| Northern Leaf Blight of Corn, caused by Setosphaeria turcica, is one of the most important diseases in corn production areas, often results in significant economic losses in epidemic years. Signal transduction pathways play a central role in sensing and transmission of external signals leading to altered cell responses, regulating the growth, development and pathogenicity of fungal pathogens. There are three extracellular pathways in fungal pathogens, MAPKs, cAMP-dependent pathway and Ca2+ pathway. All of the pathways are very important for development and various cell functions. MAPK cascades is composed of a group of protein kinase which are highly conserved in eveloution,takes a great part in the diverse cell signal transduction pathways, and regulats a variety of physiological activities, such as growth and development, virulence, osmotic regulation, mating, cell wall integrate, morphologic formation, and so on. MAPKK located in the upstream of MAPK, and it stayed in the centre part of the cascade. So, it is very important to identify the function of MAPKK in the MAPK cascade pathway in S. turcica. In this paper a MAPKK gene, STK1K, was cloned, and the functions of the gene were explored by analyzing the characteristic of RNAi mutants.And the main results was as follow.1. A 1 242 bp homologous fragments of MAPKK, named STK1K, was cloned by candidate gene approach and Genomic walking technology. Homologous analysis showed there was high homology between STK1K and HOG-MAPKK genes of other filamentous fungi.2. Based on the basic plasmid pSilent-1, an RNAi vector of STK1K was constructed. The recombinated vector was transformed to the protoplasts of S. turcica mediated by PEG 4000. 51 transformants were obtained by cultureing on PDA medium containing 50μg/mL hygromycin B. 4 mutants were determined through PCR and Southern blotting.3. By comparing the phenotypes of mutants and the wild-type strain, it was found that 4 mutants were significantly different from wild-type strains in the colour and shape of the colony, and the morphology of the hypha of the mutants changed obviously.①Mutant strain R3 showed a light dark-green in colony color, irregular colony edge, hyphae swelling, internode shortening, and granulated material in the cell of mycelium .②Mutant strain R14 showed deep-dark green in the colony color, irregular colony edge, hyphae swelling. Some hyphae were spherical, and showed granular material in the mycelium cell.③Mutant strain R35 showed gray-black in colony color, and the colony edge was neat. There were no significant morphological changes in mycelium, and there were also granular material deposited in mycelium cell.④Mutant strain R40 showed gray in colony colour, colony edge was irregular, and hyphal was transparent and irregular swelling.4. It was found that 4 RNAi mutants grew slower than the wild-type strain.5. The yield of the conidiophore of mutant strain R3, R14,R35 significantly reduced, only 1% of the wild-type strains. There was no conidiophore in strain R40.6. STK1K gene knockout vector was constructed, and the recombinant vector was transformed into the protoplasts of S.turcicum. 36 thansforments with Hygromycin B resistance were obtained. 8 positive transformants were screened by PCR.
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