Effects And Mechanism Of Elevated CO₂-induced Heat Stress Tolerance In Tomato Plants

In modern agriculture, enrichment of atmospheric CO2is gradually becoming an important tool for improving plants growth and yield. Importantly, heat stress often occurs during CO2enrichment in the green house facilities, which affects the growth, yield and quality of horticultural crops to a great extent. However, the underlying physiological and molecular mechanisms involved in plants response to heat stress under elevated CO2environment are largely unknown. In the present study, Condine Red (CR), Ailsa Craig (AC) and its abscisic acid (ABA)-deficient mutant notabilis (not) tomato genotypes were used as materials to elucidate the involvement of signal molecules such as ABA, nitric oxide (NO) and hydrogen peroxide (H2O2) in elevated CO2-induced heat stress tolerance. Salient findings are as follows:1. Firstly, the effects of CO2enrichment (800μmol mol-1) and exogenous application of ABA, NO, H2O2on heat stress (42℃for24h) were investigated in Condine Red (CR) tomato. Results showed that the plants pretreated with elevated CO2, ABA, NO, H2O2exhibited less heat injury as indicated by the higher quantum efficiency of PSII photochemistry (Phi PSII) and the lower electrolyte leakage (EL) in leaves compared with non-treated control under heat stress. Meanwhile, CO2enrichment increased the content of NO and H2O2by48%and40%, respectively with no significant changes in endogenous content of ABA compared with the plant grown under ambient CO2(380μmol mol-1). These results imply a crucial role of elevated CO2, ABA, NO and H2O2in heat stress tolerance.2. Using Ailsa and not as materials, this study investigated the interactive effects of elevated CO2and heat stress on the endogenous level of ABA, and the cellular redox state. We found that heat stress remarkably damaged the photosynthetic apparatus and caused oxidative stress, which was indicated by the decreased level of Phi PSII and the increased level of EL. Notably, not seedlings were more sensitive to heat stress compared with Ailsa, suggesting a role of endogenous ABA in heat stress tolerance. Irrespective of genotypes, the elevated CO2remarkably stimulated Phi PSII and EL in heat-stressed plants towards enhanced tolerance to heat stress with no changes in ABA content, indicating that induction of ABA level might not be involved in elevated CO2-induced heat stress tolerance. In addition, elevated CO2significantly strengthened the antioxidant capacity of heat-stressed tomato seedlings towards a reduced cellular redox state for a prolonged period, thereby mitigating oxidative stress. These results suggest that elevated CO2alleviated heat stress through efficient regulation of the cellular redox poise in an ABA-independent manner in tomato plants.3. The involvement of signal molecules such as NO and H2O2in elevated CO2-mediated heat stress tolerance was confirmed by using chemical scavenger and virus-induced gene silencing (VIGS) approach for key biosynthetic genes Exogenous applications of both NO and H2O2improved heat stress tolerance in ambient CO2and elevated CO2. CO2enrichment could effectively mitigate heat stress both in NO-scavenged or NITRATE REDUCTASE (NR)-silenced plants; however, it showed no remarkable ameliorative effects against heat stress in H2O2-scavenged or RESPIRATORY BURST OXIDASE HOMOLOGUE I (RBOH1)-silenced plants, which implied that elevated CO2-mediated heat stress tolerance might involve H2O2, but not NO. Furthermore, transmission electron microscopic observations revealed that elevated CO2-induced H2O2predominantly accumulated in apoplasts. Investigation of stomatal aperture shows that H2O2is also involved in CO2-regulated stomatal movement and subsequent heat stress tolerance. In addition, qRT-PCR and western blot analyses demonstrate that elevated CO2-induced heat stress tolerance essentially involves RBOH1-dependent HSP70up-regulation. Results of this study suggest that H2O2regulates stomatal movements and HSP70expression, which plays an important role in the CO2-induced heat stress tolerance in tomato plants.