A tomato homologue of the human protein PIRIN is induced during programmed cell death

Programmed cell death (PCD), with similarities to animal apoptosis, was induced in tomato suspension cells by the topoisomerase I inhibitor camptothecin. Previously, a differential display screening was performed to isolate genes differentially expressed during camptothecin-induced cell death. As a result, the new tomato gene Le-pirin was isolated, whose mRNA levels dramatically increase during camptothecin-induced PCD. Le-pirin mRNA accumulation is also observed when cell death is triggered by the mycotoxin fumonisin-B1, but not when the suspension cells are treated with stress-related compounds such as ethylene, methyl jasmonate or salicylic acid. The caspase inhibitor Z-Asp-CH2-DCB and the calcium channel blocker LaCl3 effectively delayed whereas ethylene greatly stimulated camptothecin-induced PCD and the accumulation of Le-pirin mRNA. The Le-pirin encoded protein shows 56% identity with the human protein PIRIN, a nuclear factor reported to interact with the human oncogene Bcl-3. Human PIRIN stabilizes the formation of quaternary complexes between Bcl-3, the anti-apoptotic transcription factor NF-kappaB and its DNA target sequences in vitro. The isolation of Le-pirin and its implication in plant PCD provides new clues on the role of putative NF-kappaB-associated pathways in plant defence mechanisms.