EXPLORING ADAPTIVE IMMUNITY IN PLANTS

In their struggle for life, plants strongly rely on inducible defense mechanisms. These defense responses become activated when a plant is attacked by harmful pathogens or insects. Induced defence involves a wide spectrum of different chemical and physical defence barriers, ranging from the induction of toxic metabolites that target the attacker's physiology, to cell wall appositions that prevent invasions by pathogenic fungi. Despite this diversity in defensive strategies, the inducible defense arsenal is not always sufficient to protect the plant against intrusion by pathogens and insects. This is why plants have evolved an additional, more sophisticated, defense system that allows them to fine-tune their inducible defense system. Upon perception of specific environmental cues, plants can develop an enhanced defensive capacity that is effective against a remarkably wide range of different stresses. Interestingly, this induced resistance is not based on direct defence activation by the inducing agent, but on a faster and stronger activation of inducible defence mechanisms at the moment the plant is exposed to stress. This sensitization for defence is called 'priming'. Because priming allows the plant to adjust its inducible defence system to the environmental conditions, it can be regarded as a form of adaptive immunity. Interestingly, stimulation of the plant's adaptive immune system through priming has already been shown to yield broad-spectrum resistance with minimal reductions in plant growth and seed set. This suggests an important ecological function of plant adaptive immunity, which increases the plant's ability to survive in hostile environments. The main objectives of this research proposal are to 1) discover novel key mechanisms by which plants exploit their adaptive immune system, and 2) critically evaluate the ecological advantages that adaptive immunity provides for plants under field conditions. To this end, a multidisciplinary approach will be followed, using state-of-the-art techniques in the field of molecular biology, plant physiology and plant ecology. The outcome of this project will be instrumental for future exploitations of plant adaptive immunity in sustainable agriculture.

Although research on plant-microbe and -insect interactions has undergone exciting developments, research on plant adaptive immunity is still in its infancy. In this pioneering stage, Arabidopsis thaliana as a model system holds many advantages due to the vast amount of molecular tools and well-characterised pathosystems that are freely accessible for this plant species. This research proposal entails different research lines that focus on various aspects of adaptive immunity in Arabidopsis. Part 1 will exploit naturally occurring variation in adaptive immunity among Arabidopsis accessions. A recombinant inbred population (RIL) from 2 accessions with opposite phenotypes will be screened for early responsiveness to low doses of defence hormones and resistance against pathogens and insects. RILs showing enhanced or repressed priming phenotypes will be analyzed through 'genetical genomics'. This novel technique combines marker-based genotyping with transcription profiling, enabling genetic dissection of the pathways controlling adaptive immunity. Part 2 is focussed on the role of transcription factors and builds on the observation that various priming-inducing signals trigger expression of defence-related TF genes. Through a combination of biochemical and molecular-genetic characterisations, this line of research will reveal how interactions between TFs shape the outcome of the adaptive immune response. Part 3 will explore the cell biology behind adaptive immunity against pathogenic fungi. A forward mutagenesis screen will identify mutants impaired in the priming for cell wall defence. Confocal laser microscopy will reveal the impact of these mutations on cytoskeleton and organelle dynamics. Part 4 will evaluate the ecological value of plant adaptive immunity. Field experiments will be done using the mutants obtained in the other parts of the project. Their ecological performance will be monitored under varying degrees of disease and herbivory pressure.