| Pollen germination and pollen tube growth is crucial for higher plant reproduction. It requires proper cell wall polysaccharide synthesis and transportation in Golgi apparatus. The Golgi apparatus is an organelle with structural polarity and important function in plant cells. The Golgi resident proteins (such as glycosylation enzymes and glycosyltransferases) must be targeted specific loci of Golgi apparatus, in order to ensure the correct synthesis and directional transport of the main components of the pollen tube wall. But the molecular mechanism involved in the regulation of the resident protein localization is still unclear. In yeast and mammalian, COG (Conserved Oligomeric Golgi) complex is a protein complexes composed of eight subunits which acts as a tethering factor play a key function in maintaining Golgi apparatus integrity and regulating retrograde transport and localization of vesicles. The COG complex also exist in plants, however, only one article reported that COG complex functions in plants. COG subunit COG7 mutation ruined the distribution of Golgi resident proteins, the mutants appeared growth retardation in seedling. We took this key complex——COG subunit COG8 as the breakthrough point to explore the molecular mechanism of the resident protein transportation and localization in Golgi and its effect on plant growth.A loss of function mutant of the Arabidopsis COG subunit COG8 gene were analyzed. By the expression pattern analysis, COG8 showed a constotutive gene expression. It is highly and specifically expressed in mature pollen grains and pollen tubes, and expressed in pollen from two nuclear period. The data of Genetic analysis showed, mutations in COG8 had slight influence on female gametophyte function, but completely blocked the genetic transmission of male gametophyte. The COG8 mutations did not affect pollen's morphology, activity or nuclear development before pollen germination, but severely inhibited pollen tube growth in vitro. There are three kinds of abnormal phenotype:1) pollen bursted at the beginning of germination; 2) pollen tube with abnormal morphology, specific performance for stubby, intumescent, twisted, furcal or pollen tube bursted after germination; 3) the growth of pollen tube is restrained, so the pollen tube is very short but normal. Through the observation of pollen subcellular structure, the Golgi morphology affected severely by mutant. Further chemical specific staining of the main components of pollen cell wall showed that, the distribution and the content of cellulose in pollen tubes is almost not affected, callose and pectin can normal synthesis but can not correct transport and distribute in the pollen tube. The phenotype of COG8 mutation can be complemented by COG8 genomic DNA, so the phenotype of cog8 mutant is due to mutations in COGS. Subcellular localization results showed that COG8 protein may function in TGN, Cis-Golgi and endosome. Above these results, COG8 may act as tethering factor involved in the retrograde transport of vesicles in Golgi apparatus in plants, then regulating the main components of pollen tube wall's polar distribution, to mediating the germination and growth of Arabidopsis pollen tube.In this study, we referenced the progress in yeast and mammalian cells to carry out the research on macromolecular complex COG subunit COG8 which may play an important function in plant growth. The vesicle trafficking pathway was linked with pollen tube polar tip growth mechanism, providing a novel approach to explore for the polar tip growth of pollen tube mechanism. |
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