ATL9, a RING Zinc Finger Protein with E3 Ubiquitin Ligase Activity Implicated in Chitin- and NADPH Oxidase-Mediated Defense Responses

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.

The prolific ATL family of RING-H2 ubiquitin ligases

“…[17][18][19][20] The structural basis of the E2-E3 recognition has been elucidated for EL5, a rice ATL. The three-dimensional structure of the RING-H2 finger domain in solution, as determined by NMR spectroscopy, basically demonstrated the same structural features of previously characterized RING domains.…”

Phosphorylation of Arabidopsis Ubiquitin Ligase ATL31 Is Critical for Plant Carbon/Nitrogen Nutrient Balance Response and Controls the Stability of 14-3-3 Proteins

“…Other ATL proteins were also reported to localize to membrane compartments, including the plasma membrane and endoplasmic reticulum (24,25). ATL55/RING1 was detected in detergent-resistant membranes, specialized microdomains in plasma membranes (24).…”

“…Recently, several ATL proteins have been reported to function in multiple environmental stress adaptation (22). ATL2, ATL9, and ATL55/RING1 are involved in plant defense response, and ATL78 mediates adaptation to cold stress (23)(24)(25)(26). Moreover, IDF1 (IRT1 (iron-regulated transporter 1) degradation factor 1)/ATL14 was recently identified as an ubiquitin ligase regulating iron uptake by the direct ubiquitination of the iron transporter IRT1 (27).…”

TheArabidopsisEDR1 Protein Kinase Negatively Regulates the ATL1 E3 Ubiquitin Ligase to Suppress Cell Death

“…Members of the ATL family all contain an N-terminal transmembrane domain followed by a RING-H2 type E3 ligase domain (Aguilar-Hernández et al, 2011;Guzmán, 2012). Functional analyses of several ATL family members in Arabidopsis indicate that they play important roles in diverse biological processes, including defense responses, growth phase transitions, endosperm development, regulation of cell death during root development, and photoperiod response (Serrano and Guzmán, 2004;Pagnussat et al, 2005;Lin et al, 2008;Sato et al, 2009;Berrocal-Lobo et al, 2010;Morris et al, 2010;Guzmán, 2012). However, there have been no prior publications describing a function for ATL1.…”

“…Other members of the ATL1 family have been shown to possess E3 ubiquitin ligase activity (Salinas-Mondragón et al, 1999;Serrano and Guzmán, 2004;Berrocal-Lobo et al, 2010). To determine whether ATL1 might function as an E3 ligase, ATL1 activity was analyzed using an in vitro ubiquitination assay (Figure 4).…”