Response of vacuolar processing enzyme in Malus hupehensis and MhVPEγ -overexpressing Arabidopsis to high temperature stress

Malus hupehensis Rehd. is often used as rootstock of apple trees. Vacuolar processing enzyme (VPE) is one type of cysteine protease with substrate specificity to residues of asparagine and aspartic acid. The VPE activity, MhVPEγ gene (GenBank number: FJ891065) expression and the amount of cell death in seedlings of M. hupehensis Rehd. all gradually increased with the increase of temperature from 26 to 42 °C. To explore the function of VPE response to temperature stress, the overexpressing vector of MhVPEγ was constructed and transformed into wild-type Arabidopsis thaliana Columbia. The responses of VPE, MhVPEγ and apoptosis to high temperature stress were determined in the Arabidopsis seedlings of wild-type, MhVPEγ -overexpressing lines and the VPE-null mutant. The leaves of MhVPEγ -overexpressing lines were smaller and VPE activity and MhVPEγ expression level were both obviously higher than the levels found in the wild-type and VPE-null mutant seedlings. Under high temperature treatment, VPE activity and MhVPEγ expression level significantly increased accompanied by leaf withering, shorter roots, lipid peroxidation and increased cell death and apoptosis rate in all types of plants. The changes of these indicators were highest in MhVPEγ -overexpressing transgenic plants, followed by the wild-type and were lowest in VPE-null mutant plants. These results suggest that MhVPEγ overexpression in Arabidopsis accelerated cell membrane lipid peroxidation, promoted cell death and reduced the tolerance to high temperature stress compared with wild-type plants.