RLP- and RLK-mediated innate immune responses in Arabidopsis and tomato triggered by PAMPs and avirulence factors

Plants are challenged routinely by biotic and abiotic factors. Biotic factors include pathogens that disclose their presence to plants by Pathogen-Associated Molecular Patterns (PAMPs), such as bacterial flagellin, lipopolysaccharide, peptidoglycon and fungal chitin. On the other hand, plants have cell surface receptors that scan the environment for incoming would be pathogens. There are three types of cell surface receptors in plants; a) completely outside the cell such as polygalacturanase inhibitor (PGIP), b) majority of the protein is outside the cell but a short tail within the cytoplasm, known as receptor-like proteins (RLP) and c) receptor part is outside but also has a large part in the cytoplasm that acts as a catalytic domain such as receptor-like kinase (RLK). These receptor proteins may function to recognize microbial PAMPs, may confer resistance to specific pathogens, and could interact with each other to activate downstream signaling. To understand resistance, we need to understand better the role of these receptors in recognition and the activation of the defense response not only because of its intrinsic interest and but also because of its important role in crop protection. The main aim of this proposal is to understand the role of cell surface receptors in activating the immune system of plants. Mutants will be used to investigate the role of these immune receptors in recognition of oomycete pathogens including downy mildew and white rusts of brasssicas and late blight pathogen of potato and tomato. We have initial data suggesting that mutation in some of these genes result in enhanced pathogen growth and disease development. For example, mutation in 3 different RLK-encoding genes allows the growth of late blight pathogen of tomato, P. infestans. Detailed studies of these genes would help us to construct a strategy to control this disease in tomato and potato.

We will use reverse genetic methods to understand how some of the Toll-related cell surface receptors function in the recognition of oomycete pathogens including H. parasitica, A. candida and P. infestans. T-DNA insertion lines of 55 RLP- and 90 RLK-encoding genes have been screened with various isolates of H. parastica and P. infestans. A couple of RLP mutants showed enhanced susceptibility to H. parasitica during adult stage indicating that these genes might play a role in age related resistance. Similarly, several RLP and RLK mutants showed enhanced susceptibility to a few different isolates of H. parasitica at the seedling stage. The most striking finding was the growth of P. infestans on some of these RLK mutants. The accepted dogma suggests that Arabidopsis-P. infestans interaction is incompatible, the plant is non-host and the pathogen is non-adapted or inappropriate. We approached with the view that a) non-host resistance is part of the innate immunity and b) cell surface receptors may recognize any incoming pathogens and the information may be processed very quickly leading to restriction of pathogen growth. We screened the RLP mutants with a P. infestans isolate and mainly HR response was observed. No mutant showing any disease development was identified. However, when the RLK mutants were screened with the same isolate of P. infestans, a variation in symptom development was observed. We adopted a scoring system from 0 to 5 where 0 is no symptom or HR, 1 is local chlorosis, 2 is extended chlorosis, 3 necrosis, 4 extended necrosis and hyphal growth and 5 is total collapse of tissues. We have identified 3 different RLK genes showing disease development indicating that they may play a role in conferring resistance to this pathogen. The third oomycete pathogen, Albugo candida, has not been explored in the screening of these mutants. Our aim in this project is 1) to investigate the identified putative mutants further by a) identifying additional T-D