The sporogenesis is part of the life cycle Haplo-diplonte in plants, in which microspores and megasporas occur. They are produced by the diploid sporophyte, which by meiosis produces a tetrad of haploid spores. This is the part of the cycle where the diploid-haploid change is made. Depending on whether it occurs in the male part (anthers) or female part (ovules), we will talk about:
- microsporogenesis
- megaporogenesis
Microsporogenesis . This process takes place in the sporagenic tissue of the anthers, the pollen stem cell nuclei lengthen just before meiosis and in the formation of the microspore tetrads the callus wall around each pollen stem cell can be detected during the initiation of meiosis and in each new young microspore of the tetrad, which is also separated by a callus layer after meiotic division II, as soon as the formation of the outer wall of the pollen begins to form, the callus begins to disappear. The meristematic layers that give rise to the different structures or organs in plants are called L1 that give rise to the epidermis ( chloroplast), L2 that form the leaves, stems and flower (sexual organs) and L3 that generate the internal tissues of the stem.
Megagasogenesis . This process takes place in the tissue of the ovary (it contains from 300 to 600 ovules) and in the stem cells of the megaspores, giving rise to reproductive cells called embryonic sacs, which contain the female gamete or ovocell. A maternal cell, macrospore or megaspore, through the meiotic division I and II forms four megaspores, the top three degenerate and the bottom becomes MacrosporeFunctional, this will give rise to the embryonic sac, which is enlarged by megagametogenesis, with three mitotic divisions occurring, without cytokinesis, which will give rise to eight nuclei that, when oriented, form the embryonic sac. One of the 3 nuclei at the micropillar end of the embryonic sac is transformed into a functional egg. There are several alternatives in the formation of the embryonic sac, the one described occurs in most Angiosperms.
Megasporogenesis in gymnosperms
In gymnosperms and flowering plants, the macrospore occurs inside the nucleus of the ovule. During megasporogenesis, a diploid cell precursor, the megaspore or megasporocyte stem cell, undergoes meiosis to initially produce four haploid cells (the megaspores). Angiosperms present three models of megasporogenesis:
- Monosporic or Polygonum type: This pattern occurs more frequently (> 70% of angiosperms) and is found in many economically and biologically important groups, such as Brassicaceae ( Arabidopsis , Capsella , Brassica ), Gramineae ( corn , rice , wheat ), Malvaceae ( cotton ), Leguminoseae ( beans , soybeans ) and Solanaceae ( pepper , tobacco , tomato , potato , petunia). This pattern is characterized by the formation of a plaque of cells after meiosis 1 and 2, resulting in four uninucleated megaspores of which three degenerate.
- Bisporic or Alisma type: This pattern is characterized by the formation of only one cell plate after meiosis I, and as a result two binucleated megaspores form, one of which degenerates.
- Tetrasporic or Druse type: In this pattern after I and II meiosis a tetranucleated megaspore is formed. Therefore, each pattern gives rise to a single functional megaspore, which contains one, two, or four meiotic nuclei, respectively. The megaspore then undergoes megagametogenesis to give rise to the female gametophyte.
