Changes in gene expression during programmed cell death in tomato cell suspensions

To identify genes involved in plant programmed cell death (PCD), changes in gene expression during PCD in a model system of suspension-cultured tomato cells were studied. In this system, cell death is triggered by treatment w ith camptothecin, an inhibitor of topoisomerase I. Cell death was accompani ed by internucleosomal DNA degradation, indicating that the cell death proc ess shares similarities with apoptosis in animals. Tomato homologues of DAD 1 and HSR203, two genes that have been implicated in PCD, were isolated. Du ring camptothecin-induced PCD tomato DAD1 mRNA levels roughly halve, while tomato HSR203 mRNA levels increase 5-fold. A differential display approach was used to identify novel genes that show changes in expression levels dur ing camptothecin-induced PCD. This resulted in isolation of two up-regulate d (CTU1 and CTU2) and four down-regulated (CTD1, CTD2, CTD4, and CTD5) cDNA clones. CTU1 shows high homology to various gluthatione S-transferases, wh ereas CTU2 is as yet unidentified. CTD1 is highly similar to Aux/IAA early- auxin-responsive genes. CTD2 corresponds to the tomato RSI-1 gene, CTD4 is an unknown clone, and CTD5 shows limited homology with a proline-rich prote in from maize. Addition of the calcium channel blocker lanthanum chloride p revented camptothecin-induced cell death. The effect of lanthanum chloride on camptothecin-induced gene expression was studied to discriminate between putative cell death genes and general stress genes. The possible role of t he various predicted gene products in plant PCD is discussed.