Maize Methionine Sulfoxide Reductase Genes ZmMSRA2 and ZmMSRA5.1 Involved in the Tolerance to Osmotic or Salinity Stress in Arabidopsis and Maize

Methionine (Met), including the free and protein-bound forms, can easily be oxidized in the presence of excess reactive oxygen species (ROS) under conditions of abiotic stress to form Met sulfoxide (MetO). This reaction can be reversed by MetO reductases (MSRs), which are known to be involved in a variety of stress response mechanisms in plants, but the functions of their maize MSRA homologs have not been reported to date. Cytoplasmic ZmMSRA2 and secretary ZmMSRA5.1 , previously shown to be induced under salinity or drought stress, belong to the MSRA subfamily. In the present study, their constitutive expression in Arabidopsis resulted in notable increases in MSR enzymatic activity; virus-induced gene silencing (VIGS) analysis in maize indicated that the silencing expression of ZmMSRA2 or ZmMSRA5.1 decreased the tolerance of seedlings to osmotic or salinity, respectively; the ectopic expression of ZmMSRA2 in Arabidopsis increased the tolerance of seedlings to mannitol, H 2 O 2 , and ABA, and that of ZmMSRA5.1 enhanced the tolerance to NaCl and H 2 O 2 . Compared with that of the wild-type, the germination rates of seeds overexpressing ZmMSRA2 or ZmMSRA5.1 were higher under osmotic or salinity stress, respectively. The effect of active ZmMSRA2 in Arabidopsis was the suppression of ROS accumulation and the increase in intracellular proline content; ZmMSRA5.1 led to improved ion transport and decreased ROS content. The expression of the maize MSRA homologs in Arabidopsis resulted in no observed changes in the transcription of Arabidopsis MSRA subfamily members, while genes of the MSRB subfamily were downregulated overall. Together, our findings suggest that ZmMSRA2 participates in osmotic stress tolerance by decreasing the ROS content, and enhancing the proline and ABA pathways, whereas ZmMSRA5.1 is involved in tolerance to salinity by reducing ROS accumulation and modifying ion transportation. Characterization of these two genes contributes to the understanding of redox metabolism in maize.