TRANSPORT OF DISASSEMBLED PROTOPLASM FROM DEGENERATED NUCELLUS INTO EMBRYO SAC AND ITS ROLE IN FEEDING THE PROLIFERATING ANTIPODALS IN WHEAT
Zhang Wei-cheng Yan Wen-mei (Shanghai Institute of Plant Physiology,Academia Sinica)Lou Cheng-hou (Beijing Agricultural University)
The wheat embryo sac is pear-shaped and deeply imbedded in fleshy nu- cellus of uneven thickness,which,in turn,is enclosed partially by two layers of integument and is in intimate connection with the procambium around the chalazal region(Text fig.1). This connection seems to be the main inlet passage of nutrient in the ovule.Accordingly the nutrient has to pass from cell to cell and to be incorporated in the nucellus before it is fed to the enlarging embryo sac.Though as a whole the nucellus is transitory in existence,esta- blishment of new peripheral layers and decline of inner layers occur at the same time.While cells in the outer layers multiply by mitosis;cells in the intermediate layers begin to exhibit “nuclear extrusion”(an indication of transcellular movement of protoplasm)which becomes more frequent in inner layers(Text Fig.2);and cells in the innermost layer,embracing the embryo sac,actively undergo disintegration,showing walls in rupture and cellular contents in disassembly and in retreat(Fig.7,8).A distinguished feature of high activity of ATPase lo- cated in extruding nucleus has been observed in chalazal region(Fig.4)and in degenerating nucellag cells(Fig.5).The embryo sac is delimited from the nucellus by an incomplete en- velop at the mycopylar end,and the envelop is reinforced by successive deposition of wall de- bris of the diminished nucellar layers(Fig.9);whereas at the chalazal end the envelop is lacking and the antipodals can communicate directly with the disintegrating layer of the thic- kened portion of nucellus.The antipodals grow steadily as the embryo approaches maturity and the number of cells can be increased 7-8 times(from 3 to 20 or more).Conceivably, proliferation of the antipodals is sustained at the expense of the disintegration of nucellar tissue. The present investigation has confirmed our previous statement that transport of disas- sembled protoplasm is involved in the feeding of antipodals by nucellus.Some electron mic- rographs are chosen to reveal details of this particular process.Some findings of special in- terest are listed below: 1.Cells that make up the nutritional pathway at the chalazal end are small,closely packed,and rich in mitochondria.Its wall is thickened irregularly by heavy deposition of el- ectron-translucent material and is interspersed with prominent bundles of plasmodesmata (Fig.2).It seems likely that the inlet passage is predominantly symplastic in nature.2.Wi- thdrawal of cell contents from the nucellar tissue at the early stage is carried out by efflux of nuclear substance(karyoplasm)through enlarged openings on the nuclear envelop,and by exokaryosis of vesicles packed with ribosome-like granules(Fig.6).These vesicles can then be trapped in the ER cavity.Breakdown of the endomembrane system follows next.A mu- ltitude of small vacuoles,vesicles(coated or not)impregnated with sap,fibrils and granules respectively,deformed mitocondria,chromatic aggregates,etc.can be found in suspension within the deteriorating cell(Fig.7,10).In addition,degradation of polysaccharides can also take place.Withdrawal of the cell content from shrinking nucellar layer and its flow into the antipodal section is through the ruptured wall where the cellulose skeleton is tur- ned loose and fluffy at the opening.The protoplasmic fragments in transport are those structures of definite submicroscopic constitution,resulted from disassembly and disinteg- ration of the protoplasm and from reassorment of protoplasmic constituents.3.The antipo- dal cells are separated from each other by partial walls riddled with cytoplasmic canals.The naked portion of the cell can be in direct access to the invading fragments,which can be utilized and incorporated by the antipodal cell and participate in the building of new cell possibly by reorgnization(Fig.12a)which may be a special mode of cell proliferation in antipodals.Amitosis of antipodal nucleus also has been observed(Text Fig.3). Discussion is made in regards to the physiological significance of the nutritional supply in form of protoplasmic fragment and of the self-propelling mobility of the fragment.Alth- ough the antipodals still proliferate to some extent even after fertilization,they meet the same fate as its predecessor,the nucellus,and soon vanish during the establishment of en- dosperm. In food transport,the interconversion of polymer-monomer of saccarides,etc.is fre- quently involved.In the present case of material transport,interconversion at higher levels plays a dominant role as shown in the assembly and disassembly of protoplasmic constituen- ts and in the orgnization and disorgnization of ephemeral tissue.