Mechanism Research Of Apoplastic H₂O₂Mediated Elevated CO₂-induced Salt Tolerance In Tomato

The concentration of atmospheric CO2is continuously being increased due to extensive use of fossil fuel since the industrial revolution. In addition to elevation in atmospheric CO2concentration, the global climate change also attributes to a series of problems such as high temperature, sea level upsurge and soil salinization. From view point of plant biology, CO2is a substrate for photosynthesis and thus its concentration has an extensive impact on plant growth, development and stress responses. Soil salinization not only decreases crop yield, but also reduces crop quality. In recent years, this problem has become more devastating particularly in protected cultivation and thus threatening the sustainable development of facility agriculture. Although there are a lot of studies on the effects of CO2enrichment on growth and development of plants, the effect of elevated CO2on salt tolerance and the underlying mechanisms are largely unknown. In this study, tomato (Solanum lycopersicum L.) was used as plant material to elucidate the effect of elevated CO2on salt tolerance by using chlorophyll fluorescence, fluorescence microscope, transmission electron microscope (TEM), atomic absorption spectroscope (AAS) and other techniques. This study unveiled the important underlying mechanisms of elevated CO2-induced salt stress tolerance in tomato.. The salient features of current study are as follows:1. Firstly, we noticed that elevated CO2could confer salt tolerance in tomato, which was characterized by increased CO2assimilation rate and biomass accumulation as well as decreased relative electrolyte leakage in plants grown under elevated CO2(760μL/L) as compared with ambient (380μL/L) counterpart. The detection of Na and K content in roots, stem and leaves by AAS revealed that CO2-enrichment decreased the content of Na and Na/K with insignificant effect on K content. It is note worthy that CO2-enrichment-induced salt stress tolerance is also associated with upregulation of RBOH1expression and subsequent ROS accumulation.2. In order to understand the involvement of apoplastic H2O2in CO2-enrichment-induced salt tolerance, we constructed RBOH1-silenced plants using VIGS technique. It was observed that ROS accumulation in roots, stem, petiole and apoplast due to CO2enrichment and salt treatment was significantly decreased in RBOH1-silenced plants. CO2enrichment increased biomass accumulation, CO2assimilation rate and water potential, but decreased relative electrolyte leakage and cell death in pTRV plants under salt treatment. In addition, CO? enrichment decreased Na content and Na/K in pTRV, but not in the RBOH1-silenced plants following salt treatment. These results suggest that apoplastic H2O2plays a critical role in CO2-induced salt stress alleviation in tomato.3. Further investigation revealed that the stomatal aperture, stomatal conductance and transpiration rate of pTRV-RBOH1plants were higher than pTRV plants. The Na content in xylem sap was increased by silencing RBOH1, however, CO2enrichment decreased Na content in xylem sap of pTRV plants. In addition, we noticed that CO2enrichment induced salt stress-responsive genes expression, more obviously under salt stress in pTRV plants. Such upregulation in gene expression was abolished by silencing RBOH1. These results suggest that apoplastic H2O2mediates elevated CO2-induced salt stress tolerance by regulating stomatal movement, the Na delivery from roots to the shoots and salt-responsive genes expression.