Ectopic expression of a human cysteine aspartate-specific protease gene <i>caspase-3</i> induces cell death and affects the infection of tomato mosaic virus in <i>Nicotiana</i> plants

In animals, genes of the cysteine aspartate-specific protease (caspase) family play a crucial role in inducing cell death. Genes homologous to animal caspase genes have not been found in plants, and to date, there are no examples of the ectopic expression of animal caspase genes inducing cell death in plants. In this study, we investigated whether cell death could be induced by expressing the human caspase-3 gene in plants. Wild-type caspase-3 (V266), inactive type (V266H), and constitutively active type (V266E) genes were expressed in leaves of Nicotiana benthamiana and/or Nicotiana tabacum using two types of Agrobacterium-infiltration-type virus vectors and one type of mechanical inoculation-type virus vector. In all cases, cell death symptoms were induced by V266 and V266E but not V266H. Similar results were obtained even when the caspase-3 gene was split into coding regions for subunit 1 and subunit 2 and co-expressed. As for virus infection, expression of V266 and V266E suppressed the infection of the tomato mosaic virus (ToMV). This suppression effect was particularly pronounced when the constitutively active type V266E was expressed using a virus vector of ToMV. With V266E, cell-to-cell movement of the virus was inhibited and long-distance movement did not occur. Furthermore, the expression of V266 and V266E tended to increase the mRNA levels of defense-related genes in N. benthamiana. These results suggest that the expression of the human caspase-3 gene induces cell death in plants, affects plant gene expression, and exerts an inhibitory effect against ToMV infection.