The Response Mechanism To Hypersaline Stress In Pyropia Yezoensis

Pyropia yezoensis(previously called Porphyra yezoensis),belongs to the genus of Pyropia,is a common rhodophyta of intertidal zone.It’s also an important economic marine algae in China.Py.yezoensis can survive the severe water loss that occurs during low tide,making it an ideal species to investigate the acclimation mechanism of intertidal seaweed to the special extreme environments.To study the mechanisms of resistances to abiotic stresses in Py.yezoensis,we carried out research in the following aspects:1.Because of the lack of xanthophylls cycle,some other photo-protective mechanisms are much more important for Py.yezoensis,such as cyclic electron transport.Firstly,we determined the effects of high salinity on photosynthesis using increasing salinity around algal tissues.CEF increased and remained at a stable state in 50‰-120‰ salinity and decreased sharply when Py.Yezoensis were treated with 150‰ or higher salinity.Inhibitors experiments illustrated that there were three different cyclic electron transportation pathways at least.To figure out how those different CETs were coordinated,we had cloned the gene sequence(HM370553)of ferredoxin-NADP+ oxidoreductase(FNR).Then,a phylogenetic tree was constructed and the evolutionary relationship among different FNRs indicated that the Py.yezoensis FNR showed closer relation to the cyanobacterial FNR.Due to the structure-function correlation in organism,Py.yezoensis FNR was proposed to be involved in the NAD(P)H-dependent Fd+ reduction under severe salinity conditions.Thus,through the connection between different donors bridged by FNR,electrons were channeled towards distinct routes according to the different metabolic demands.FNR was also directly involved in certain CET pathways.This made the electron transfer in the chloroplasts became more flexible and contributed greatly to acclimation of Py.yezoensis to the extreme variable environments in the intertidal zone.2.The antioxidant mechanisms which consists of a variety of antioxidases and antioxidant molecules in Py.yezoensis might play a very important role to survive the extreme variable environments.We treated Py.yezoensis with different hypersaline seawater and determined the time variation of photosynthetic parameters,antioxidases activity and the content of antioxidants.Our results showed that Py.yezoensis can cope with a short period of hypersaline or mild hypersaline with minimal disruption of their total antioxidants pools.This could be achieved by increasing the activities of antioxidases,and/or increasing the rate of antioxidants regeneration.With the increasing salinity and the extension of time,it is insufficient for the primary antioxidants pools to scavenge the generated ROS.Thus,antioxidant pools increased and play an important role in ROS scavenging.GSH and total glutathione pool increased quickly to cope with the tempory ROS burst.3.The phytohormone abscisic acid(ABA)is involved in the adaptive stress response and regulates the activation of anti-oxidization system,which has been studied thoroughly in land plants.However,its function in marine algae remains unclear.The algal tissues were treated with increasing salinity and ABA content was determined.Our results showed that the ABA content in high salinity stressed Py.yezoensis presented a significant up-regulation compared with that of the control,especially in 150‰ salinity group.The results strongly suggested that there were some positive effects which mediated by ABA.To find out the possible ABA synthesis pathway in Py.yezoensis,thallus of the algae were treated firstly with the inhibitors which was related to the biosynthetic pathway of ABA.After that,ABA content was determined using 150‰ salinity seawater around thallus tissues.The results showed that sodium tungstate and naproxen reduced ABA synthesis significantly,which meant the synthesis of ABA in Py.yezoensis was through the carotenoid biosynthetic pathway.Paclobutrazol showed the same effect on ABA content,indicating that IPP(the precursor of ABA synthesis)was derived from MVA pathway.In addition,exogenous ABA could decrease the accumulation of MDA when Py.yezoensis was treated with increasing salinity,indicating that there might be an antioxidant pathway mediated by ABA in Py.yezoensis.Still,the effect of ABA on antioxidases activity needs to be studied further.