L14 Plant Reproduction

一、Mitosis & Meiosis

Life Cycle of A Flowering Plant

Both gametophytes, the male and the female, represent the haploid generative phase of the life cycle of higher plants

Eukaryotic Chromosome

The eukaryotic chromosome is a segregation device

1. centromere – A constricted region of a chromosome that includes the site of attachment (the kinetochore) to the mitotic or meiotic spindle.

• It may consist of unique DNA sequences or highly repetitive sequences and proteins not found anywhere else in the chromosome.

2. acentric fragment – A fragment of a chromosome (generated by breakage) that lacks a centromere and is lost at cell division.

Mitosis

1. Procedure

Eukaryotic cell division for vegetative cells

Cell division that results in two daughter cells each the same as the parent nucleus, typical of ordinary tissue growth

Prophase

Chromosomes begin to condense

1. Chromosome condenses with two chromatids held together at centromere
2. Developing bipolar spindle
3. Developing spindle pole

Metaphase

Chromosomes align alone cell center

Chromosomes alligned halfway between poles

Anaphase

Separated chromatids being pulled toward poles

Telophase

Cell wall forms

Cytokinesis

Cell division complete

2. Cytoskeletal Organization

Cytoplasmic strands containing actin filaments span the vacuole, linking the cytoplasm surrounding the nucleus with the cortical cytoplasm.

During prophase, a cortical preprophase band (PPB) of microtubules circumscribes the future plane of cell division.

Actin filaments are distributed throughout the cell cortex during prophase; some are aligned together with the microtubules of the PPB.

3. Centromeres

Regional centromeres contain a centromeric histone H3 varient and repetitive DNA

Centromeres are characterized by a centromere-specific histone H3 variant and often contain heterochromatin that is rich in satellite DNA sequences.

The function of the repetitive DNA is not known.

A model of the overall structure of a regional centromere

Plant Centromeres: Tandem Repeats

All centromeres have short tandem repeats (approx. 170 bp unit length) of different, but extensive, length, and transposon-like moderately repeated sequences surrounding the tandem repeat sequences.

Distribution of centromere-specific repetitive sequences of rice in a region of more than 630 kb. Short tandem repeat blocks (“RCS2”) in one region, plus retrotransposon-related sequences (RIRE3, RIRE8 and RIRE7) and the RCE1 sequence.

Meiosis

1. Location of Gametogenesis

Pollen (花粉) in the anthers (花药)

Ovules ([植]胚珠；幼籽；[动]卵细胞) in the gynoecium (雌蕊群, 雌蕊)

2. Pollen Development

Pollen development in young anther tissues: Sporangial layer → pollen spore mother cells (2N) → a “tetrad” (four) of microspores (1N)

Mitosis of microspores to produce two cells/nuclei

• Vegetative nucleus.

• Generative nucleus.

Bi-nucleated pollen:

• Only vegetative and generative nuclei at dehiscence.

Tri-nucleated pollen:

• Mitosis of generative nucleus occurs before dehiscence.

3. Bicellular & Tricellular Pollen

The majority of flowering plants produce bicellular pollen, many plants such as Arabidopsis, rice, wheat and maize produce advanced, but often short-lived, tricellular pollen grains (A) Bicellulartomato and (B) tricellular oilseed rape pollen stained with DAPI. Nuclear DNA within the vegetative (V) and generative (G) or sperm (S) cells are highlighted.

In both cases, the generative cell undergoes mitosis to form two sperm cells for double fertilization. (the difference is the timing of the mitosis – before or after anthesis/germination

4. Ovule Development

Ovule development in young carpel:

The nucleus of the megaspore undergoes three successive mitotic divisions forming eight nuclei. The megaspore enlarges into an oval shaped structure called the embryo sac. The eight nuclei of the embryo sac arrange themselves in 3 groups.

This diploid megaspore mother cell increases in size and undergoes meiosis to form a linear tetrad of 4 haploid megaspores, 3 of which degenerate and the 4th becomes the functional megaspore.

Double Fertilization

Double fertilization to create the embryo and endosperm

The unfertilized ovule contains the haploid egg cell and the double haploid central cell. Seed development follows double fertilization of the egg and central cells. Growth of the seed is driven by rapid divisions of the triploid endosperm derived from the central cell, accompanied by elongation of the maternal integuments.

In the mature seed, the embryo has grown to full size, replacing most of the endosperm, and the integuments have differentiated into the seed coat..

After Fertilization

Seed development

1. Seed Germination

In most monocots, the single cotyledon remains underground.

The growing shoot emerges while protected by a sheath.

In some dicot species, the cotyledons emerge above ground, protecting the stem and first foliage leaves.

In other dicots, the cotyledons stay underground and provide a food source for the growing seedling