1.4.7S
GENETIC DISSECTION OF SYSTEMIC ACQUIRED RESISTANCE: CIM MUTANTS

K MALECK1, B CADE1, P EPPLE2, Klaus HAHLBROCK3, J DANGL3 and J RYALS4

1Novartis Crop Protection Inc., 3054 Cornwallis Road, Research Triangle Park, NC 27709, USA; 2Department of Biology, Coker Hall 107, Chapel Hill, NC 27599, USA; 3MPI für Züchtungsforschung, Carl-von-Linne Weg 10, D- 550829 Köln, Germany; 4Paradigm Genetics Inc., 303 Brittany Place, Cary, NC 27511, USA

Background and objectives
In order to elucidate the molecular events triggering the expression of systemic acquired resistance (SAR), we aim towards a genetic dissection of the pathway. Several different mutant types have been described in the literature that are either compromised for SAR (nim/npr) or constitutively activated for SAR (lsd, cpr, cim) Here we report the identification and characterization of mutants that fall in the class of cim (i.e. lesion-free, constitutive immunity).

Results and conclusions
A transgenic line of Arabidopsis thaliana was created using an inducible promoter of the best marker gene for SAR, encoding the pathogenesis-related protein 1 (PR1), linked to the firefly luciferase reporter gene. Plants activated for SAR that contained this construct in their genome could easily be identified by looking for glowing plants upon induction of SAR. A selected transgenic line was characterized for the induction pattern of the transgene upon chemical and biological induction, and mutagenized by EMS. In a screening of 250,000 plants, over 800 plants that expressed the reporter gene in a constitutive manner were identified. These plants, likely also to express SAR, were further investigated. Most of them were genetically stable and indeed showed all the characteristics of plants that were activated for SAR, such as increased resistance to pathogens, high levels of salicylic acid and strong expression of all SAR marker genes. Mutants could be classified based on their leaf morphology and appearance: 90% of the mutants exhibited spontaneous necrosis formation which, consecutively, might induce SAR (mutants of the lsd type). Several mutants with increased SAR gene expression and resistance did not exhibit lesion formation. These cim mutants were characterized in detail. They can be distinguished based on differential resistance to virulent pathovars of Pseudomonas syringae, Peronospora parasitica and Erysiphe cichoracearum. Gene expression patterns of these mutants, biochemical characteristics and epistasis studies enable further understanding of SAR. Genetic mapping of some mutants is in progress.