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PLANT GENETICS

 

M.Tevfik DORAK

 

 

Plant taxonomy

Plant Kingdom has about 260,000 species divided into two Phyla (or divisions in plants) (see Plants Kingdom):

1. Bryophyta (non-vascular plants, lower plants): They lack a vascular system for the internal conduction of water, minerals and food (lower plants), and depend on direct contact with surface water. This group includes mosses, liverworts and hornworts. There is always an alternation of generations between morphologically distinct sporophyte and gametophyte. The familiar leafy plant of Bryophytes is the sexual, gamete-producing (gametophyte) generation of their life cycle.

2. Tracheophyta (vascular plants, higher plants): This group consists of plants that have a vascular system, i.e., xylem and phloem (water/mineral and food-conducting tissues, respectively). Tracheophyte leafy plants are the asexual, spore-producing, diploid (sporophyte) generation of their life cycle. One Subphylum, Pteropsida, consists of the following Superclasses:

i. Filinicae: Ferns. They do not reproduce by seeds but by spores like the Phylum Bryophyta. Alternation of generation is typical of ferns and Bryophyta.

ii. Gymnosperms: Cone-bearing woody seed plants. Includes cycads, gingko, conifers (pines, cedars) and gnetophytes.

iii. Angiosperms: Flower plants (divides into monocots and dicots). The gymnosperms and angiosperms are collectively called Spermatophyte (seed-bearing) plants. In this group, the gametophyte (haploid) generation does not occur as an independent plant (as in ferns). The vestigial gametophytes are contained in the sporophyte tissue as a few nuclei and can only be seen by a microscope (the embryonic sac and the pollen grain). The sporophyte embryo is contained in a seed which is dispersed from the plant. The angiosperms, therefore, cannot produce asexual spores and there is no obvious alternation of generations. The haploid pollen and ovule produced by a flower are thought to contain the remains of the gametophyte generation which was typical of the ancestors of the angiosperms (up to and including ferns). Link to Plant Evolution and Classification in Kimball's Biology Pages.

Plant evolution

Evolution of eukaryotes from a presumed bacteria-like ancestor is one of the major events in evolutionary history. They have a distinct nucleus, organelles involved in energy metabolism (mitochondria and chloroplast), extensive internal membranes and a cytoskeleton of protein fibres and flaments. Chloroplasts (photosynthesis) in green plants and algae originated as free living bacteria related to the cyanobacteria [the chloroplastic DNA is more similar to free-living Cyanobacteria DNA than to sequences from the plants the chloroplasts reside in]. The eukaryotic mitochondria (ATP synthesis) are endosymbionts like chloroplasts. Mitochondria were acquired when aerobic Eubacteria were engulfed by anaerobic host cells. As they conferred useful functions like aerobic respiration (mitochondria) and photosynthesis (chloroplasts), these organelles have been retained as endosymbionts. This must have happened after the nucleus was acquired by the eukaryotic lineage. The origin of eukaryotic nucleus is almost certainly autogenous and not a result of endosymbiosis. Mitochondria are believed to have originated not from cyanobacteria but from an ancestor of the present-day purple photosynthetic bacteria that had lost its capacity for photosynthesis (chloroplasts from an ancestral Cyanobacterium).

All land plants evolved from the green algae or Chlorophyta. In the period before the Permian (the Carboniferous), the landscape was dominated by seedless ferns and their relatives. Vascular plants first appeared in Silurian (439-409 Mya). After the Permian extinction, gymnosperms became more abundant. They evolved seeds and pollens (encased sperm). Angiosperms evolved from gymnosperms during the early Cretaceous about 140-125 Mya. They further diversified and dispersed during the late Cretaceous (97.5-66.5 Mya). Water lilies are one of the most ancient angiosperm plants. Currently, over three quarters of all living plants are angiosperms. The angiosperms developed a close contact with insects which promoted cross-pollination and resulted in more vigorous offspring. Their generation time to reproduce is short, and their seeds can be dispersed by animals. For these reasons, the angiosperms were able to travel and disperse all around the world. The important events in the evolution of the angiosperms were the evolution of showy flowers (to attract insects and birds), the evolution of bilaterally symmetrical flowers (adaptation for specialized pollinators), and the evolution of larger and more mobile animals (to disperse fruits and seeds). See also A Brief History of Life.

Polyploidy is an important mechanism in the evolution of plants. It is a situation in which the organism has more than two (2n) sets of chromosomes. It can be 3n, 4n or more. A high proportion (47%) of angiosperms are polyploid. It arises as a result of meiotic irregularities and results in sterile progeny which can still reproduce asexually. The original South American potato is a tetraploid (4n). Many of the common food plants (strawberries, apples, potatoes) are polyploid as this results in larger flowers and fruits (as well as larger cells, thicker and fleshier leaves). The wheat now grown for bread (T. aestivum) is hexaploid (6n = 42 chromosomes). Polyploidy can be induced by treatment of colchicines experimentally. Triploid offspring can be produced by crossing a colchicine-induced or naturally occurring tetraploid and a diploid. Odd number polyploids are sterile because they cannot segregate chromosomes evenly into gametes in meiosis. Sterility caused by triploidy is useful to produce seedless fruit that is easier to eat (banana) or better tasting (less bitter cucumbers). Polyploidy is a common mechanism for sympatric speciation (reproductive isolation without geographical isolation).

