| Drought is considered as one of the most important environmental stress factors that limit the crop growth and yield, how to resolve the drought problem and improve the crop yield have become the focus that people concern. C4 plants possess a unique CO2 concentrating mechanism have higher photosynthetic capacity and higher nitrogen and water use efficiency than C3 plants under high light intensity, low CO2 concentration and drought conditions. In terms of theory, introducing C4 model into C3-type crop can greatly improve the traits such as yields amd stress resistance. Phosphoenolpyruvate carboxylase (PEPC, EC.1 1.31) is one of the key enzymes of C4 photosynthetic pathway. Currently, it has achieved the maize C4-pepc gene into the rice and high level expression through transgenic technology. Studies showed that pepc transgenic plants had relatively high Pn and improved oxidative resistance under drought sress, however the molecular mechanisms of tolerance yet to be established. It’s a worthy studying scientific issue how it is respond to external tolerance stress, resulting in different levels of adaptation mechanisms.Hydrogen peroxide (H2O2) is one of reactive oxygen species (ROS) in plants, but also viewed as an important signal molecule that regulates various developmental process. Many previous studies have found that H2O2 involved in leading to many biotic and abiotic stress signals. Calcium has an important physiological role as a second messenger regulates many physiological processes in plant growth and development, including drought signal abscisic acid (ABA) transmission, regulating stomatal opening and closing levels, inducing stress-related gene expression, and enhancing the ability of tolerance of plants. In this paper, transgenic rice with overexpressing maize pepc (PC) and Kitaake (WT) were used as materials, the changes of physiology, enzymes, gene and signal transduction components et al. were observed at plant level under PEG-6000 simulated drought condition.and the response of signal molecules to PEG stress was explored at cellular level, also the relationship among the signal molecules. The main results are as follows:1. Pn in WT was enhanced significantly (P＜0.05) by different concentrations of exogenous calcium (0.1,1 and 5 mM) through spraying the tested materials leaves; Unlike the WT, it’s found that Pn in PC was not promoted by spraying the calcium solution, and the suppression was observed to some extent. And the suppression of Pn had no relationship with stomatal factors. Meanwhile, endogenous calcium content in WT was significant lower than that of in PC (P<0.05). PC has a unique response to calcium, it may be related to its higher endogenous calcium content. It also provides a new perspective to research on the drought resistance mechanism for us.2. The decline of relative water content of WT was greater than that of PC after 1st of 15% PEG-6000 treatment. The net photosynthetic rate (Pn) of WT was decreased significantly by PEG-6000 treatment after a day, but no notable manifestation was observed in PC. Moreover Pn in PC recovered faster after 2nd of rewatering, while not in WT. The stomatal conductance (Gs) of WT and PC was also reduced remarkablly with PGE-6000 treatment. It follows that the damage to WT was more serious than PC, and PC had better recovery capacity as following rewatering.3. In order to illuminate the mechanism of drought resistance, further analysis the changes of antioxidant system, related genes expression and signal transduction components et al. The results revealed that the ascorbate peroxidase was enhanced during the two days of PEG-6000 treatment in PC, but not in WT. With PEG-6000 treatment, both H2O2 and Ca2+content were increased, and for the second day the improvements of PEPC activity and the expression of C4-pepc and NAC6 in PC were observed. Also the protein kinase activity was enhanced for the second day with PGE-6000 treatment in PC. The results revealed that the mechanism of photosynthetic capacity which maintained stability under PEG-6000 stress in PC due to the regulation of the H2O2 and Ca2+ content in vivo, which involved in the regulation of the expression of transcription factors,C4-pepc and antioxidant enzyme genes at the transcriptional level, and at the translational level positive regulating PEPC and antioxidant enzyme activity throught the protein kinases, resulting in lowering the increase of MDA and in favor of reducing the inhibition of photosynthesis by drouguht stress.4. In order to further verify the above hypothesis, the response of signal molecules to PEG stress was also observed at cellular level. The results revealed that the level of H2O2 was significantly increased in PC cells under 20% PEG stress for 1 and 4h (P< 0.05), but no changes in WT; For Ca2+ concentration, it exhibited the tendency of firstly reduced and then increased both in WT and PC cells under PEG stress, but different time for increased Ca2+ between WT and PC. Meanwhile, the level of H2O2 was not reduced by the application of cPTIO (a scavenger for NO) and (a chelating agent for free calcium)/ruthenium red (an inhibitor for outflow of calcium channel) in PC, while NO content and Ca2+ concentration were reduced by the application of DMTU(a scavenger for H2O2) in PC. Therefore, both NO and Ca2+ served as a downstream component of H2O2.In conclusion, the resisatance mechanism of transgenic rice with overexpressing maize pepc under drought stress due to the regulation of the H2O2 and Ca2+ content in vivo, then the transcription and translation of genes were regulated and the antioxidant capacity was enhanced, which in favor of reducing the inhibition of photosynthesis by drouguht stress. |
PDF Full Text Request