Effect Of Overexpression Of Ss.sAPX On Salt Tolerance Of Arabidopsis Thaliana

Abiotic stress is one of the major enviromental stresses for crop yield and quality worldwide, which severely affects the plant growth and development. Biotic and abiotic stress conditions (such as temperature, soil water content, and soil salinity) produce excessive concentrations of reactive oxygen species (ROS), causing protein denaturation and lipid peroxidation. Because plants are frequently subjected to abiotic stress, they have developed several strategies to avoid and alleviate injury by ROS. Antioxidative defense systems include non-enzymatic and enzymatic components. Non-enzymatic components include ascorbate, reduced glutathione, phenolic compounds, and several other compounds. Enzymatic components include superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and so on.The chloroplast, where light harvesting and tranformation occur, is the organelle in which most ROS are generated under light condition. The electron transfer chain of the chloroplasts is the best-documented source of H2O2, and APXs are the key enzymes responsible for removing H2O2. Chloroplastic APXs include stromal APX (sAPX) and the thylakoid membranes (tAPX). Although researchers recently have examined preliminarily the role of the APX isoforms in protecting against oxidative stress induced by abiotic stress by transgenic plants, so far, all of which focused on the APX of nonhalophytes such as tobacco, rice and Arabidopsis, little study has been done on the role of chloroplastic APXs in the halophytes. Moreover, the results were controversial. Therefore, the research of the relationship between halophyte chloroplate APX and stress tolerance is important, which needs to be further studied. The role of APXs in protecting against oxidative stress tolerance may differ depending on plant species, plant developmental age and stress intensity.Suaeda salsa L. is a leaf succulent euhalophyte that may have evolved the unique salt-tolerant mechanism and may have an effective antioxidant system to protect against oxidative stress induced by salt treatment. In addition, the effect of NaCl on seed germination of Arabidopsis has been studied, but all focused on the reponse to salt tolerance at gene expression and physiological level. Little is known about the ionic effect and osmotic effect. To clarify the contribution of Ss.sAPX in the regulation of ROS levels and plant protection against oxidative stress induced by salt tolerance, we produced Arabidopsis lines overexpressing Ss.sAPX and investigated the performance of wild type (WT) and two of these transgenic lines of Arabidopsis under different NaCl conditions. The main results are showed as follows:The transgenic plants grew as well as WT plants under normal conditions (no NaCl added) in MS medium. Although the growth of both WT and transgenic lines was inhibited when NaCl (≥120 mmol/L) was added, germination, cotyledon growth, survival rate, root length and total chlorophyll content were all greater in the transgenic lines than in the WT. Overexpressing Ss.sAPX improves salt tolerance of Arabidopsis.The seed germination of WT plants and Arobidopsis lines overexpressing Ss.sAPX were examined under NaCl, LiCl and iso-osmotic mannitol treatments, and Na+, K+, proline content, antioxidant enzymes activities, H2O2 and MDA contents of their cotyledons under salt stress were also determined. Iso-mannitol and NaCl had the similar effect, which supports the hypothesis that ion toxicity was not the main cause of germination inhibition, and the osmotic component of salt stress was the main factor. Level of MDA and H2O2 was significantly lower in the transgenic Arabidopsis than in the wild type. Correspondingly, the transgenic lines had higher total APX activity than the WT. Interestingly, transgenic Arabidopsis plants and wild type did not differ in both Na+, K+, proline contents and SOD, CAT activities under salt stress. In contrast, the overexpression of Ss.sAPX in transgenic Arabidopsis markedly enhanced salt tolerance by maintaining a low level of H2O2, which consequently protected the transgenic plants from damage caused by oxidative stress, not by the alleviation of ionic effect and osmotic effect.