In this article we will discuss about:- 1. Callose Wall and Meiocytic Syncytium 2. Ultra-Structural Changes in Microsporocytes 3. Cytokinesis.
The sporogenous cells generally function directly as microspore mother cell, also called microsporocyte or pollen mother cell. They undergo meiotic division to give rise to microspore tetrads.
This division is of tremendous importance in the life cycle of the Pteridophytes and Spermatophytes, as it gives rise to the haploid gametophytes. The necessary meiotic stimulus is believed to originate in the vegetative shoots and is highly specific in nature, i.e. being effective on the microsporocytes only, and not even on the tapetal cells through which it must pass in order to reach the microsporocytes.
Meiosis halves the chromosome number of the microsporocytes and in the process produce four microspores with the haploid number of chromosomes.
This is achieved by two successive nuclear divisions viz., meiosis I, or the heterotypic division, and meiosis II, or the homotypic division. While describing the prophase of meiosis I it is usual to recognize five distinct stages, viz., leptotene, zygotene, pachytene, diplotene, and diakinesis.
During leptotene, the chromosome has two chromatids that appear long, thread like and without definite structure. Leptotene passes on to zygotene where pairing of homologous chromosomes takes place to produce the bivalents.
Synaptonemal complex between the homologous chromosomes reflects the pairing process. Using the technique of density gradient centrifugation, SDS-PAGE, and immunoblotting it has been possible to show that in Lilium longiflorum the synaptonemal complex of microsporocyte is composed of more than 20 proteins, of which 5 are specific for the complex.
Separation of the paired chromosome partner denotes the beginning of the diplotene stage. As meiosis advances from diplotene to diakinesis the chromosomes attain their maximum contraction. Prophase is now followed by the remaining regular stages of meiosis. The nuclei of the two dyads quickly complete the meiosis II, thus generating four microspores in a tetrad.
Contents
1. Callose Wall and Meiocytic Syncytium:
The plasmodesmata interconnect the microsporocytes with one another and with the tapetum. Following the entry of the microsporocytes into the meiosis, the plasmodesmatal bridges with the tapetum is snapped.
After the discontinuity of the plasmodesmata connection among the microsporocytes, it is covered by a primary wall made up of cellulose, and shortly before meiosis this wall disintegrates and is replaced by a massive deposit of callose (β-1,3- glucan), outside the plasma membrane.
Callose deposition starts at the corners of the cells between the plasma membrane and the original wall. However, the primary wall persists in Allium tuberosum and Cyclamen persicum until late tetrad stage. The possible reason for the delay is to form a barrier in the entry of macromolecules in the microsporocytes, thus ensuring autonomous development of microspores.
The deposition of the callose is initially incomplete, leaving many gaps through which there is an establishment of massive cytoplasmic channels between the microsporocytes. These channels are 1 -2 pm in diameter and attain their maximum development in the zygotene- pachytene stage. Thus at this stage the highly interconnected mass of microsporocytes in the locule, form a large meiocytic syncytium (Fig 1.7).
This massive coenocyte provides a channel for the transport and distribution of metabolites. Further it imposes a mutual influence of one cell over the other, thus helping to maintain a close synchrony during meiosis among the large number of microsporocytes in the anther locule.
At the close of meiotic prophase the callose walls of the microsporocyte lock up, and the cytoplasmic channels are cut off and now the microsporocytes go through the rest of the meiosis as isolated cells.
At the end of metaphase I or II, the callose wall around microspore mother cells become continuous. The isolation of the microspore mother cells and then the microspores, by a callose wall seems essential for the normal development of the pollen grains.
Failure of callose wall development or its early breakdown results in pollen sterility. Table-1.2 shows the formation of walls and intercellular connections in tapetum, meiocytes and meiospores during microsporogenesis.
The accurate determination of the timing and pattern of DNA synthesis during microsporocyte meiosis is markedly complex. Studies involving autoradiography and microspectro- photometry have established that the main DNA synthesis period for the microsporocytes begins shortly before leptotene and continues into leptotene.
For example in Lilhim longiflorum autoradiography of 32P incorporation, followed by enzyme digestion and acid hydrolysis, for eliminating RNA and phosphoproteins, identified a preleptotene period of DNA synthesis.
