Characterisation of a conserved protein acting as key positive regulator in plant innate immunity

Plants lack the adaptive immunity mechanisms of jawed vertebrates, so rely on innate immune responses for their defence. As sessile organisms they are subject to changing environmental conditions including constant pathogen attack. However, would-be pathogens have first to encounter constitutively present barriers such as wax layers or rigid cell walls. If a pathogen can overcome these barriers, they are then subject to recognition by plant cells. Plants lack circulating cells specialized in microbe recognition, such as macrophages. Instead, each cell is able to recognise and respond to pathogens autonomously. In addition, systemic signalling can be triggered in response to microbial stimuli that prepare naïve tissue for imminent attack. Overall, plant innate immunity is very efficient and most plants are resistant to most microbes. Part of this success is due to an amazing spectrum of recognition specificities encodes by all cells. Plants initially sense microbes via perception of pathogen-associated molecular patterns (PAMPs) by pattern-recognition receptors (PRRs) located on the cell surface. PAMPs are conserved, indispensable molecules that are characteristic of a whole class of microbes, therefore are difficult to mutate or delete. They are also referred to as microbe-associated molecular patterns (MAMPs), as they are not limited to pathogenic microbes. This first level of recognition is referred to as PAMP-triggered immunity (PTI). To infect host plants successful pathogens have evolved strategies either to evade recognition, or to suppress the subsequent signalling steps. In many cases, suppression of PTI involves secretion of virulence effectors by the pathogens. In a dynamic co-evolution between plants and pathogens, some plants have evolved resistance proteins (R proteins) to recognise these effectors directly or indirectly. This so-called effector-triggered immunity (ETI) is often accompanied by local cell death known as the hypersensitive response (HR). In turn, pathogens have evolved effectors capable of suppressing ETI, and so the arms-race between host and pathogens continues. In this model, PTI is the first facet of active plant defence and can therefore be considered as the primary driving-force of plant-microbe interactions. We need to understand PTI properly not only because of its intrinsic interest, but because many of the effector targets will be PTI components. Despite the importance of PTI, we still know little about PAMP perception and signalling. We are using the plant model species Arabidopsis thaliana to study PTI. We have identified a protein conserved in plants, but also in mammals, insects and worms, as being a key regulator of PAMP responses and bacterial disease resistance. Our preliminary results indicate that this protein is required for several PAMP perception systems. We will address if this protein directly regulates PAMP perception or acts on the level of downstream signalling following binding of the PAMP to its receptor. We will also determine which other PAMP perception systems are dependent on this protein. Finally, experiments to explore the molecular function of this protein will be performed.

A key aspect of active defence mechanisms is the recognition of pathogen-associated molecular patterns (PAMPs) by pattern-recognition receptors (PRRs). The Arabidopsis leucine-rich repeat receptor kinases (LRR-RLKs) FLS2 and EFR recognize the bacterial PAMPs flagellin and EF-Tu, via the peptidic epitopes flg22 and elf18, respectively. Although certain signalling events occurring after flg22 and elf18 perception have been identified, the mechanisms linking receptor activation and intracellular signalling remain largely unknown. Genetic and interaction screens have been crucial for the understanding of other RLK systems in plants. We have identified many Arabidopsis mutants that are strongly affected in their response to elf18. Surprisingly, most of these mutants were not affected in their response to flg22, suggesting distinct genetic requirements for EFR and FLS2 expression, function, or downstream signalling. The current proposal is based on the exciting finding that Arabidopsis plants mutated in the protein SDF2 are affected in elf18 responses, but are also more susceptible to bacterial infection. Although sdf2 mutants do not seem to be affected in flg22 responses, they are more susceptible to bacteria compared to efr mutants. This suggests that SDF2 regulates other PRRs. SDF2 is therefore a key positive regulator of PTI. Despite being conserved among eukaryotes, the molecular function of SDF2 is still unknown. We are therefore in a unique position to study SDF2 in a biological context. Using a range of bioassays following treatment with purified PAMPs or pathogens, we will determine which aspects of PTI are affected in sdf2 mutants (perception or signalling). A major question to answer is which other PAMP perception systems depend on SDF2, and why SDF2 is not required for FLS2 function. Finally, we will use a panel of molecular, biochemical and cell biology techniques to understand how SDF2 regulates plant innate immunity at the molecular level.