1  Epistatic analysis for the characterization of the Arabidopsis mutant cpr22

 

To investigate the signaling pathway to induce pathogen resistance responses at the molecular level, a genetic screen was undertaken resulting in the isolation of the Arabidopsis mutant cpr22. cpr22 expresses several defense genes such as the spathogenesis related (PR) genes constitutively and exhibits elevated levels of the signaling molecule salicylic acid (SA). Furthermore, cpr22shows hypersensitive response like spontaneous cell death without pathogen infection. This indicates that cpr22 can be used as a tool to study the pathogen resistance pathway in Arabidopis. Interestingly, cpr22 expresses the plant defensin gene PDF1.2, which is induced by Jasmonic acid (JA)/ ethylene, suggesting that the JA/ethylene signaling pathway is also activated in cpr22 (Yoshioka et al. 2001). For further characterization of the cpr22 mediated-signaling pathway, we are currently conducting epistatic analyses with a number of Arabidopsis mutants which have deficiencies in various components of the pathogen resistance signaling pathways.


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2  Identification of novel components in the pathogen resistance signaling             pathwayusing suppressor mutants of cpr22 and bioinformatics approaches

 

To identify other novel components of the defense signaling pathway, EMS-treated cpr22 plants were screened for suppressors of the cpr22 mutation. Through this screening, we have isolated more than 70 suppressor mutants. Currently we are characterizing those suppressor mutants. Through this project, it is anticipated that several novel components in pathogen signaling pathways will emerge. Additionally we are analyzing intragenic mutants for structure-functional studies of CPR22 (ATCNGC11/12) as well as cyclic nucleotide gated ion channel in general. Our study on one intragenic mutant, #73 revealed an important residue for this channel function (Baxter et al., 2008 in press).


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3  Characterization of the molecular mechanism of CPR22 to induce pathogen     resistance responses.

 

To investigate the resistance signaling pathways activated by pathogen infection, we identified the Arabidopsis thaliana mutant constitutive expresser of PR genes22 (cpr22), which displays constitutive activation of multiple defense responses. Here, we identify the cpr22 mutation as a 3-kb deletion that fuses two cyclic nucleotide-gated ion channel (ATCNGC)–encoding genes, ATCNGC11 and ATCNGC12, to generate a novel chimeric gene, ATCNGC11/12 (Yoshioka et al., 2006) Genetic, molecular, and complementation analyses suggest that ATCNGC11/12, as well as ATCNGC11 and ATCNGC12, form functional cAMP-activated ATCNGCs and that the phenotype conferred by cpr22 is attributable to the expression of ATCNGC11/12. However, because overexpression of ATCNGC12, but not ATCNGC11, suppressed the phenotype conferred by cpr22, the development of this phenotype appears to be regulated by the ratio between ATCNGC11/12 and ATCNGC12.
      The spontaneous cell death induced by expression of CPR22 (ATCNGC11/12) was further characterized by molecular and cellular analyses and it was found that this cell death is apoptosis-like programmed cell death (Urquhart et al., 2007). We are currently using a transient expression system to understand the signal transduction pathway to induce programmed cell death as well as plant immunity.


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4  Characterization of environmental effects on pathogen resistance                       responses

 

Through characterization of cpr22, we found that the environmental condition, especially humidly and temperature affect the SA dependent cpr22 phenotypes. High RH and/or high temperature condition were found to suppress cpr22-mediated pathogen resistance responses presumable due to a repression of SA accumulation. After our report, the same kind of phenomena was reported from several other laboratories, suggesting that the putative environmental factor might be a common element in the SA signaling pathway. We are currently studying the relationship between environmental conditions and pathogen resistance signaling, especially from the plant hormonal point of view. We are going to analyze the kinetics of several plant hormones simultaneously under various stress conditions.


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5  Study of pathogen resistance in monocotyledonous plants using the model      plant, rice

 

Read more about Dr. Moeder's work here.

 


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