Study On The Non-photochemical Quenching And The Function Of Four Light Harvesting Complex-like Encoding Genes In Ulva Linza

Ulva linza is one of the main cause of the green tide and it inhabits the intertidal zone where is strongly influenced by different kinds of physical stress, such as high light and desiccation. To cope with these stress conditions, U. linza has evolved stress tolerance strategies, especially the photoprotection mechanism to protect its photosystems against high light-induced photodamage. Light is necessary for photosynthesis, but excess light can cause photoinhibition and even result in cell death. For this reason, photosynthetic organisms have evolved multiple protection mechanisms with protection from the damaging effects of light.Non-photochemical quenching(NPQ) is one of the most important photoprotection mechanisms in photosynthetic organisms when they are exposed to excessive irradiation.Light-harvesting complex(LHC) family members are known to play important roles in this mechanism, and is associated with an enzymatic xanthophyll cycle(XC) which is controlled by the light-driven transthylakoid proton. The exact mechanism of NPQ is different between photosynthetic mechanisms. In this syudy, we investigated NPQ in the green alga U. linza coupled with inhibitors to alter the amplitude of the transthylakoid proton gradient(Δp H)and/or de-expoxidation of xanthophyll cycle(XC) under high light conditions. The data demonstrates that NPQ started with a rapid initial rise within the first minute of illumination,followed by a decline before a further rise in quenching. During the first phase, NPQ was triggered and completely controlled by Δp H. NPQ was triggered strengthened and modulated by zeaxanthin in the second phase which Δp H gets involved in. NPQ relaxation was slower in U. linza when compared to plants and other green algae, and it may be mainly caused by the slow conversion of zeaxanthin to violaxanthin.In the present work, we identified four light harvesting(Lhc)-like genes(Elip L1, Elip L2,Cbrx and OHP), which encode proteins that are relatives of light-harvesting complex(LHC)proteins in U. linza. Maximum PSII photochemical efficiency(Fv/Fm) and NPQ of cells subjected to different conditions(high light, desiccation) were examined by Dual-PAM. The lower Fv/Fm as well as the associated induction of NPQ was observed under high light and desiccation. The data showed that U. linza was under stress and NPQ plays an important rolein the thermal dissipation of excess absorbed energy. Under high light and desiccation conditions, while the NPQ increased, after which they began to decline. These results demonstrated that U. linza was responsed to short-term high light and desiccation conditions efficiently, but it may be beyond the limits of the algae with the extension of time.Primers for full-length c DNA of Elip L1, Elip L2, Cbrx and OHP were designed from the transcriptome sequencing. The full-length of Elip L1, Elip L2, Cbrx and OHP which were obtained by PCR was same as the genes of transcriptome sequencing. There were three possible transmembrane helices in ELIP and CBR, and one in OHP. According to phylogenetic relationship, we found that the Elip genes evolved separately in green algae and land plants, and the Cbr and Elip genes originated from one ancestor. These four protein belongs to LHC family.The m RNA levels of the four genes and the expression of the four proteins were identified by RT-q PCR and western blot, respectively. The m RNA levels of the four genes increased and reached maximum within 3 h under high light, and then rapidly returned to a low level. By contrast, these four genes displayed their highest expression levels at 6 h under desiccation stress. The up-regulation of Elip L1, Elip L2, Cbrx and OHP, which was induced by high light and desiccation, was accompanied by a high level of NPQ. These findings indicated that Elip L1, Elip L2, Cbrx and OHP, might play a primary role in reducing the excited state of excess photons to avoid photodamage and the four proteins participate the photoprotect of U.linza. Meanwhile, up-regulation of Cbrx was more significant than the other three genes under both conditions, so it might play a more important role in thephotoprotection mechanism. When compared with the m RNA expression data, the protein levels were not consistent, showing a slight delay under both conditions. These results suggested putative photoprotection functions for Elip L1, Elip L2, Cbrx and OHP in U. linza under both high light and desiccation stresses.