Studies On Cytoplasmic Structure Remolding Of Developing Microspore Mother Cells In Althaea Rosea

Meiosis is a pivotal node in the life cycle of plants,controlling the transition of diploid(sporophyte)generation to haploid(gametophyte)generation of eukaryotes. During this phase the microspore mother cells(MMCs)have experienced intense and complicated structure changes or cell events.All these changes indicated that the male meiocytes were in the process of structural reorganization or remolding.However, among many previous investigations concerning plant male meiosis,most have focused on nuclear events(related to chromatin/chromosome).There were a few papers dealing with cytoplasm reorganization.In addition,these studies were based on very few plant species,and the observations differed with regard to plant and/or author.To further characterize the mechanism of cytoplasmic structure remolding during plant meiosis,the male meiocytes of Althaea rosea(L.)Cavan which never studied before were examined.We used paraffin sections,thick resin sections, ultrathin sections and fluorescence staining under the combination of light and electron microscopy to study the detailed process of cytoplasmic remolding during early stage of male meiosis in Althaea rosea(L.)Cavan.The major results showed below:Light microscopic observation of the safranine-bright green stained paraffin and toluidine blue-stained thick resin sections of young anthers revealed that the anthers of A.rosea consisted of two instead of the normal four sporangia,one per lobe. Viewed from transversal sections,each sporangium of the anther during sporogenous phase contains 2-4 MMCs.On longitudinal sections,1-2 rows of meiocytes could be observed in each sporangium,which looked spherical,hemispherical,crescent, rectangular,or irregular.The meiocytes were extremely voluminous(about 10-20 fold larger than the wall cells),and considerably variable in morphology and division plane.The cell nucleus located in the centre of the cytoplasm or near the CWs,and contained a very large ringed nucleolus.In addition,we also observed some special features:1)The CWs between meiocytes had some unequal-length branches which elongated into the deeper cytoplasm of MMCs.2)The shapes of meiocytes were very irregular;some meiocytes could produce protuberances that extended into another cell. When transversally sectioned,the protuberance extended to another cell looked like a small cell lying in another cell.3)The CWs between meiocytes and tapetum cells remained integrated;however,the CWs between some meiocytes were discontinuous at one or several site(s)(called nicks)which varied between 0.2-3.0μm in length,and some darkly stained particles(may be the plastids or mitochondria)can cross these nicks.4)Occasionally,we observed that chromatin spheres of one meiocyte can extend into another meiocyte through the nick on the wall.Transmission electron microscopy(TEM)observation showed that during meiosis of male meiocytes in Althaea rosea(L.)Cavan,the major part of the ribosome population is removed;plastids and mitochondria have experienced structural aberration.All these changes have reported before in male meiocytes of other plant.Most importantly,we observed some very interesting changes never reported before in meiocytes of Althaea rosea(L.)Cavan.1)The endoplasmic reticulum(ER)cistema in cytoplasm tightly appressed,and the whole ER turned into long and flat double-membrane structure during pre-meiosis.2)One or two double-membrane structure developed from ER showed paralleled arrangement, encapsulated a portion of cytoplasm and elongated,finally resulted in a number of huge(0.5-1.0μm diameter)spherical cytoplasmic compartment or inclusions lined by single or double layer(s)of membrane.The membrane-bound inclusions(MBIs)lined by single or double layer(s)of membrane did not contain plastid or mitochondria,but contained many ribosomes and granular,lamellar and fibrillar structures.3)In the double-layered MBIs,the inner membrane could split from the outer membrane at one or several sites,which can enfold gradually and form a new small one which lined by single or double layers of membranes.4)The single or double-layered MBIs can phagocytize ambient MBIs by outer membrane invagination.5)More interestingly, the MBIs formed by ER could associate with the CC on CWs to release their contents into the cytoplasm of another cell or directly extend from one cell to another through the CC,finally resulting that the MBIs located at both ends of one CC(one at each end).Moreover,it is often observed that 1-5 MBIs with different sizes located around CCs or tightly combined with the CC.After pachynema,the association between CCs and MBIs disappeared.Taking all the data together,it is suggested that the structural reorganization of ERs and the formation of MBIs could represent a process of cell macroautophagy in male meiocytes.The MBIs could represent a kind of macrophagosomes;this phenomenon was never reported before in other plants.The cytoplasm contents (maybe the non-useful or redundant cytoplasm constituents)sequestered in MBIs could fuse with vacuole and use the resident hydrolases to hydrolyze the enclosed materials or transport to another cell which lacked the constituents mentioned above by the association with CCs.The autophagy could lead to the uniform distribution of cytoplasm between all male meiocytes in the sporangium and eliminate the non-identity of cytoplasm of the male meiocytes caused probably by the random asymmetric division observed at sporogenous cell phase.The purpose is to ensure production of a large number of identical functional male gametes which required for successful fertilization and relatively stable inheritance.