14-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

14-1Copyright 2005 McGraw-Hill Australia Pty Ltd PPTs t/a Biology: An Australian focus 3e by Knox, Ladiges, Evans and Saint

Part 3: Plant form and function

Chapter 14: Reproduction, growth and development of flowering plants


Introduction

• Plant life cycles are characterised by an alternation of generations


Fig. 14.1a: The life cycle of a homosporous plant

Copyright © Professor Pauline Ladiges, University of Melbourne


Fig. 14.1b: The life cycle of a heterosporous plant

Copyright © Professor Pauline Ladiges, University of Melbourne


Alternation of generations

• Many mosses and ferns are homosporous, in that their sporophytes produce only one type of haploid spore

• Flowering plants are heterosporous as they produce, by meiosis, separate male and female spores, each of which undergoes mitosis to produce male and female gametophytes


The angiosperm flower

• In angiosperms, the sporophyte is the dominant generation

• The sporophyte produces flowers, which are the sites of sexual reproduction

• A flower is a specialised shoot composed of four whorls of leaves, grouped around the tip of the flower stalk or receptacle

• These whorls, beginning with the outermost, are the sepals, petals, stamens and carpel


(cont.)

Fig. 14.2a: Longitudinal section of a flower of oilseed rape, Brassica napus

Copyright © E Evans


Fig. 14.2b: Top view and longitudinal section of a typical flower


The angiosperm flower (cont.)

• Stamens are the male reproductive organs• A stamen consists of a filament upon which is

borne an anther• A carpel is the female reproductive organ• A single carpel consists of a stigma, style and

ovary• In some species of flowering plants, a number of

carpels are fused together to form a gynoecium


Anthers and carpels

Development of angiosperm gametophytes involves meiosis and mitosis


Fig. 14.3: Development of pollen and embryo sac (top)


Fig. 14.3: Development of pollen and embryo sac (middle)


Fig. 14.3: Development of pollen and embryo sac (botttom)


The anther

• An anther consists of two pollen sacs, each containing a large number of multicellular pollen grains

• Pollen grains are the sperm-producing male gametophytes

• Pollen forms when a unicellular microspore undergoes mitosis to produce a small generative cell and a larger vegetative cell

• When pollen lands on a stigma, it germinates to produce a pollen tube


The carpel

• The stigma may be wet or dry and either smooth or covered in elongated cells known as papillae, which trap pollen

• The pollen tube of a germinating grain grows down through the style into the ovary

• An ovary contains ovules, within each of which is an embryo sac

• A pollen tube enters the ovule via the micropyle, and releases sperm into the embryo sac, fertilising the egg


Double fertilisation

• Each pollen tube contains a tube nucleus and two sperm nuclei

• The egg sac of an ovule contains an egg cell, situated near the micropyle, and two polar nuclei contained within a large central cell

• At fertilisation, one sperm fuses with the egg cell to form a diploid zygote

• The other sperm fuses with the polar nuclei to form triploid endosperm, which will support the growth of the embryo and in some cases the growth of a germinating seedling


Apomixis

• Some plants have the capacity to reproduce without fertilisation e.g. apomictic species

• Apomicts produce a diploid megaspore that does not undergo meiosis, but instead divides by mitosis to produce an embryo, which then develops in the same way as sexually-produced embryos

• The absence of meiosis means that apomictic plants are identical to one another lack of genetic variation

• Apomixis is common among successful species and provides a means of rapid reproduction


Pollination in flowering plants

• Pollination, the first step in the chain of events leading to fertilisation, unites male and female gametophytes

• Most flowering plants have close interactions with insects, birds or other animals that convey pollen directly between flowers

• Plants (e.g. grasses, she-oaks) that lack such mutualisms may be wind-pollinated, compensating for the randomness of this form of dispersal by releasing large quantities of pollen

(cont.)


Pollination in flowering plants (cont.)• Plants, which are essentially fixed in place, have

evolved mechanisms that prevent cross-species pollination

• The stigma recognises pollen belonging to the same species and either prevents the pollen from other species from germinating, or blocks pollen tube growth down the style

• The flowers of most species contain both male and female reproductive organs—they are bisexual—and some of these may self-fertilise

(cont.)


Pollination in flowering plants (cont.)• Many plants have evolved mechanisms that

encourage fertilisation between separate individuals of the same species (cross-fertilisation)

• Cross-fertilisation maintains genetic variation in offspring, which is advantageous in unpredictable or changing environments

– monoecious species: male and female organs occur on separate flowers of the same plant

– dioecious species: male and female flowers occur on separate plants


Enhancing cross-pollination

• In plants with bisexual flowers, a variety of mechanisms may prevent self-fertilisation

– some species produce flowers that go through separate male and female phases

– others have flower structures that inhibit self-pollination e.g. ‘pin’ and ‘thrum’ flowers on separate primrose plants


Preventing self-fertilisation

• Self-incompatibility is the most common means by which plants prevent self-fertilisation

