Overexpression of cyanoalanine synthase 1 improves germinability of tobacco seeds under salt stress conditions

Seed germination is the most critical phase in the life cycle of plants and salt tolerance during seed imbibition is essential for germination and subsequent plant growth in saline soil. In this study, we found for the first time that overexpression of tobacco cyanoalanine synthase 1 (CAS1), i.e. an enzyme for cyanide detoxification, improved the germination rate of tobacco seeds under salt stress conditions. Measurement of cyanide content showed that there was a quick cyanide production during seed imbibition and germination, but less cyanide accumulated in CAS1 overexpressing seeds than in WT seeds, especially under salt stress conditions. CAS1 activity increased during seed imbibition and germination, reaching levels two- or three-times higher under stress conditions of 50 mM and 100 mM NaCl than under normal conditions. Moreover, salt stress inhibited the respiration of seeds, whereas CAS1 overexpressing seeds maintained higher levels of total respiration and alternative oxidase (AOX) pathway respiration than WT seeds. Notably, inhibiting the AOX pathway with 2 mM salicylhydroxamic acid (SHAM) significantly repressed the germination of WT seeds, and also impaired the CAS1 overexpressing-mediated seed germinability under both normal and salt stress conditions. These findings indicate that overexpression of CAS1 helps to improve tobacco seed germination under salt stress conditions, but requires the assistance of AOX.