Whereas microspectrophotometric measurement showed a 4C DNA amount in the microsporocytes before their entry into meiosis, a transient 2C amount after meiosis I, and a 1C amount in the microspores.
What prompts sporogenous cells to cease mitotic divisions and enter the meiotic cycle is not known. There is a prediction that meiosis is triggered by the synthesis of some factors in tissues other than the sporogenous cells. These substances have trival names as meiosis determinants, or meiosis-inducing substances.
Few attempts have been made to clone meiosis specific genes from microsporocytes and identify the protein products they encode. However, none of the genes so far characterized can be considered to be meiosis specific because they encode common proteins associated with cell metabolism such as HSP (heat shock proteins), serine proteases, proteins involved in DNA repair, and leucine zipper proteins.
Thus the action of specific genes that control the entry of cells into and their exit from meiosis is still a matter of distant dream. The cytoplasm of meiocytes undergoes profound changes during meiosis and there is a significant fall in the cytoplasmic RNA.
Functions of Callose Wall:
The possible functions of the callose envelope are:
i. To control major features, such as the arrangement of apertures, which are probably related to the geometry imposed by the callose wall.
ii. To isolate the young microspore from influences of tapetal cells during early stages of development. This isolation enables the young microspores to deposit a primexine without interaction between them and the tapetum. Knox (1984) thus emphasized that the callose wall serves to separate the gametophyte from the sporophyte.
iii. Participates in the development of wall ornamentation. Godwin (1968) described the action of callose wall as a template, defining the position of apertures and the deposition of the primexine in Ipomoea.
iv. Protects the meiocytes from dehydration under condition of deficient water supply.
v. It isolates and insulates meiocytes for the normal completion of meiosis. This isolation provides avenues for two major events, viz., transition from sporophytic phase to gametophytic phase, and the expression of the gametophytic genome which is essential for achieving the limited function of gamete formation and their discharge in the embryo sac.
vi. After its break down it serves as a source of soluble carbohydrates for the developing pollen.
2. Ultra-Structural Changes in Microsporocytes:
Nuclear and chromosomal rearrangements during meiosis produce interesting changes in the ribosome population, nucleoli, plastids and mitochondria.
Studies conducted by Dickinson and Heslop-Harrison (1977) in the microsporocyte of Lilium longiflorum showed that there is an almost complete elimination of ribosomes in the early meiotic prophase (zygotene-pachytene), followed by their restoration at the metaphase-anaphase stages of meiosis I indicates that there is a close relationship between the ribosome change and RNA content of the microsporocyte.
Further the replenishment of the ribosome population is believed to be related to the formation of RNA- enriched accessory nucleoli that flood the microsporocyte cytoplasm at the anaphase of meiosis I.
The plastids show clear changes during meiosis. They undergo cyclic dedifferentiation and re-differentiation. The dedifferentiation changes involve the loss of starch, pleiomorphism, and erosion of internal structure by the late zygotene stage. Dedifferentiation is delayed until metaphase of meiosis I and starts off with the transient appearance of granule- double membrane associations in the stroma.
After the release of microspores from the tetrad, the membrane-particle associations disappear and the plastids constitute their normal structure. In the mitochondria there is a structural simplification by the zygotene stage and the recovery of the cristae structure in the early-stage microspores (Table 1.3).
There is a general belief from the ultra-structural point that the transition of the microsporocyte to the gametophyte phase results in the elimination of much of the sporophytic programme, and the installation of a programme for gametophytic functions.
3. Cytokinesis:
The microsporocytes after the meiotic division undergo cytokinesis, by any of the two processes, viz., simultaneous or successive.
i. Simultaneous Wall Formation:
In this case the first meiotic division is not accompanied by wall formation, as a result after meiosis I a binucleate cell is formed. The two haploid nuclei undergo the second meiotic division synchronously by virtue of their common cytoplasm. Later callose walls are laid down as centripetally growing furrows, which meet in the centre of the cell to produce a tetrad, e.g., Drimys winteri. (Fig 1.8)
ii. Successive Wall Formation:
Immediately after the first meiotic division wall is laid down centrifugally to form a dyad. The cell plate is formed in the center and then extends laterally. It is followed by the deposition of callose on either side of the plate. The cells of the dyad undergo second meiotic division followed by callose wall formation in the same as the first division, thus resulting in a tetrad, e.g. Commelina subulata. (Fig 1.9).
