Molecular Basic Study Of Chloroplast Division Between Diploid And Tetraploid Arabidopsis Thaliana

The chloroplasts are one kind of the most important cellular organelles that not only perform photosynthesis but also synthesize amino acids,lipids and phytohormones in plants.Therefore,the chloroplasts directly affect plant photosynthetic efficiency and further do the accumulation of plant biomass.The chloroplast proliferation is achived by itself division,and the frequency of its division is influenced by multiple factors.There is a positive relationship between cell size and the division frequency and number of chloroplast.Plant polyploids usually increase the cell size and the number of chloroplasts.Therefore,studying the relationships of cell enlargement and chloroplast division in polyploid plant can not only provide information for the revealing the regulatory mechanism of chloroplast division,but also have an important significance for the breeding of economic plants.In order to investigate the relationships between cell enlargement and chloroplast division and to study the effect of ploidy changes on chloroplast division,in this research,we compared the differences of relative mRNA levels of cell division genes,cell expansion genes and chloroplast division genes between diploid and tetraploid Arabidopsis thaliana by quantitative RT-PCR.We also to comparathe difference of the leaf morphological characteristics and microstructure between diploid and tetraploid Arabidopsis thaliana employing the methods of leaf phenotype analysis and semi-thin section.The main results are as follows:1.In diploid Arabidopsis thaliana,during the development of the first pair of true leaves,there were two expression peaks of AtEXPA1 and AtEXPA8,occurring on 5th day and 11 th day after the first pair of true leaves budding,respectively.Correspondingly,in tetraploid Arabidopsis thaliana,on 5th day and 11 th day after the first pair of true leaves budding,there were also two expression peaks of AtEXPA1 and AtEXPA8.This suggested that there are two rapid cell enlargement stages during the development of the first pair of true leaves of diploid and tetraploid Arabidopsis thaliana.2.Compared to the diploid Arabidopsis thaliana,the expression of AtCYCB1;1 gene was down-regulated in tetraploid Arabidopsis thaliana.Cyclin regulatory proteins are important for cell division,whose decreasing activity might be one of reasons to result in the continually increasing cell size of tetraploid.3.Compared to diploid Arabidopsis thaliana,the expressions of AtEXPA1 and AtEXPA8 genes in tetraploid Arabidopsis thaliana were up-regulated at high expression stages.Especially on 11 th day after true leaves budding,the expressions of these two genes in tetraploid Arabidopsis thaliana were significantly higher than in diploid.Correspondingly,the mesophyll cell size of tetraploid Arabidopsis thaliana were larger than that of diploid on the 11 th day after true leave budding.These results indicated that the effect of cell enlargement in tetraploid Arabidopsis thaliana is stronger than in diploid.4.Comparing the expressions levels of 7 chloroplast division genes(AtARC5,AtARC6,AtPDV1,AtPDV2,AtFtsZ2-1,AtFtsZ2-2 and AtFtsZ 1-1)of diploid and tetraploid Arabidopsis thaliana,we found that the expressions levels of all the 7 genes of tetraploid were higher than that of diploid to some extent.These results show that the chloroplast division frequency of tetraploid Arabidopsis thaliana is higher than that of diploid,which may be the main reason to produce more chloroplasts in each mesophyll cell of tetraploid than that in diploid.Taken together,the cell division and cell expansion are coordinated with chloroplast division to regulate the development of leaves.The cell expansion directly affacted the chloroplast division and proliferation.The chromosome doubling plant could reduce the frequency of cell division and increase the cell size.The cell enlargement further promoted chloroplast division and eventually led to an increase in the number of chloroplasts.