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Unique features of the plant life cycle and their consequences

Nature Reviews Genetics volume 4, pages 369–379 (2003)Cite this article

Key Points

  • Plants lack a germline, express many genes in the haploid phase of the life cycle and have a flexible developmental plan. These features allow stringent selection in the haploid phase against deleterious mutations that could accumulate during somatic growth.

  • During reproduction, flowering plants experience two fertilization events: the egg and sperm joining to make the zygote, and a second sperm fusion with the (typically) diploid central cell to produce a triploid endosperm.

  • In most plants, the growth of the embryo depends on the nutritive support of the endosperm.

  • Parent-of-origin effects on gene expression in flowering plants are, so far, restricted to the endosperm.

  • Immediately after fertilization, maternal alleles are required for the development of normal endosperm, and, therefore, for viability of the embryo.

  • During the growth of the endosperm, paternal alleles and a correct ratio of maternal:paternal chromosomes are required to support a robust endosperm.

  • Both maternal and paternal alleles are, therefore, crucial for successful plant reproduction in the seed.

  • This conclusion is not absolute; there are many instances of the vegetative production of ectopic plantlets and apomixis (embryo development without fertilization in a seed) that illustrate that diverse plant cell types can organize to form a new individual.

Abstract

Continuous development, the absence of a germline, flexible and reversible cellular differentiation, and the existence of haploid and diploid generations — both of which express genes — are characteristics that distinguish plants from animals. Because these differences alter the impact of mutations, animals and plants experience varied selection pressures. Despite different life-cycles, both flowering plants and multicellular animals have evolved complex sensing mechanisms that act after fertilization as 'quality checks' on reproduction, and that detect chromosome dosage and the parent of origin for specific genes. Although flowering plant embryos escape such surveillance in vitro, embryo success in the seed often depends on a healthy endosperm — a nutritive tissue that is produced by a second fertilization event in which maternal and paternal gene contributions can be monitored immediately after fertilization and throughout development.

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Figure 1: Comparison of plant and animal life cycles.
Figure 2: Seedlings produced along the Kalanchoë leaf margin.
Figure 3: Relative contribution of embryo and endosperm to mature seeds.
Figure 4: Microgametophyte development and B-A chromosome behaviour.

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Authors and Affiliations

  1. Department of Biological Sciences, Stanford University, Stanford, 94305-5020, California, USA

    Virginia Walbot

  2. Department of Plant Biology, Carnegie Institution of Washington, 260 Panama Street, Stanford, 94305, California, USA

    Matthew M. S. Evans

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DATABASES

MaizeGDB

ig

R

TAIR

DDM1

DME

emb30

FIE

FIS2

MEA

SIN1

FURTHER INFORMATION

Virginia Walbot's laboratory

Zea mays Database (ZmDB)

Glossary

DOUBLE FERTILIZATION

The process by which two cells in a megagametophyte fuse with two sperm (typically from the same pollen grain) to produce both a diploid embryo and an accessory organ — the endosperm. Double fertilization is characteristic of angiosperms, but also occurs in other taxa in which the result is usually the production of two embryos.

ENDOSPERM

A tissue, found in flowering plants, which is generated by the fusion of the central cell of the megagametophyte and a sperm. In most angiosperms, the endosperm is triploid, with two genome equivalents from the maternal line and one from the paternal line; however, there are many exceptions to this general rule.

AUTOTROPHIC

Able to independently acquire a nutrient.

INFLORESCENCE

The reproductive portion of a plant that bears a cluster of flowers in a specific pattern.

SOMA

The cells of the body that cannot undergo meiosis. In plants, this comprises the entire plant body until the late specification of reproductive cells in flowers. By contrast, animals have a somatic body and a germline that differentiates early in development; at reproductive maturity, the germ cells proliferate, undergo meiosis and the meiotic products differentiate into gametes.

APOMIXIS

The production of seed without embryo fertilization, which can involve direct embryogenesis from somatic cells or the development of meiotic products into embryos.

GYNOGENETIC

An individual that develops from a cell in the megagametophyte (typically the egg) and, therefore, contains only maternal chromosomes.

ANDROGENETIC

An individual that develops from a sperm and, therefore, contains only paternal chromosomes.

DICOT

A flowering plant with two embryonic initial leaves, known as cotyledons.

MONOCOT

A flowering plant with a single cotyledon in the embryo.

POLYCOMB

A class of proteins — originally described in Drosophila melanogaster — that repress the expression of the genes with which they are associated. There are several classes of polycomb proteins and in higher plants they are organized into gene families.

PHENOCOPY

A mimic of a phenotype that is caused by a known mutation.

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Walbot, V., Evans, M. Unique features of the plant life cycle and their consequences. Nat Rev Genet 4, 369–379 (2003). https://doi.org/10.1038/nrg1064

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