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. campestris (X. campestris)-inoculated Arabidopsis thaliana cells cultured in the presence of cycloheximide. AtMYB30 is a single-copy gene, and the encoded protein contains a MYB domain highly homologous to other plant and animal MYB proteins. Analyses of transcript levels in A. thaliana plants, or in cultured A. thaliana cells infected with either virulent or avirulent strains of the pathogens X. campestris and Pseudomonas syringae pv. tomato, showed that maximal levels of transcription of this gene occurred during the hypersensitive response. Furthermore, in A. thaliana mutants affected in the control 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. However, AtMYB30 expression could not be detected in the Isd1 mutant, which was 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 controlled cell death, with a role in the initiation of cell death rather than in the limitation of its extent. Our results further indicate that the Isd mutants constitute an appropriate genetic model for studying the role of this gene in hypersensitive cell death, and their relation to different steps of the pathway(s) leading to cell death.