Poly(γ-glutamic acid) enhanced tolerance to salt stress by promoting proline accumulation in Brassica napus L.

Poly(γ-glutamic acid) (γ-PGA) is a new plant growth regulator with extensive application prospects. The effects of γ-PGA on rape seedlings under salt stress and the mechanism of action were investigated via hydroponic experiments. A salinity model was simulated by exposing the roots of rape seedlings to 100 mM NaCl solution for 48, 96 and 144 h. We determined that the growth of rape seedlings treated with NaCl was significantly inhibited. However, after the application of γ-PGA under NaCl stress, the dry weights of the entire plant, the shoot and the root were increased by 37.4, 38.8 and 34.1 %, respectively, at 144 h compared with those of the NaCl group. Moreover, the K⁺/Na⁺ ratio, proline content and antioxidant enzyme activity were all evidently enhanced, and the malondialdehyde content was significantly reduced in samples treated with γ-PGA. Proline metabolism regulation genes, including pyrroline-5-carboxylate synthetase genes (BnP5CS1 and BnP5CS2) and proline dehydrogenase gene (BnPDH), were also upregulated and downregulated by γ-PGA, respectively. Results showed that γ-PGA improved resistance to salt stress in rape seedlings by activating the proline synthesis pathway and promoting proline accumulation.