Defense Responses Of Two Red Seaweeds Induced By Marine Oligosaccharides

Porphyra lives in open ocean where environment is complex, changeful and fullof pathogen. Broad scale diseases in marine agriculture will lead to large economicalloss. Oligosaccharides working as elicitors in both high plants and algae have beenreported by many researches. The present thesis investigated the elicitor activities ofoligo-porphyran and oligoagar in Porphyra yezoensis and Gracilaria vermiculophylla,respectively.Water-extractions and alkaline solution-extractions were employed to obtainnatural polysccharides and oligosaccharides in cell wall of P. yezoensis,Oligosaccharides with different Mw (molecular weight) were obtained by the acidhydrolysis of porphyran from P. yezoensis. The elicitor activities of oligo-porphyranwere investigated. Results showed that oligo-porphyran induced the generation ofH2O2from P. yezoensis. Active oligo-porphyran was seperated by anion-exchangechromatography. Compound of oligo-sulphated galacans with degree ofpolymerization (DP) from1to4induced the expression of cellular oligosaccharidesoxidase, and enabled the P. yezoensis to resist rotting caused by dense incubation.Oligo-sulphated galacans (DP4~13) compound might activate NADPH oxidase,leading to oxidative burst in P. yezoensis.In order to learn new research methods and enforce the communication withother international research groups, the last experimental part of this thesis wascarried out in Kiel, Germany. We investigated the oligoagar (OA)-induced defence responses of G. vermiculophylla and compared the different defence responsesbetween the native (Qingdao and Rongcheng, China) and the invasive (Kiel andNordstrand, Germany) populations. We found that OA induced the generation ofH2O2from G. vermiculophylla which was related to the activation of OA oxidase: OAoxidase catalyzed the oxidation of OA, generating aldehyde and H2O2. After24htreatment of150umol/L OA, G. vermiculophylla had a significant expression of OAoxidase which was related to cellular iron-containing protein and energytransformation. However, OA-treatment couldn’t change the number of G.vermiculophylla surface bacteria, but OA changed the bacterial composition of algalsurface. Both invasive and native populations could response to OA with the similarquantities of OA oxidase expression, but the enzyme activities of native populationswere lower than that of invasive populations because of their less release of aldehydeand H2O2. The surface bacteria in the native and invasive populations are different inquality and quantity. OA reduced the degree of bacterial diversity of native populationfrom Qingdao which had much more bacteria than that in invasive population fromNordstrand, but increased that of invasive population which had fewer bacteria.In the present research, two red seaweeds were chosen and their defenceresponses to the oligosaccharides derived from their cell wall polysaccharides wereinvestigated. For both of the two chosen algae, H2O2was induced by oligosaccharidesby a same mechanism, i.e. the activation of oligosaccharides oxidase. However, thepatterns of H2O2generation were different: the release of H2O2from P. yezoensissustained for more than one day, while that from G. vermiculophylla lasted only forhalf an hour. These results confirmed that defence responses varied in differentspecies, and also indicated that the generation of H2O2in these two red seaweeds maybe not due to the activation of only one enzyme or one signal pathway. Other causesfor H2O2release and the further analysis of oligosaccharide oxidase’s structure,function and relative signal pathways are worthy to be done in the future.