Abbreviations used in this paper:CsA, cyclosporin A; FKBP, FKS06-binding protein; Gal, 2% galactose supplemented with 0.2% sucrose; Glc, 2 % glucose; GST, Schistosomajaponicura glutathione S-transferase; HSE, heat shock response element; Hsp, heat shock protein; PPIases, peptidylprolyl-cis, trans-isomerases. in protein folding and protein assembly events in vivo (Parsell and Lindquist, 1993) because in vitro these enzymes catalyze interconversion of the cis and trans isomers of the bond between the carboxyl group of the preceding amino acid and the imino nitrogen of proline in peptide and protein substrates (Kofron et al., 1991;Stein, 1991). It was discovered several years ago that there are two separate types of PPIases. This realization was made possible by investigations into the molecular mode of action of antibiotics that act as immunosuppressants in humans. These drugs effectively block the specific signal transduction pathways required for activation and proliferation of T lymphocytes (for review see Schreiber et al., 1993).The first PPIase isolated (Fischer et al., 1984) was shown to be the intracellular receptor for an immunosuppressant, cyclosporin A (CsA); and hence, a member of this class of PPIase is called a cyclophilin (Fischer et al., 1989; Taka-

Voltage-dependent anion channels (VDACs) are highly conserved proteins that are involved in the translocation of tRNA and play a key role in modulating plant senescence and multiple pathways. However, the functions of VDACs in plants are still poorly understood. Here, a novel VDAC gene was isolated and identified from alfalfa (Medicago sativa L.). MsVDAC localized to the mitochondria, and its expression was highest in alfalfa roots and was induced in response to cold, drought and salt treatment. Overexpression of MsVDAC in tobacco significantly increased MDA, GSH, soluble sugars, soluble protein and proline contents under cold and drought stress. However, the activities of SOD and POD decreased in transgenic tobacco under cold stress, while the O2- content increased. Stress-responsive genes including LTP1, ERD10B and Hxk3 were upregulated in the transgenic plants under cold and drought stress. However, GAPC, CBL1, BI-1, Cu/ZnSOD and MnSOD were upregulated only in the transgenic tobacco plants under cold stress, and GAPC, CBL1, and BI-1 were downregulated under drought stress. These results suggest that MsVDAC provides cold tolerance by regulating ROS scavenging, osmotic homeostasis and stress-responsive gene expression in plants, but the improved drought tolerance via MsVDAC may be mainly due to osmotic homeostasis and stress-responsive genes.

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