Abscisic acid, H2O2 and nitric oxide interactions mediated cold‐induced S‐adenosylmethionine synthetase in Medicago sativa subsp. falcata that confers cold tolerance through up‐regulating polyamine oxidation

Summary S ‐adenosylmethionine synthetase ( SAMS ) is the key enzyme catalysing the formation of S ‐adenosylmethionine ( SAM ), a precursor of polyamines and ethylene. To investigate the potential role of SAMS in cold tolerance, we isolated Mf SAMS 1 from the cold‐tolerant germplasm Medicago sativa subsp. falcata and analysed the association of SAM ‐derived polyamines with cold tolerance. The expression of Mf SAMS 1 in leaves was greatly induced by cold, abscisic acid ( ABA ), H 2 O 2 and nitric oxide ( NO ). Our data revealed that ABA , H 2 O 2 and NO interactions mediated the cold‐induced Mf SAMS 1 expression and cold acclimation in falcata . SAM , putrescine, spermidine and spermine levels, ethylene production and polyamine oxidation were sequentially altered in response to cold, indicating that SAMS ‐derived SAM is preferentially used in polyamine synthesis and homeostasis during cold acclimation. Antioxidant enzyme activities were also induced in response to cold and showed correlation with polyamine oxidation. Overexpression of Mf SAMS 1 in tobacco resulted in elevated SAM levels, but polyamine levels and ethylene production in the transgenic plants were not significantly changed. Compared to the wild type, transgenic plants had increased levels of apoplastic H 2 O 2 , higher transcript levels of genes involved in polyamine synthesis and oxidation, and higher activities of polyamine oxidation and antioxidant enzymes. The results showed that overexpression of Mf SAMS 1 promoted polyamine synthesis and oxidation, which in turn improved H 2 O 2 ‐induced antioxidant protection, as a result enhanced tolerance to freezing and chilling stress in transgenic plants. This is the first report demonstrating that SAMS plays an important role in plant tolerance to cold via up‐regulating polyamine oxidation.