Cloning of a cystatin gene from sugar beet M14 that can enhance plant salt tolerance.

An open reading frame encoding a cysteine protease inhibitor, cystatin was isolated from the buds of sugar beet monosomic addition line M14 (BvM14) using 5'-/3'-RACE method. It encoded a polypeptide of 104 amino acids with conserved G and PW motifs, the consensus phytocystatin sequence LARFAV and the active site QVVAG. The protein showed significant homology to other plant cystatins. BvM14-cystatin was expressed ubiquitously in roots, stems, leaves and flower tissues with relatively high abundance in developing stems and roots. It was found to be localized in the nucleus, cytoplasm and plasma membrane. Recombinant BvM14-cystatin expressed in Escherichia coli was purified and it exhibited cysteine protease inhibitor activity. Salt-stress treatment induced BvM14-cystatin transcript levels in the M14 seedlings. Homozygous Arabidopsis plants over-expressing BvM14-cystatin showed enhanced salt tolerance. Taken together, these data improved understanding of the functions of BvM14-cystatin and highlighted the possibility of employing the cystatin in engineering plants for enhanced salt tolerance.