Study On Cytological Regulation Mechanism Affecting The Volume Of Mesophyll Cells And The Chloroplasts Division In Diploid And Tetraploid Arabidopsis

Chloroplast is a specific organelle of plant cells.It can transform light energy into biological energy through photosynthesis to store energy for life activities.At the same time,it also bears the functions of synthesizing lipids and other energy storing material.Chloroplast quantity,morphology,structure and biological activity are closely related to the growth and development of higher plants.Studies have shown that chloroplast proliferation and cell expansion are closely related.In higher plants,cell size and chloroplast quantity show a positive correlation.However,there are no clear data showing that the expansion of mesophyll cells can directly affect(or stimulate)the division of chloroplasts.According to the theory of endophytic symbiosis,the chloroplast originated from the symbiotic relationship between cyanobacteria and plant cells.However,the chloroplasts are unable to carry out their own division,and their dividing mechanism is controlled by the protein family encoded by the host cell core.Studies have shown that the FtsZ protein family(FtsZ1 and FtsZ2 in dicotyledonous plants)plays an important role in the chloroplast division.It shows that the intimal division ring participates in the chloroplast spills,and the protein family is firstly assembled in the process of division ring formation.Mesophyll cells contain more than ten or even hundreds of chloroplasts in higher terrestrial plants.Even though the cells are at the peak period of chloroplast division,these chloroplast divisions do not occur at the same time.In terrestrial plants,there is a system closely related to the division and differentiation of plastids.At the same time,this system is closely related to the state of cells,which determines the form and quantity of plastids.In this paper,a variety of experimental methods were employed to observe,record and analyze the mesophyll cells of Arabidopsis thaliana with different ploidy and developmental periods,and to explore the relationship between cell size and chloroplast division and quantity during the mesophyll development.The main results and conclusions are as follows:1.During the growth and development of Arabidopsis leaves,the mesophyll cells gradually grew and expanded.According to statistical observations,there were two high-speed elongation periods of mesophyll cells in diploid and tetraploid Arabidopsis.Among them,diploids were 3-7 days(60.0%)and 9-11 days(73.8%)since true leaves budding,and the increase rate in the second high-speed elongation period was higher than that in the first high-speed elongation period.In tetraploidy,the high-speed elongation period,respectivly,were 5-7 days(110.4%)and 13-15 days(84.5%),and in contrast to diploids,the first high-speed expansion period was higher than that in the second high-speed expansion period.2.In diploid Mesophyll cells,we observed two high-speed division period of chloroplasts,which were 5-7 days(30.9%)and 11-15 days(64.7%)since true leaf budding,and the second high-speed division period were higher.In the tetraploid mesophyll cells,the chloroplasts also had two high-speed division period,which were 6-7 days(67.5%)and14-15 days(43.7%)since true leaf budding.On the contrary to diploids,the first high-speed division period increased more than the second high-speed division period.This study revealed for the first time that in diploid and tetraploid Arabidopsis mesophyll,two high-speed elongation period happened before two high-speed division period of chloroplast did.We speculate that the elongation of mesophyll cells may be the direct cause of chloroplast division.3.In 3-day-old true leaves,the number of chloroplasts per cell in average was 12(diploid)and 15(tetraploid),and the cell area wasn't significant different.In 17-day-old true leaves,the number of chloroplasts per cell in average was 70.46(diploid)and 88.78(tetraploid),and the cell area was 3232.17?m~2(diploid)and 4851.17?m~2(tetraploid)(50 mesophyll cells were taken as statistical samples for each period and each ploidy).The number of chloroplasts in tetraploid mesophyll cells was significantly higher than that of diploids.In mature true leaves,the size of tetraploid mesophyll cells was significantly higher than that of diploids.It can be seen that the increase in ploidy led to an increase in the volume of mesophyll cells,which indirectly led to more chloroplasts in tetraploid mesophyll cells than that in diploids.4.Transmission electron microscopy(TEM)were used to observe the ultra-structure of the chloroplasts of Arabidopsis thaliana,it was observed that at the late stage of chloroplasts division,an plastid division ring appeared in the middle position of the chloroplast.At the same time,the starch grains in the tetraploid chloroplasts are higher in quantity and volume than in diploids(same period).Using the immunofluorescence to mark the Z-ring,it can be clearly observed that the dividing chloroplasts had different shapes such as long ellipse,circle,and dumbbell,and it can also be observed that the Z-ring is in the middle positon of the chloroplast,which fully proves that FtsZ protein family is involved in the process of the chloroplast division.