| Plasma membrane intrinsic proteins (PIPs) have the primary role in mediating subcellular transport of H2O, CO2, and other small substrates, and they also respond to environmental cues, including pathogen attack and stimulation by pathogen-associated molecular patterns (PAMPs). As a special type of PAMP, harpin proteins produced by Gram-negative plant pathogenic bacteria are multifunctional in plants, inducing disease resistance and plant growth enhancement. The effects take place upon molecular recognition of harpins by plant receptors located at plasma membrane. The molecular interaction triggers transduction of phytohormone or non-hormone signals, which function in turn to regulate disease resistance or growth enhancement. However, mechanisms that underpin harpin sensing remain largely unknown. This study was attempted to demonstrate the regulatory role of AtPIP1;4, which was identified in Arabidopsis thaliana through mutant screening, in the plant resistance to a bacterial pathogen. This study was also sought to elucidate the roles of the phytohormones ethylene and gibbrellin in the promoting effect the harpin protein Hpalxoo, produced by the rice bacteria blight pathogen Xanthomonas oryzae pv. oryzae, on plant growth found in previous studies. Both aspects of researches detailed in this thesis were anticipated to offer a basis for further analyses of harpin signal recognition, possibly by a PIP protein, and subsequent signaling pathways.Plant growth and plant water transport and balance in the plant are closely related. Water transport though cell membrane with three ways:free diffusion through the cell membrane, transport across with transmembrane proteins, passive transport though water channel. Aquaporin is one of the major intrinsic protein (be the intrinsic protein, MIP) family members, which can selectivity transport the water and small molecule though cell membrane efficiently, and its main function is intercellular transport and in-or extracellular transport of water. The previous studies have shown that when harpin applied to plants, they can induce resistance to pathogens and insects, confer drought tolerance, and promote plant growth. We attempt to study the influence of AtPIP1;4 to disease resistance in Arabidopsis, and for the future study to explore the disease-resistant mechanism. In addition, we also studied the role of ethylene and gibberellin in HpalXoo signaling.1. Defence disease resistance in AtPIP1;4-expressing transgenic ArabidopsisAquaporins are intracellular channels for transport of water and other small molecular substances, such as reactive oxygen species and small molecule metabolites, which can influence plant growth and drought resistance. The role of aquaporin in plant resistance to diseases and the reaction mechanism are not clear at present. In this study, transgenic Arabidopsis that overexpressing the AtPIP1;4 gene was generated; homozygous progenies of atpipl;4 mutants were screened. Both genotypes were compared with the wild-type plant in terms of response to inoculation with the bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Duplicate experimental data suggested that AtPIP1;4 overexpression led to enhanced resistance to the pathogen, whereas atpip1;4 mutant plants were more susceptible, in comparison to the wild-type plant.2. The Hpalxoo signal transduction coregulated by ethylene and gibberellinIncreasing studies have indicated that harpin induces plant growth enhancement through activating the ethylene signaling and gibberellin signaling pathways. Both pathways regulate the expression of EXP genes while the gene expression often contributes to plant growth. The present study elucidated that the Hpal treatment caused an upregulation in expression levels of several genes that are involved in ethylene responses of plants. To determine whether both hormones act together to regulate the effects of Hpal on plant growth and associated physiological responses, Arabidopsis mutants etrl and ga5-1, which are defected in ethylene perception and gibberellin biosynthesis, respectively, were investigated in comparison with the wide-type (WT) plant, regarding their responses to Hpal treatment. Extents by which Hpal performed to enhance plant growth, induce EXP expression, and promot photosynthesis were partially impaired, but not cancelled, in both etrl and ga5-1 mutants. Similar results were observed in WT plants treated separately with the ethylene signaling inhibitor 1-MCP and the gibberellin biosynthesis inhibitor PBZ. However, the effects of Hpal were cancelled by the application of 1-MCP to ga5-1, by the applicationg of PBZ to etrl, or by a combinative application of 1-MCP and PBZ to WT plants. These analyses suggest that ethylene and gibberellin coregulate the effects of Hpal on plant growth and related physiological responses. |
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