Overexpression of cysteine protease gene from Salix matsudana enhances salt tolerance in transgenic Arabidopsis

Salix matsudana is a salt-tolerant arbor tree species. We are interested in understanding its mechanisms of salt-tolerance. Here, we isolated a full-length 1080-bp cDNA for a salt stress-responsive cysteine protease gene from S. matsudana, designated SmCP. The deduced 359-amino-acid SmCP protein was essentially identical to cysteine proteases of other plant species and was predicted to contain inhibitor-I29 and peptidase-C1A domains with three active catalytic sites found in eukaryotic cysteine proteases. Expression profile analysis revealed that SmCP expression is constitutive and overexpression in roots takes place under salt stress. Expression of SmCP in Escherichia coli cells led to enhanced salinity tolerance. In addition, transgenic Arabidopsis thaliana lines overexpressing SmCP displayed enhanced tolerance to salt stress, with increased germination rates, antioxidant enzyme (superoxide dismutase) activity, chlorophyll content and ion flux in the root, and reduced malondialdehyde (MDA) levels and electric conductivity compared with control plants. These results indicate that SmCP likely plays an important role in salt tolerance in S. matsudana.