What is Meant by the Life Cycle of Plants? 

All the organisms that belong to the Kingdom Plantae are known as plants. They are multicellular and eukaryotic and can synthesize their food by photosynthesis, known as autotrophic organisms. They are classified based on certain characteristics, such as the plants bear flowers; some have only naked seeds and do not contain flowers. Some plants neither have seeds nor flowers, such as ferns and mosses.

Phases of a Plant's Life Cycle

Plants complete their life cycle in two phases or generations that are morphologically and, in many cases, chromosomally different. This cycle of reproduction is known as the plant life cycle or the alternation of generation. Generally, this cycle includes both sexual and asexual phases and different types of events during the plant's life. All the events that take place from the formation of the zygote to the formation of the gamete complete the reproductive cycle of plants.

The plant life cycle includes the diploid (sporophyte) and haploid (gametophyte) phases. Generally, the term sporophyte is used for spore-producing plants, and the term gametophyte is used for plants that produce gametes. The haploid gametophytic body produces gametes by mitosis, and the production of meiospores in the diploid sporophyte takes place by meiosis. The diploid phase gives rise to the haploid phase by meiosis. The haploid gametes then fuse to produce a zygote that marks the beginning of another diploid phase. 

Sporophyte

The diploid multicellular phase, which is part of the plant's life cycle, is known as the sporophyte. During the formation of the sporophyte, the formation of the diploid zygote takes place by the fusion of two haploid gametes. Generally, organisms that have an 'n' number of chromosomes are named haploid organisms. Since gametes are haploid, when two gametes of the same species fuse, then the formation of the diploid zygote takes place that contains 2n number of chromosomes. Thus, the zygote contains double the amount of DNA (deoxyribonucleic acid) but the DNA codes for the same type of proteins in the same organism. The sporophyte formation occurs through multiple mitotic divisions of the zygote, and thus the sporophyte retains its 2n condition.

During the maturation of the sporophyte, an essential event occurs during the alternation of generations. A major organ called sporangia is produced from the sporophyte. The sporangia are specialized reproductive organs used to produce single-celled, haploid spores. After the maturation, these cells are discharged into the open environment, which may be air or water. When the spores find a favorable or stable environment for reproduction, they germinate into the gametophyte phase. 

In seed plants such as gymnosperms and angiosperms with well-developed roots, stems, leaves, cones, and flowers, the sporophyte phase is more remarkable than the gametophyte phase.

Gametophyte

A gametophyte is another phase that is present in the alternation of generations and produces the haploid spore. Technically, the spore is a new organism containing only half the DNA respective to the parent organism. The spores divide many times by the mechanism of mitosis and thus form a new multicellular individual known as the gametophyte. Thus, the sporophyte phase produces spores, whereas the gametophyte generation creates haploid gametes. The gametangia are a type of special organ that produces gametes. The gametes that are produced by gametangia are spread into the environment or transferred between plants.

As the two opposite gametes come close together, they fuse and form another zygote. This type of zygote eventually becomes a sporophyte, and thus the plant life cycle continues.

Seed

Seeds are formed during the reproductive phase. After maturation, the ovule grows into the seed, and the ovule's integument grows as a seed coat. Seeds play an important role in the production of plants and their nourishment. The seed is a medium of reproduction and thus helps maintain the different types of plants. A mature seed contains an embryo that is nourished by the reservoir food of the seed. During the process of germination of the seed, the radicle and plumule are formed, and finally, the radicle grows as root, and the plumule grows as the shoot. In certain plants, the shoots are differentiated as stem, leaves, and flowers. The flower is the reproductive organ and plays an important role in the process of reproduction. The color of the flower depends on the nature of the pollinators, such as insects or birds. The period during which the seed becomes metabolically inactive is known as dormancy of the seed. Many factors are responsible for seed dormancy, such as hormones (abscisic acid) and environmental factors.

Types of Plant Life Cycles 

Based on certain characteristics, plants' life cycles are classified in three ways:

1. Haplontic life cycle 

In the haplontic process, the diplontic sporophyte phase is shown only by the one-celled zygote. There is no free-living sporophyte. The gametophyte that represents the photosynthetic phase of the haplontic process is the dominant phase. Zygotic meiosis is a type of meiosis of a zygote that occurs immediately after the process of karyogamy. Karyogamy is a process in which the mating of two nuclei of the cells takes place. At the end of the diploid phase, many haploid cells have generated that divide by mitosis and give rise to different types of cells that grow into larger, multicellular plants. The formation of a zygote takes place when the mating of two opposite gametes of these cells occurs.

In the complete cycle, zygotes are the only diploid cells, and mitosis occurs only in the haploid phase.

Since the cells or organisms produced from mitosis are haplonts, this life cycle is named the haplontic life cycle. Some green algae such as Spirogyra show the haplontic life cycle.

2. Diplontic life cycle

In the diplontic process, the zygote division takes place mitotically and thus produces a multicellular individual. The diploid phase is the dominant phase represented by the independent and photosynthetic phase. Some specific cells of the multicellular diploid undergo meiosis to form gametes. The formation of gametes takes place directly by the process of meiosis, hence it is named gametic meiosis. The haploid cells divide multiple times by mitosis to produce several haploid cells, as observed in yeasts where the haploid phase is not paramount. In most cases of diplonts, mitosis occurs only in the diploid phase, resulting in gametes' formation. Further, these gametes fuse to form a diploid zygote.

In the complete process of the cycle, gametes are the only haploid cells that are formed. Moreover, mitosis only takes place in the diploid phase. Since the individual is a diplont, so the gametic meiosis is known as a diplontic. Generally, this kind of life cycle is not common in plants. However, certain brown algae, such as Sargassum and Fucus, show this type of life cycle.

3. Haplo-diplontic life cycle

In the haplo-diplontic process (also named diplobiontic), the haploid spores (meiospores) are produced by the diploid sporophyte through meiosis. Because meiosis directly produces spores, this process is also known as sporic meiosis. The meiospores germinate and grow into the multicellular haploid gametophyte through mitosis. The gametophytic bodies produce gametes by mitosis. Fusion of gametes gives rise to a diploid zygote. In sporic meiosis or intermediary meiosis, the zygote division takes place mitotically that produces a multicellular diploid sporophyte. This sporophyte produces haploid spores through meiosis. These spores form the haploid structure called a gametophyte by the process of mitosis. The gametophyte undergoes mitotic division to produce gametes. The nature of the gametophyte varies in different plants; it may be short-lived and small in certain cases while it may be dominant and long in other cases. In this type of process, both the sporophyte and gametophyte are multicellular but show differences in the condition of dominance. This type of life cycle is found in all land plants, such as bryophytes, pteridophytes, gymnosperms, angiosperms, and certain algae such as Polysiphonia and Ectocarpus.

Context and Applications

This topic is significant in many bioscience examinations for both undergraduate and graduate courses, especially for: 

• Bachelor of Science (Honors) in Botany
• Bachelor of Science in Life Sciences
• Bachelor of Science in Biotechnology
• Bachelor of Science in Biochemistry
• Master of Science in Biochemistry
• Master of Science in Biotechnology 
• Master of Science (Honors) in Botany

Related Concepts

1. Metagenesis
2. Heterogeneric
3. Alternation of generation
4. Macrogametophyte
5. Megagametophyte
6. Gametophore
7. Spore mother cell