Analysis Of The Relationship Between LeZE And Photoprotection Under Salt Stress

Salt stress is one of the most important environmental factor reducing plant growth and productivity. The decrease in growth observed in many plants subjected to salinity stress is often associated with a decrease in their photosynthetic capacity. The decrease in photosynthesis induced by salt stress enhances the amount of excess excitation energy. Photosynthetic apparatus might be damaged even under moderate light if the excess light energy cannot be appropriately dissipated and consequently secondary stress, such as reactive oxygen species (ROS) damage often occur.In order to avoid severe photoinhibition, there are several photoprotective mechanisms in higher plants. The irradiance-dependent xanthophyll cycle plays an important role in the protection of plants under environmental stress. It involves interconversions between the three pigments, violaxanthin (V), antheraxanthin (A) and zeaxanthin (Z). The cycle is catalyzed by two enzymes, violaxanthin de-epoxidase (VDE:EC1.10.99.3) and zeaxanthin epoxidase (ZE: EC1.14.13.90). When the energy is excessive, V is converted to Z via A in the presence of ascorbate and an acidic lumen generated by the proton pump. This reaction is thought to occur in the lumen of thylakoids, catalyzed by VDE, and ZE catalyzes the reverse reaction thought to be located in the stromal side. Non-photochemical quenching (NPQ) which depends on the xanthophyll cycle protects the photosynthesis apparatus from inactivation and damage caused by excess excitation energy. Zeaxanthin is the exclusive xanthophyll that accumulates under excess light, by de-epoxidation of existing violaxanthin in the xanthophyll cycle. It is widely thought to play a photoprotective role by dissipation of excessive light energy as heat.In order to investigate the physiological effects and functional mechanism of zeaxanthin in the tomato at salt stress-induced photoinhibition and photooxidation, wild type (WT), sense transgenic lines (S1, S2, S3) and antisense transgenic lines (A1, A2, A3) of tomato were used to determine the ratio of Z/(V+A+Z) and (Z+0.5A)/(V+A+Z) and growth performance, net photosynthetic rate, chlorophyll fluorescence parameters, the production of chloroplastic reactive oxygen species (ROS), lipid peroxidation, relative electrolyte leakage and the content of D1protein. The results showed as follows: (1) Constitutive accumulation of zeaxanthin in tomato alleviates salt stress-induced photoinhibition and photooxidation. The ratio of Z/(V+A+Z) and (Z+0.5A)/(V+A+Z) in antisense transgenic plants were maintained at a higher level than those in WT plants before and after salt stress, but the value of NPQ in WT and transgenic plants was not significantly different under salt stress. However, the maximal photochemical efficiency of PSII (Fv/Fm) and the net photosynthetic rate (Pn) in transgenic plants decreased more slowly under salt stress. Furthermore, transgenic plants showed lower level of hydrogen peroxide (H2O2), superoxide anion radical (O2-) and ion leakage, lower malondialdehyde (MDA) content. Compared with WT, the content of Dl protein decreased slightly in transgenic plants under salt stress. These results suggested that the constitutive accumulation of zeaxanthin in transgenic tomatoes can alleviate salt stress-induced photoinhibition due to the antioxidant role of zeaxanthin in the scavenging of singlet oxygen and/or free radicals in the lipid phase of the membrane.(2) Overexpression of zeaxanthin epoxidase gene enhances salt stress-induced photoinhibition and photooxidation in tomato. The ratio of Z/(V+A+Z) and (Z+0.5A)/(V+A+Z) in sense transgenic plants were lower than those in WT plants both before and after salt stress. Although the NPQ in both of WT and sense transgenic plants increased markedly under salt stress, the increase of NPQ was more obvious in WT than in sense transgenic plants. The maximal photochemical efficiency of PSII (Fv/Fm) and the net photosynthetic rate (Pn) decreased in both WT and sense transgenic plants salt stress, but the decrease of Pn and Fv/Fm in sense transgenic plants was more significant than that in the WT. We also observed the growth of sense transgenic plants was restrained more serious than WT under salt stress. Moreover, the sense transgenic plants showed higher level of hydrogen peroxide (H2O2), superoxide anion radical (O2-) and ion leakage, more malondialdehyde (MDA) content. Compared with WT, the content of Dl protein decreased more obviously in sense transgenic plants under salt stress. These results suggested that overexpression of LeZE impaired the function of the xanthophyll cycle and aggravated salt stress-induced PSII photoinhibition and photooxidation in tomato.In conclusion, the depletion of LeZE could not affect NPQ under salt stress but the constitutive accumulation of zeaxanthin in transgenic tomatoes can alleviate salt stress-induced photoinhibition due to the antioxidant role of zeaxanthin in the scavenging of singlet oxygen and/or free radicals in the lipid phase of the membrane. Overexpression of LeZE decreased the level of de-epoxidation, and then the thermal dissipation capacity and reactive oxygn species scavenging capacity under salt stress-induced photoinhibition and photooxidation were decreased. As a result, the sensitivity of PSII to salt stress-induced photoinhibition and photooxidation was enhanced.