• This genetically-controlled recognition system stops eggs from being fertilised by pollen from the same plant

• If pollen is deposited on the stigma of a flower on the same plant, a biochemical block prevents the pollen from forming a pollen tube and fertilising an egg

• Recognition of ‘self’ pollen is based on genes for self-incompatibility, called S-genes


Self-incompatibility

• As a pollen grain is haploid, it will be recognised as ‘self’ if its S allele is the same as either of the two S alleles of the diploid stigma


Fig. 14.13: Genetics of self-incompatibility


Seed development

• After fertilisation, a zygote undergoes a series of rapid cell divisions to form an embryo

• In dicotyledons (e.g. beans) the embryo continues to develop and generates two seed leaves (cotyledons) between which is situated the shoot apical meristem

(cont.)


Fig. 14.16a: The zygote divides into a two-celled proembryo


Seed development (cont.)• Mitotic divisions of the triploid endosperm nucleus

ultimately generate liquid endosperm, which as it forms cell walls, solidifies and expands

• In this state, endosperm is the major nutritive tissue of the seed, rich in lipids or carbohydrates

• As a seed matures, it enters dormancy, a state of extremely low metabolic rate with deferral of growth and development

• Dormancy increases the likelihood that when the seed germinates, it will be under conditions (light, temp. etc.) that most advantage the seedling


Fruit development

• As seeds develop from ovules, other changes occur in the flower, including swelling of the ovary to form a fruit, which protects the seeds and assists in their dispersal

• Fruits normally ripen at about the same time as its seeds are completing their development

• In cereals and grasses, the fruit contains a single fertilised ovule and develops into a grain

• If a flower is not pollinated, fruit will not normally develop and the flower will shrivel and drop


Seed germination• Germination of seeds relies on imbibition, which is

the uptake of water resulting from the low water potential of the dry seed

• As the seed expands, it ruptures the seed coat, providing oxygen to the embryo and triggering metabolic changes that enable growth to restart


Organogenesis

• Plant growth describes the irreversible increase in mass that results from cell division and expansion

• Development, on the other hand, is the sum of all the changes that together define the plant body

• Most plants demonstrate indeterminate growth, growing for as long as they remain alive

• In contrast, most animals and certain plant organs, such as leaves and flowers, undergo determinate growth, ceasing to grow after they attain a certain size

(cont.)


Organogenesis (cont.)

• Growth involves the production of new cells by repeated mitotic division, together with enlargement of existing cells

• These cells will differentiate into a range of cell types, each of which will assemble into the three-dimensional structures characteristic of mature organs

• Growth and development of new organs begins in specialised regions of cells found at the tips of shoots and roots—the apical meristems


The shoot apex• The shoot apical meristem produces stems and

leaves, and also flowers when the plant enters its reproductive phase

• The apex of the shoot is a spherical dome of meristematic cells that divide to produce leaf primordia, structures that develop into leaves


Fig. 14.19: Shoot apical meristem


The root apex

• The root apical meristem of flowering plants contains a zone of rapidly dividing cells that gives rise to the mature tissues of the root

• Include root hairs, which arise by elongation of an epidermal cell, and lateral roots, which arise deep within the tissues of more mature parts of the root


Asexual reproduction in plants

• Many plants have the ability to clone themselves by asexual, or vegetative, reproduction

• Some plants, such as strawberries and Spinifex grass, have stolons, long stems that grow horizontally along the soil surface, forming roots and leaves that eventually form independent units

(cont.)


Asexual reproduction in plants (cont.)• Some species have the ability to form shoots from

underground storage organs such as corms and bulbs, or from root tubers

• Rhizome-producing species such as bracken, and those that have horizontal roots, such as wattle, also have the capacity to reproduce vegetatively

• These clones are genetically identical to the parent • Plants have the ability, under suitable conditions,

to generate an entire plant from a single cell—a property known as totipotency


Biotechnology and plants

• Plant biotechnologists use a number of in vitro methods to generate new plant varieties

• Tissue culture is a propagation technique in which one or a few cells are grown on artificial media, containing nutrients and hormones, to generate large numbers of plants

• Via manipulation of the hormonal balance, the callus (a mass of dividing undifferentiated cells) that forms can be induced to develop shoots and roots with fully differentiated cells


Genetic engineering of plants

• Tissue culture techniques are now used to produce genetically-modified (transgenic) plant species

• Desirable plant traits can be introduced into crop plants to increase disease resistance, improve nutritional value and increase crop survival in adverse environments

• The gene that codes for the plant trait is identified and isolated, and then incorporated into the nuclear DNA of a host cell

(cont.)


Genetic engineering of plants (cont.)• Transformation is the process by which the

genetic makeup of a single cell is altered• This process uses vectors such as bacterial

plasmids to transfer the gene


Fig. 14.26a: Transferring cloned genes

14-1 copyright 2005 mcgraw-hill australia pty ltd ppts t/a biology: an australian focus 3e by knox,...

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