if any might be classed as ‘effectors’ that control the plant.

Several of the speakers at the meeting focussed on nonhost resistance and the importance of PAMP (pathogen-associated molecular pattern) perception and transduction (Paul Schulze-Lefert, Max Planck Institute, Cologne, Germany; Shauna Somerville, Carnegie Institution, Stanford, CA, USA). Essential elements of the genetic and molecular basis for resistance (e.g. the pen genes) in Arabidopsis against usually nonpathogenic powdery mildews are now known. In this area recent advances have shown how plant receptors are poised to alert their hosts of potential attacks (Lipka et al., 2005). The role of bacterial flagellin and its perception (Zipfel et al., 2005) was also mentioned repeatedly by speakers (Georg Felix, Botanisches Institut der Universität, Basel, Switzerland). Perhaps one of the most impressive of these talks was that which convincingly demonstrated the internalisation of the Arabidopsis flagellin receptor in response to ligand binding (Silke Robatzek, Botanisches Institut der Universität, Basel, Switzerland). Another advance was shown in understanding how plants and microbes adhere to surfaces, for example, by studying how bacteria adhere to the plant surface (Jan Kijne, Leiden University, Leiden, the Netherlands; Laus et al., 2005) or organise into complex biofilms on a substrate (Ann Hirsch, University of California, Los Angeles, CA, USA). In the case of the latter, several symbiotic genes were shown to be important for biofilm formation (Fujishige et al., 2006).

Scientific legacy and the future

Finally, there is mounting evidence that a surprisingly wide variety of interactions, ranging from nematodes to endosymbionts to rhizospheric bacteria, involve, directly or indirectly, a common set of genes (Sanchez et al., 2005), proteins and cell mechanisms (Vivienne Gianinazzi-Pearson, CMSE-INRA, Dijon, France). A central role has been very clearly attributed to calcium-dependent signalling pathways, with CCaM-dependent kinases representing a key step in the molecular interaction (Kanamori et al., 2006) whose constitutive activation can even lead to root nodule formation in the absence of any exogenous signal (Allan Downie, JIC, Norwich, UK). Between expositions of such novel results the meeting also evoked its recent past in the form of a moving commemoration of Georges Truchet, who has made an outstanding contribution to the field of plant–microbe interactions. His most significant legacy was probably his involvement in the discovery of the nodulation (Nod) factors (Truchet et al., 1991), which demonstrated the concept of a molecular dialogue between plant and microbes, something which we are still unravelling today.

The jesters amongst us speculated that Chac, the Mayan rain god, had cast a curse on this conference. If so, let us only hope that Vulcan will spare the next XIII IS-MPMI congress that will be held in Sorrento – not a million miles away from Pompeii!

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