Angiosperm Life Cycle

Plants, like animals, can reproduce sexually (as well as asexually). In animals, the organism’s body is always diploid (except in male bees, wasps, and ants) and produces haploid gametes through meiosis. Plants, unlike animals, have two different generations during their life cycle. A diploid generation produces haploid spores through meiosis, and a haploid generation produces gametes through mitosis.

Flowers are the structures that produce and bear this haploid generation in flowering plants (angiosperms). We will describe the general structure of angiosperms, how the two generations alternate in their life cycle, how this is related to their sexual reproduction, and the differences with gymnosperm reproduction.

Angiosperm structure

Angiosperms are vascular plants, a group of plants that present tissue differentiation including vascular tissue for nutrient transport. The other groups of vascular plants are gymnosperms (pines, cycads, cypresses) and seedless vascular plants (ferns, clubmosses, and horsetails). All vascular plants share a basic body plan and structure of organ systems and tissues.

Angiosperm organ systems

An angiosperm plant has two organ systems.

Fig 1 - The angiosperm plant body is organized in an underground root system and an aerial shoot system.

• Root system: the underground system of a plant composed of roots. Its main functions are to anchor the plant to the soil, absorb water and minerals, and sometimes store nutrients.
• Shoot system: the aerial part of the plant composed of stems, leaves, and flowers. The vegetative or non-reproductive organs include the leaves where photosynthesis occurs (providing the food for the plant), and the stems bear the leaves and flowers, and give structural support to keep them upright. On the other hand, the flowers are the reproductive structures that bear the gametes (sexual reproductive cells) and where sexual reproduction takes place in angiosperms.

Angiosperm plant tissues

The plant organs are made of several tissues, which are divided into three main types:

• Dermal tissue: like our skin, it is the outer cover of the plant organs that gives protection from the environment and allows gas interchange with the environment.
• Vascular tissue: it forms the transport system of the plant, connecting the root and the shoot systems. Water and minerals absorbed by the roots and organic molecules synthesized through photosynthesis are transported throughout the plant.
• Ground tissue: all other plant tissues that are not dermal nor vascular are ground tissues (also called fundamental tissue). They are below the dermal tissue and surrounding and inside the vascular tissue. Ground tissues have diverse functions including support, storage, and photosynthesis.

Besides these differentiated or specialized tissues, there are regions of undifferentiated cells in the plant that continuously divide. These are regions of plant growth called meristems.

Angiosperm life cycle

All plants and some algae have a similar life cycle known as alternation of generations, meaning that they have a haploid stage and a diploid stage. The diploid stage, also known as the sporophyte (diploid non-sexual phase or generation) alternates with the gametophyte stage (haploid sexual generation). The main steps of a plant life cycle, starting with the zygote formation, can be summarized as follows:

• The male gamete (usually carried in the pollen) fertilizes the female gamete (the ovule) resulting in a diploid zygote. In angiosperms, this occurs in the female part of the flower (specifically inside the ovary).
• The zygote grows through mitosis becoming the multicellular diploid sporophyte (division through mitosis does not change the number of chromosomes).
• At maturity, a tissue called sporangium is formed in the sporophyte flower. It contains a sporocyte cell (spore mother cell) that produces haploid spores through meiosis (division through meiosis reduces the number of chromosomes from 2n to n).
• These haploid spores undergo mitosis, forming the multicellular haploid gametophyte.
• The gametophyte produces the haploid gametes (sexual reproductive cells) through mitosis.

These stages and the names of tissues or structures are common for all plants and algae life cycles (except for the flower part that is specific to angiosperms). Each group of plants has variations or modifications to this basic cycle, and some specific structures differ (like cones for gymnosperms and flowers for angiosperms).

Angiosperm life cycle diagram

The diagram below (Fig. 2) shows the general life cycle of a plant, you can identify in this cycle the main stages described above. Remember that there are male and female gametes (sperm and eggs respectively) and that they come from separated male and female gametophytes. Thus, gametophyte development has variations for each sex.

Fig 2 - Diagram of the cycle of angiosperms

The angiosperm life cycle includes two main parts: since the sporophyte is the non-sexual generation, it corresponds to the growth phase of a plant (some plants can reproduce asexually though). The gametophyte phase corresponds to the sexual reproduction of a plant which we describe below.

Angiosperm reproduction cycle

The reproduction cycle in plants corresponds to the sexual generation, the haploid gametophyte. In angiosperms, sexual reproduction occurs in the flower since this structure produces the gametophytes. Below we describe the process for the development of male and female gametophytes separately. The general process is the same, but with some variation for each sex.

Development of female gametophytes and gametes

The female gametophyte is produced inside the ovary (the swollen base of a carpel). An ovary can have one or more ovules. An ovule is composed of a tissue called the megasporangium (plural microsporangia) enclosed by the integuments (two layers of protective tissue that have a small opening called micropyle). One cell in the megasporangium differentiates into a megasporocyte (megaspore’s mother cell). Each megasporocyte enlarges and undergoes meiosis, producing 4 haploid daughter cells that are the megaspores (the first female haploid cells in the plant life cycle). Three of these megaspores usually degenerate and only one survives.

Fig 3 - Diagram of female gametophyte development in angiosperms

Fig 4 - Micrograph of a Lilly ovary (cross section), the dashed lines delineate one of the three fused carpels with two ovules showing in this section, one with a visible megasporocyte. Source: Ilse Anahi Carrascos, CC BY 4.0 via Wikimedia Commons

The surviving megaspore then goes through mitosis three times, but without cytokinesis (the splitting of the cytoplasm), resulting in one large cell with eight haploid nuclei. Membranes form around six of the nuclei, resulting in six small cells and the original large central cell, with the two remaining nuclei. These seven cells and eight nuclei form the embryo sac that is the female gametophyte. The eight nuclei have specific locations inside the embryo sac:

• Three cells stay near the micropyle, one being the egg cell (female gamete). This makes sense as the pollen tube that delivers the sperm enters through the micropyle. The other two, called synergid cells, seats one on each side of the egg and are thought to attract the pollen tube.
• Other three cells, called the antipodal cells, remain on the opposite side of the micropyle. Their function is unknown.
• The remaining two nuclei, called polar nuclei, stay at the center of the large cell. These also participate in the fertilization process.

Development of male gametophytes and gametes

Fig 7 - Cross section of an antherSource: Bruce Kirchoff via flickr

Table 1: The differences between the life cycles of gymnosperms and angiosperms.