A novel myb oncogene homologue in Arabidopsis thaliana related to hypersensitive cell death

A novel myb oncogene homologue (AtMYB30) has been isolated by differential screening of a cDNA library prepared from Xanthomonas campestris pv. campes tris (X. campestris)-inoculated Arabidopsis thaliana cells cultured in the presence of cycloheximide. AtMYB30 is a single-copy gene, and the encoded p rotein contains a MYB domain highly homologous to other plant and animal MY B proteins. Analyses of transcript levels in A. thaliana plants, or in cult ured A. thaliana cells infected with either virulent or avirulent strains o f the pathogens X. campestris and Pseudomonas syringae pv. tomato, showed t hat maximal levels of transcription of this gene occurred during the hypers ensitive response. Furthermore, in A. thaliana mutants affected in the cont rol of cell death initiation (Isd3, Isd4 and Isd5), constitutive expression or expression in lesion-positive plants was observed, while in suppressors of the mutations Isd5 and Isd4, AtMYB30 transcripts did not accumulate. Ho wever, AtMYB30 expression could not be detected in the Isd1 mutant, which w as hyperresponsive to cell death initiators and unable to limit the extent of cell death, whatever the environmental conditions. The results presented here suggest a strong correlation between AtMYB30 and genetically controll ed cell death, with a role in the initiation of cell death rather than in t he limitation of its extent. Our results further indicate that the Isd muta nts constitute an appropriate genetic model for studying the role of this g ene in hypersensitive cell death, and their relation to different steps of the pathway(s) leading to cell death.