Studies characterizing the roles of several ubiquitin E3 
ligases in defense have begun to provide clues about the regulation of pathogen-induced signaling. For instance the rice resistance protein Xa21 has been shown to interact with an E3 ubiquitin ligase XB3. Interaction Gomisin-D between XB3 and Xa21 is required for the accumulation of the XA21 protein and is necessary for Xa21-mediated resistance to Xanthomonas oryzae pv. oryza in rice. A RING-finger type protein from pepper CaRFP1 was shown to physically interact with PR-1 protein in leaves of plants after infection with both bacterial and fungal pathogens. Over-expression of CaRFP1 in transgenic Arabidopsis conferred disease susceptibility to Pseudomonas syringae pv. tomato and reduced PR-2 and PR-5 expression suggesting that CaRFP1 is an E3 ligase that targets PR proteins. E3 ligases also appear to play a prominent role in elicitormediated defense responses. In particular, members of the ATL gene family have been shown to be activated by elicitors and to play important roles in defense pathways. The Arabidopsis ATL gene family contains 80 members and is a conserved group of RING zinc-finger proteins that encode putative E3 ubiquitin ligases. ATL2 and ATL6 in Arabidopsis and EL5 in rice, all encoding RING-finger type E3 ligases, have been shown to be rapidly induced in response to the elicitor chitin. Recent work by Hondo et al. demonstrated that the tomato ortholog of Arabidopsis ATL2, LeATL6, responded to cell wall protein fraction elicitor from the biocontrol agent Pythium oligandrum and appeared to regulate the jasmonic aciddependent defense gene expression. In a screen for chitinresponsive genes in Arabidopsis, we identified an ATL family member, ATL9, that responded strongly to chitin treatment. Loss-of-function mutations in this gene resulted in increased susceptibility to the powdery mildew pathogen, Golovinomyces cichoracearum. Our results here confirm that ATL9 is an E3 ubiquitin ligase and show that it is localized to the endoplasmic reticulum. ATL9 expression is induced by infection with G. cichoracearum and ATL9 function is required for basal defense against this biotrophic pathogen. Interestingly, ATL9 expression appears to be dependent on NADPH oxidases and mutations in ATL9 lead to an impairment in the ability of plants to produce reactive oxygen species after infection. Expression profiling of atl9 revealed a complex interplay between chitin-mediated signaling and other defense pathways. Recent work has highlighted the ubiquitin-proteasome system 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. In the current study we have shown that ATL9 is a 4-(Benzyloxy)phenol RING-type E3 ubiquitin ligase strongly induced by chitin. The ATL gene family encodes a group of proteins that share three specific characteristics: 1) rapid induction after elicitor treatment, 2) a highly conserved RING-H2 zinc-finger domain with at least six cysteines and three histidines conserved and 3) at least one amino-terminal transmembrane domain. Most members of the ATL gene family are predicted to function as single subunit E3 ubiquitin ligases with seventeen members of the ATL family known to be expressed in Arabidopsis. Although determination of a common function for them is still in progress, mutations in members of the ATL family have been shown to have an altered defense response to pathogens. The ATL2 gene was shown to be specifically induced by chitin but not by other elicitors of classic defense pathways. Constitutive over-expression mutants of ATL2 induced high levels of pathogen-related genes such as NPR1-1 and the phenylpropanoid biosynthetic enzymes phenylalanine ammonia lyase and chalcone synthase. The EL5 gene in rice is also a member of the ATL family and is rapidly and transiently induced by chitin. EL5, like ATL9, is an E3 ubiquitin ligase and is hypothesized to play a role in defense responses through protein turnover via the UPS.