Comparative Analysis Of Nonphotochemical Quenching In Ulva Prolifera Responding To High Light,Light Quality And Salinity Stress

Ulva prolifera is a familiar macro-alga lived in intertidal zone.It has attracted more attentions in recent years due to the green tide occurred persistently along Qingdao bay.The green tide destroyed the ecosystem of the sea and has cased huge economic losses.The outbreak of U.prolifera is closely related to its high ability against high light,high salinity and other stress factors.It has been proved that the green tide occurred in Qingdao is much related to the Porphyra yezoensis culture in Jiangsu province of China.After fall off from the raft,U.prolifera is floatted to the open sea.In this work,we first measured the physiological activities of U.prolifera in the green tide occurring area in situ.The results show that the physiological activities of floating thalli kept higher although decreased a little compared with adhering thalli.The photosynthetic activities varied in different ocean areas.In total,the photosynthetic activities decreased gradually from south to north area.In addition,we calculated the percentages of sporangium and suggested that it formed quickly when floating on the sea.These all contributed to the occurrence of green tide.The floating thalli would subjected to high light and high salinity stresses.Moreover,light qualities under seawater were different from that in atmosphere.These were all stress to U.prolifera.There must exist efficient pathway of U.prolifera to against adversity stress.In the following work,we mainly focused on high light,high salinity and light quality stress.We first treated U.prolifera with high light in laboratory and analyzed the photosynthetic activities and protein compositions of thalli.The result show that NPQ value increased obviously,accompany by the increased Zeaxanthin content.However,neither PsbS nor LhcSR was up-regulated.We suggested that these two key protein for NPQ in U.prolifera were quite different from Physcomitrella patens and Chlamydomonas reinhardtii.Besides,we founded that the composition of the minor LHC proteins in U.prolifera is CP26 and CP29,but without CP24,which is different from higher plants.We deduced that,it was only the lacking of CP24 that resulting in the defect in fast NPQ induction.In conclusion,in order to active the NPQ mechanism quickly,U.prolifera keep enough PsbS,LhcSR and Zeaxanthin within the cells.This is a result of U.prolifera adapting to intertidal zone.We further treated thalli with different light qualities for 2 days.Our results show that thalli treated with long wavelength light,far red light(FR),show high NPQ value and Zeaxanthin content compared with white light and short wavelength light.However,the photosynthetic activity of thalli under FR decreased,indicating FR as a stress factor.Our WB experiments indicated that LhcSR was up-regulated while PsbS was not when U.prolifera exposed to FR condition.This was further conformed by RT-PCR analysis.This was to say FR induced the synthesis of Zeaxanthin and the up-regulation of LhcSR to promote NPQ.We compared this in Physcomitrella patens and no similar phenomenon was found.This indicated a special NPQ mechanism existing in U.prolifera.Next,we separate the photosynthetic complexes with sucrose density gradient and identified that LhcSR was bonded to light harvesting complex II(LHCII)trimer and minor LHC but not PSI-LHCI complex.Moreover,under FR the LHCII trimer became larger and the ratio of chlorophyll b/a in PSI-LHCI.Our results declared the special role of LhcSR compared with PsbS in NPQ induction and put forward a new direction of LhcSR study.We further verified these results with transcriptomic analysis,and found calcium signal pathway play important role in U.prolifera responding to far red light.Phytochrome encoding gene was explored and we finally built a complete far red signal transduction pathway in U.prolifera.Finally,we explored the salt stress upon U.prolifera and Dunaliella salina.We found different YI and YII changes when subjected to salt stress in U.prolifera and Dunaliella salina.The pentose phosphate pathway may play different roles in these two algae.In Dunaliella salina,the pentose phosphate pathway provides NADPH and ATP for glycerol synthesis to modulate intracellular osmotic stress.Besides,it was shown that 9% NaCl induced the increasement of NPQ and Zeaxanthin content.However,neither PsbS nor LhcSR was proved up-regulated.This was much similar with Physcomitrella patens when subjected to salt stress.This was considered as a special pathway to induce NPQ and respond to salt stress,which deserved more studies.In total,NPQ inductions under these three stress were quite different,although there exist same similarities.This was consider the result of U.prolifera to survive the complex environment in intertidal zone.