Overexpression of KcTrxf in tobacco enhances salt tolerance through the regulation of ROS homeostasis under NaCl stress

Thioredoxins (Trxs) play a crucial role in mediating the redox state in living cells; however, little is known how Trxs mediate salt tolerance in woody species. The aim of this study was to clone putative Trxf from non-secretor mangrove Kandelia candel and analyze its role in NaCl tolerance. Salt treatment enhanced NPTs (non-protein thiols) contents and increased transcription of Trxf in K. candel. The KcTrxf gene contained an open reading frame of 585 bp encoding a putative thioredoxin protein with 194 amino acids. Sequence and localization analyses revealed that KcTrxf is a typical thioredoxin in the chloroplast. To clarify the role of KcTrxf in salinity tolerance, it was transferred to tobacco, a model experimental system for functional genetics. KcTrxf-transgenic lines were more salt tolerant than wild-type (WT) tobacco. In the WT tobacco, salinity (100 mmol/L NaCl) resulted in a marked reduction of chlorophyll content, and chlorophyll a/b ratio. The salt damage in WT resulted presumably from lipid peroxidation and ROS accumulation. However, the salt treatment caused less damage in KcTrxf-transgenic plants because of the up-regulated activity of catalase and ascorbate peroxidase. Moreover, KcTrxf-transgenic plants were able to increase the activities of MDAR (monodehydroascorbate reductase) and GR (glutathione reductase) in chloroplast AsA-GSH cycle, leading to an increase in the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) and NPTs in the leaves. Therefore, KcTrxf-transgenic plants could better scavenge the salt-elicited reactive oxygen species (ROS) in leaf cells, compared to WT plants. In this way, reduction of chlorophyll content, and chlorophyll a/b ratio of KcTrxf-transgenic plants was consequently less restricted. As a result, KcTrxf-transgenic plants maintain photosynthesis and growth under salinity in the longer term.