Vacuolar processing enzyme is essential for mycotoxin-induced cell death in Arabidopsis thaliana

Some compatible pathogens secrete toxins to induce host cell death and promote their growth, The toxin-induced cell death is a pathogen strategy for infection. To clarify the executioner of the toxin-induced cell death, we examined a fungal toxin (fumonisin B1 (FB1))-induced cell death of Arabidopsis plants. FB1-induced cell death was accompanied with disruption of vacuolar membrane followed by lesion formation. The features of FB1-induced cell death were completely abolished in the Arabidopsis vacuolar processing enzyme (VPE)-null mutant, which lacks all four VPE genes of the genome. Interestingly, an inhibitor of caspase-1 abolished FB1-induced lesion formation, as did a VPE inhibitor. The VPE-null mutant had no detectable activities of caspase-1 or VPE in the FB1-treated leaves, although wild-type leaves had the caspase-1 and VPE activities, both of which were inhibited by a caspase-1 inhibitor. γVPE is the most essential among the four VPE homologues for FB1-induced cell death in Arabidopsis leaves. Recombinant γVPE recognized a VPE substrate with K_m = 30.3 μM and a caspase-1 substrate with K_m = 44.2 μM, which is comparable with the values for mammalian caspase-1. The γVPE precursor was self-catalytically converted into the mature form exhibiting caspase-1 activity. These in vivo and in vitro analyses demonstrate that γVPE is the proteinase that exhibits a caspase-1 activity. We show that VPE exhibiting a caspase-1 activity is a key molecule in toxin-induced cell death. Our findings suggest that a susceptible response of toxin-induced cell death is caused by the VPE-mediated vacuolar mechanism similar to a resistance response of hypersensitive cell death (Hatsugai, N., Kuroyanagi, M., Yamada, K., Meshi, T., Tsuda, S., Kondo, M., Nishimura, M., and Hara-Nishimura, I. (2004) Science 305, 855-858).