Alpha Lipoic Acid Application Promotes Water-Deficit Tolerance by Modulating Osmoprotectant Metabolism-Related Genes in Maize

Alpha lipoic acid (ALA) is a potent antioxidant molecule that has positive effects on plant growth and the adaptation of plants to environmental stresses. However, the physiological and molecular mechanisms of the effect of exogenous ALA on the tolerance of plants to environmental stresses associated with osmolyte metabolism are not yet known. Therefore, this study was designed to assess the potential role of exogenous ALA in the water-deficit tolerance mechanism related to the osmolyte metabolism of maize (Zea mays L.) seedlings in hydroponic conditions. Three-week-old maize seedlings were treated or not treated with ALA (12 μM) and exposed to water-deficit stress generated by addition of 10% polyethylene glycol (PEG6000). The results showed that exogenous ALA enhanced the proline content and the transcript level of the pyrroline-5-carboxylate synthetase (P5CS) gene while decreasing the transcript level of the proline dehydrogenase (ProDH) gene under PEG-induced water-deficit stress. With ALA treatment, the contents of polyamines and transcript level of the S-adenosylmethionine decarboxylase (SAMDC) gene increased while the transcript level of the diamine oxidase (DAO) gene decreased. In addition, the content of soluble sugar and transcript level of the sucrose synthase (SUS) gene increased in ALA-treated maize seedlings under the PEG-free and PEG-induced water-stress conditions but the transcript level of sucrose phosphate synthase (SUPS) did not change. These findings provide evidence that ALA application to plants under water deficit can be a useful strategy for enhancing their stress tolerance by regulating the metabolism of some osmolytes or signal intermediate compounds such as proline, soluble sugar, and polyamine and thus by reducing water loss.