Plant-Microbe Interactions

The research program of the PMI group is concentrated mainly on Arabidopsis thaliana as a model plant species to unravel molecular and ecological aspects of the plant immune system and the way beneficial micro-organisms affect host immunity. Three lines of research are being pursued.

• Unraveling the complexity of induced defense signaling in plant immunity (Corné Pieterse)
• Microbial ecology of disease suppression by beneficial soil micro-organisms (Peter Bakker)
• Molecular basis of disease susceptibility and pathogen virulence (Guido van den Ackerveken)
• Mycorrhizal ecology (Marcel van der Heijden)

Unravelling the complexity of induced defense signaling in plant immunity (Corné Pieterse)

An important question in plant defense signaling research is how plants integrate signals induced by pathogens, beneficial microbes and insects into the most appropriate adaptive response. Molecular and genomic tools have been used to uncover the complexity of the induced defense signaling network that evolved during the arms races between plants, parasites, herbivores and beneficial organisms. Microarray and phytohormone studies with Arabidopsis and various microbial and herbivore attackers revealed that salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) are main regulators of plant defense responses, but that the final outcome of the resistance response is strongly affected by attacker-specific factors and pathway cross-talk. Cross-communication between SA, JA and ET signaling was shown to enable the plant to finely tune its immune response. As described in a series of papers we were able to unravel molecular mechanisms involved in this defense pathway cross-talk.  A review article entitled “Networking by small-molecule hormones in plant immunity” recently appeared in Nature Chemical Biology (Pieterse et al., 2009) and provides an in depth update on this emerging topic in plant defense signaling research. 

Non-pathogenic rhizobacteria are able to help the plant to defend itself by activating a systemic, broad-spectrum resistance called rhizobacteria-induced systemic resistance (ISR). In the past years we investigated the molecular mechanisms of how beneficial soil-borne rhizobacteria are able to stimulate the plant’s immune system, thereby protecting the plant against a broad spectrum of plant pathogens and even insect herbivores. We unraveled many molecular details of the ISR signaling pathway. Microarrays have shown that ISR is based on priming for enhanced defense-related gene expression, rather than on direct activation of defense. Ecological studies revealed that priming for enhanced defense has significant fitness benefits when plants are grown under pathogen pressure. A recent review on this topic entitled “Plant immune responses triggered by beneficial microbes” recently appeared in Current Opinion in Plant Biology (Van Wees et al., 2008).

Microbial ecology of disease suppression by beneficial soil micro-organisms (Peter Bakker)

Through mutant analysis and complementation the mechanism(s) responsible for disease suppression by beneficial rhizobacteria have been defined. Several microbial determinants have been demonstrated to trigger ISR in Arabidopsis, rice, bean and tomato. Bacterial mutants that are no longer inducive, as well as complementation analysis enabled isolation and characterization of the genes involved. It has been established that several microbe-associated molecular patterns (MAMPs), such as specific siderophores, the outer membrane lipopolysaccharide (LPS), and flagella can play a role in induction in a bacterial strain – host plant-specific manner. This research line fulfills the aim to place molecular mechanisms in a microbial ecological perspective. A recent review entitled “Induced systemic resistance by fluorescent Pseudomonas spp.” in Phytopathology (Bakker et al., 2007) provides an overview of this field of research.

Molecular basis of disease susceptibility and pathogen virulence (Guido van den Ackerveken)

 In this line of research, the role of the host plant in supporting infection by biotrophic pathogens is central. The interaction of the downy mildew pathogen Hyaloperonospora arabidopsidis with Arabidopsis is used as a model. The downy mildew pathogen causes disease in many economically important crop plants. Hence, knowledge on downy mildew resistance is of potential interest to apply in crop plants. Through map-based cloning, several Arabidopsis genes have been identified that, when mutated, give a dmr (downy mildew resistant) phenotype. For instance, in the dmr1 mutant homoserine kinase activity was found to be reduced leading to the accumulation of homoserine that leads to resistance via a novel, so far unknown mechanism (Van Damme et al., 2009: Plant Cell). The dmr6 mutant has a loss of function mutation in an oxidoreductase leading to the activation of a so far undiscovered mechanism of plant defense (Van Damme et al., 2008; Plant J. 54: 785-793). Without exceptions, the dmr mutations revealed downy mildew resistant phenotypes that are based upon previously undiscovered molecular mechanisms. Hence, the approach taken provides novel insights into the functioning of the plant’s immune system. Moreover, the approach taken provides promising possibilities for obtaining downy mildew resistant crop plants. Hence, this line of research is of interest for Dutch plant breeding companies. 

In addition, this line of research also investigates mechanisms by which the downy mildew pathogen is able to suppress host immunity. This is a rapidly emerging field of research for which research on the Arabidopsis-H. arabidopsidis interaction is highly suitable. H. arabidopsidis produces a large number of effector proteins that are thought to play a role in immune suppression. These oomycete  effectors have conserved motifs in their amino acid sequence. Hence, genome mining and bioinformatics are excellent tools to gain insight into this phenomenon. The group participates in the H. arabidopsidis sequencing project and collaborates internationally to make important advances in this emerging theme in plant defense research.  

Mycorrhizal ecology (Marcel van der Heijden)

Almost all plants form intimate symbiotic associations with mycorrhizal fungi. Mycorrhizal fungi form extensive mycelial networks in the soil and forage effectively for minerals such as nitrogen and phosphorus that are delivered to the plant roots. Plants often benefit from infection by these fungi and show enhanced growth. Plants can obtain up to 80% of nitrogen and up to 90% of their phosphorus demand from mycorrhizal fungi. Mycorrhizal fungi in turn receive photosynthates from the plant and the result is an association between two completely different organisms. Our research focuses on arbuscular mycorrhizal fungi (AMF), the most common group of mycorrhizal fungi. In particular we investigate the agricultural and ecological significance of mycorrhizal fungi and mycorrhizal fungal diversity. For more information click here.