Singlet Oxygen And Small GTPase Ran In Plant Defense Response

Plants may experience several distinct stresses either concurrently or at different timesthrough the growing season. The stresses can be divided into biotic stresses and abioticstresses. Biotic stresses involve pathogenic (include fungi, bacteria and virus), animal andweeds. Abiotic stresses involve the environment conditions such as illumination, atmosphere,moisture and soil which endanger the plant. In order to resist these stresses, plants employmany defense mechanisms. In addition to the constitutive protect system itself possesses (e.g.cell wall, cerolipoid, cuticula et al), the plant defense responses based on the successfulrecognition of stress is very important for survival.Several signal molecules and the signal transduction have been recently implicated inplant defense response. But some crucial problems have not been resolved, especially themechanism of plants to resist the abiotic stress. We took the cultured suspension cells ofPanax ginseng as our experimental material, studied the generation of singlet oxygen whichinduced by fugal elicitor, testified the singlet oxygen acted as a signal molecule in plantdefense responses. We also found abiotic stress induced OsRAN2 expression change in rice.Overexpression OsRAN2 in Arabidopsis, the seeds were hypersensitive to abiotic stress andthe germination was seriously affected. We suspected OsRAN2 might involve the signaltransduction induced by abiotic stress. The main results are as follows:(1) Fugal elicitor induced singlet oxygen generation, ethylene release and saponin synthesis incultured cells of Panax ginseng C. A. Meyer.Singlet oxygen is a high-energy molecular oxygen species. As one of the most activeintermediates involved in chemical and biochemical reactions, singlet oxygen plays essentialroles in plant responses to UV and strong light. Here, we report that Cle, an elicitor derivedfrom fungal cell walls, induces the generation of singlet oxygen in cell cultures of ginseng,Panax ginseng. Cle treatment also triggers the activation of plasma membrane NADPHoxidase and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO), subsequently leading toethylene release and increased saponin synthesis, as shown by increased mRNA expression ofsqualene synthase (SQS) and squalene epoxidase (SQE), and accumulation of β-amyrinsynthase (β-AS). Suppression of Cle-induced singlet oxygen generation or inhibition ofethylene production blocks saponin synthesis, whereas treatment of ginseng cells withethylene or singlet oxygen induces the synthesis of saponin. Together, these results indicatethat singlet oxygen also acted as a signal molecule in plant defense response, Cle-inducedproduction of both singlet oxygen and ethylene is required for saponin synthesis, and thatsinglet oxygen may function upstream of ethylene during Cle-induced saponin synthesis.(2) Abiotic stress induce OsRAN2 expression change.The small GTPase Ran is an evolutionarily conserved eukaryotic protein, mainlydistributed in nuclear, having masses of 25 kD. The analysis of molecule structure revealsRan is one number of the superfamily of Ras, so it is named Ran (Ras-related nuclear protein).Ran has roles in nuclear transport, mitotic spindle assembly and nuclear envelope assembly.When the rice seedlings were gravistimulated by placed horizontally in the absence of light,we found in the leaf sheath segment (LSS) OsRAN2 expression declined and only expressedin the top leaf sheath (tLSS). Exogenous IAA, ABA or NaCl also induced OsRAN2expression declined in LSS of rice seedlings. Overexpression OsRAN2 in Arabidopsis, theseeds were hypersensitive to ABA, NaCl and PEG, the germination were seriously affected.So we suspected OsRAN2 might involve in the signal transduction induced by abiotic stresses.