Abstract
Pathogen associated molecular patterns (PAMPs) are signals detected by plants that activate basal defenses. One of these PAMPs is chitin, a carbohydrate present in the cell walls of fungi and in insect exoskeletons. Previous work has shown that chitin treatment of Arabidopsis thaliana induced defense-related genes in the absence of a pathogen and that the response was independent of the salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) signaling pathways. One of these genes is ATL9 ( = ATL2G), which encodes a RING zinc-finger like protein. In the current work we demonstrate that ATL9 has E3 ubiquitin ligase activity and is localized to the endoplasmic reticulum. The expression pattern of ATL9 is positively correlated with basal defense responses against Golovinomyces cichoracearum, a biotrophic fungal pathogen. The basal levels of expression and the induction of ATL9 by chitin, in wild type plants, depends on the activity of NADPH oxidases suggesting that chitin-mediated defense response is NADPH oxidase dependent. Although ATL9 expression is not induced by treatment with known defense hormones (SA, JA or ET), full expression in response to chitin is compromised slightly in mutants where ET- or SA-dependent signaling is suppressed. Microarray analysis of the atl9 mutant revealed candidate genes that appear to act downstream of ATL9 in chitin-mediated defenses. These results hint at the complexity of chitin-mediated signaling and the potential interplay between elicitor-mediated signaling, signaling via known defense pathways and the oxidative burst.
Highlights
Plants defend against pathogens using an innate system of defense that has both constitutive and inducible components
Several receptors associated with these Pathogen associated molecular patterns (PAMPs) have been characterized including: the FLS2 receptor that recognizes flagellin [9]; the EFR receptor, which perceives the first 18 amino acids of bacterial elongation factor Tu (EF-Tu) [7], chitin oligosaccharide elicitor binding protein (CeBiP) [10], a transmembrane protein with two extracellular Lysine motifs (LysM) that is involved in chitin recognition and the LysM-RLK CERK1 that is required for chitin-initiated responses and downstream signalling [10,11]
Recent work has highlighted the ubiquitin-proteasome system (UPS) and its associated E3 ubiquitin ligases as regulators of the plant defense response and it is clear that these proteins play an important part in disease resistance [12,44]
Summary
Plants defend against pathogens using an innate system of defense that has both constitutive and inducible components. Inducible plant defenses depend on pathogen recognition and fall into two major classes; specific gene-for-gene interactions and more general Pathogen or Microbe-Associated Molecular Pattern (PAMP or MAMP)-associated responses. Common PAMPs such as the oligosaccharide chitin (b-1, 4 linked N-acetylglucosamine) and the bacterial proteins flagellin and elongation factor Tu (EF-Tu) are known to activate strong defense responses [4,5,6,7,8]. Several receptors associated with these PAMPs have been characterized including: the FLS2 receptor that recognizes flagellin [9]; the EFR receptor, which perceives the first 18 amino acids of bacterial elongation factor Tu (EF-Tu) [7], chitin oligosaccharide elicitor binding protein (CeBiP) [10], a transmembrane protein with two extracellular Lysine motifs (LysM) that is involved in chitin recognition and the LysM-RLK CERK1 that is required for chitin-initiated responses and downstream signalling [10,11]
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