seed physiology

angiosperm plant life-cycle
http://8e.devbio.com/image.php?id=525
Figure 4 Life cycle of an angiosperm, represented here by a pea plant (genus Pisum). The sporophyte is the dominant generation, but multicellular male and female gametophytes are produced within the flowers of the sporophyte. Cells of the microsporangium within the anther undergo meiosis to produce microspores. Subsequent mitotic divisions are limited, but the end result is a multicellular pollen grain. The megasporangium is protected by two layers of integuments and the ovary wall. Within the megasporangium, meiosis yields four megaspores—three small and one large. Only the large megaspore survives to produce the embryo sac. Fertilization occurs when the pollen germinates and the pollen tube grows toward the embryo sac. The sporophyte generation may be maintained in a dormant state, protected by the seed coat.

mitosis of microspores (1n) to microgametophyte / pollen grain (1n) yields
- vegetative cell (forms the pollen tube) (1n)
- generative cell 2 sperm (1n) cells

3 sets of mitosis (no cytokinesis) of megaspores (the only one that persists) (1n) to megagametophyte (2^3 = 8 in total)
- 3 antipodal cells (1n)
- 2 polar nuclei in the centre (each 1n)
- 2 synergid cells (1n) in egg aparatus
- 1 egg cell (1n) in egg aparatus

double fertilization (happens when pollen tube carrying 2 sperm cells reaches the egg aparatus):
- one sperm fuses with egg cells forming 2n zygote forming 2n embryo
- one sperm migrates from the synergid cell and fuses with the 2 polar nuclei forming 3n endosperm

key processes during seed maturation so seeds maintain vigor and longevity?
1. accumulation of food reserves
2. hardening of the seed coat (testa) (for structural support, interacts with wind, pathogens, prevent of loss by leaching) and accumulation of antimicrobial compounds
3. mechanisms of seed dispersal - flyers
4. synthesis of protective compounds that allow the seed to withstand some degree of water loss and adverse environmental conditions eg. antioxidants, antimicrobial, resistance to pathogens
5. dormancy - can happen for many years, needs signals or conditions to break the dormancy (eg. conifers at moist and low temperatures) or smoke

parts of the seed
1. embryonic root (radicle) and shoot
2. food reserve tissues or organs (cotyledons - legumes, endosperm - cereals and megagametophyte - pines)
3. seed coat (testa)


timing of events: stages of seed development
1. histodifferentiation - cell division
2. seed expansion (maturation) - genes for synthesis of reserves (proteins, lipids, carbohydrates) are expressed, deposition (depositing) of reserves
3 . maturation (drying) / dessication - water loss, embryo becomes quiescent state (metabolically inactive)

in situ - in place

gram-negative has lost the cell wall so does not retain the purple dye.
Most pathogenic bacteria in humans are gram-negative organisms. Classically, six gram-positive genera are typically pathogenic in humans. Two of these, Streptococcus and Staphylococcus, are cocci (coconut-shaped bacteria).

nutrients like sugar (sucrose) are transported to seed from parent plant via phloem, for peas, they are stored in the pods then later remobilized to the germinating seeds

classification of seed storage proteins:
- albumins - soluble in water
- globulins (7s vicilins and 11s legumins) - soluble in salt
- prolamins - soluble in alcohol
- glutelins - soluble in diulte acid and alkaline