Plant biology

Plants are eukaryotic, multicellular organisms. A plant cell differs from an animal cell in that they have rigid cell walls composed of cellulose, chlorophyll containing plastids (chloroplasts), and are able to photosyntesize. Fungi are different because they lack chlorophyll and chloroplasts, their cell walls contain chitin. Algae were formerly thought to be plants because of their rigid cell wall and photosynthesizing ability. They are, however, currently placed in the Kingdom Protoctista because of the variety of cell pigments, cell wall types and different forms and structures. See Plant Anatomy and Physiology.

 

Plant reproduction

Asexual reproduction: Potato (tubers), strawberry (runners), iris (rhizomes) and gladiolus (corms) are common examples.

Sexual reproduction: Because land plants are immobile, alternation of generations has evolved in some groups to allow fertilization. The plants that possess leaves, roots, stems and flowers are sporophytes (asexually reproducing). This generation gives rise to the gametophyte generation (sexually reproducing). One spore type (microspore) develops into a male gametophyte and the other kind (megaspore) into a female gametophyte. The female gametophyte remains protected in the carpel of the flower. When fertilized, an embryo is formed (a seed) which is a young sporophyte able to form a new organism when germinated.

The female gametophyte (ovum) of a flowering plant is formed in the ovules at the centre of the flower. After meiosis, an embryo sac forms (this is the female gametophyte generation of the alternation of generations in flowering plants). This consists of the ovum and several other haploid cells. The male gametophyte is the pollen grain. Two haploid sperm are produced in each pollen grain. The pollen grain should reach to the stigma of the recipient -female- flower. Following pollination, it germinates and a pollen tube grows down into the ovule carrying the two nonmotile sperm. One of these fuses with the ovum to form a zygote while the other fuses with two other cells of the embryo sac to form a triploid nutritive tissue called endosperm. This is called double fertilization and is unique to plants. The zygote divides to give rise to two cells. One will form the embryo and the other a supportive structure (suspensor). Embryonic differentiation starts but does not proceed for long. When the development ceases, the embryo becomes packaged in a seed, specialized for dispersal.

Pollination: This can be achieved via self-pollination (autogamy) or cross-pollination (allogamy). Most plants reproduce by both self and cross-pollination. Cross-pollinating plants produce better-quality seeds and more varied (adaptable) offspring. Because of the advantages of cross-pollination, most plants have evolved mechanisms to prevent self-pollination. One of them is production of some chemicals that prevent pollen from growing on the stigma of the same flower, or from developing the pollen tubes in the style (self-incompatibility system). Some plants produce only one kind of (either male or female) flowers (dioecious) and some are dichogamous (the two separate sex organs develop at different times in the same flower: protandry or protogyny). Cross-pollination can be achieved by wind, insects (honey bee), bats and birds. One feature that developed as a result of insect pollination is pollen-tube competition. When a number of pollen grains is deposited on a stigma, each pollen grows a pollen tube to reach the ovule. Whichever reaches first, it fertilizes the ovule. The fastest growing pollen tube usually carries the best genes and results in a more vigorous offspring. Therefore, apart from avoidance of self-fertilization, there is selection for the best cross-pollinating pollen as well (sexual selection in plants). See also Pollination Adaptations.

Unique genetic features of plants

Ability to photosynthesize

Totipotency of plant cells

Hermaphroditism and ability to reproduce both sexually and asexually

Double fertilization

Polyploidy

Alternation of generations

Mitosis in haploid state

 

Agricultural biotechnology

To create plants with altered characters, gene transfer has been used extensively in recent years. In principle, it is the same procedure as used in other organisms. Plants are especially suitable for genetic modification because most plant cells are totipotent. This means that a plant can be generated from a single genetically modified cell (i.e., it would not require fertilization). The most commonly used carrier vector in plant genetic modification is the Ti plasmid of Agrobacterium tumefaciens. The 30 kb-long T-DNA part of this plasmid is able to integrate into plant chromosomes. This plasmid can be used to transfer up to 40 kb inserted DNA into a protoplast (a plant cell whose cell wall has been destroyed enzymatically). The engineered protoplast has the ability to act as a (fertilized) germ cell and to regenerate into a whole plant (totipotency). An alternative gene transfer method for plant cells is penetrating the cells with DNA coated gold and tungsten spheres fired from a special gun (this technique is called biolistics). The common purposes of genetic modification in plants are induction of resistance to insects, viruses, herbicides and commercial benefit (such as higher yield, seedless fruits, long-lasting tomatoes). Plants can also be used as bioreactors to produce desired recombinant proteins. See also Gene Therapy.

Further Reading

Flower Power by B Furlow (New Scientist, 9 Jan 1999, pp.22-26)

Internet Directory for Botany    Virtual Library of Botany    Botanical Glossaries

Plant Biology Course Notes

Botany Course    Plant Molecular Genetics Course Notes    Plant Biology (mini-essays)

Arabidopsis thaliana Website    Maize Genome Database    Kew Gardens (London)

Plant Genetics Links    Plant Genetics Educational Material

M.Tevfik Dorak, B.A. (Hons), M.D., Ph.D.

Last updated on 10 July 2005

Last edited on 14 July 2013

 

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