1.2.19
THE ROLE OF THE ARABIDOPSIS GENE ACCELERATED CELL DEATH 2 IN PROGRAMMED CELL DEATH AND THE PLANT DEFENCE RESPONSE

JM MACH and JT GREENBERG

Department of Molecular Genetics and Cell Biology, University of Chicago, Chicago, IL 60637, USA

Background and objectives
When a plant recognizes a pathogen, it initiates many defences, including phytoalexin synthesis, cell-wall crosslinking, defence gene transcription and programmed cell death in the region of the infection. In accelerated cell death (acd) [1] or lesions simulating disease (lsd) [2] mutants, plant defences are initiated in the absence of pathogen. In order to further understand the process of plant programmed cell death, we have begun coupled molecular and genetic characterizations of acd2 mutants. Our goals are to isolate the ACD2 locus by map-based cloning and to find genes downstream of ACD2 by isolating mutations that modify the acd2 mutant phenotype. Molecular isolation of the ACD2 locus may provide insights to its function, if it is similar to previously isolated proteins; also, cloning the ACD2 gene will provide valuable reagents for further studies.

Because the acd2 mutations are recessive, we hypothesize that ACD2 acts to repress cell death in wild-type plants. Thus, mutations that suppress the acd2 mutant phenotype may be required to activate or execute cell death; mutations that enhance the acd2 phenotype may be required to repress or inactivate cell death in the wild-type plant.

Results and conclusions
We have localized the ACD2 locus to a 70-kb interval on chromosome 4. Current results from the cloning will be presented. We have isolated an overlapping series of cosmids covering the region. We are introducing those into mutant plants by Agrobacterium-mediated transformation to look for complementation.

In the modifier screen, we have screened 2200 M2 plants from mutagenized acd2 mutant plants. We have isolated eight putative enhancers and four putative suppressors. We are currently determining whether these act as dominant or recessive modifiers, whether they have a phenotype in the absence of an acd2 mutation, and whether they correspond to any previously isolated mutations. Current results of the screen will be presented.

References
1. Greenberg JT et al., 1994. Cell 77, 551-563.
2. Dietrich RA et al., 1994. Cell 77, 565-577.