Reproduction, Seeds and Propagation

Reproduction, Seeds and Propagation

Diploid (2n) somatic cell Two diploid (2n) somatic cells

Telophase Anaphase Metaphase Prophase I One pair of homologous chromosomes (homologues) II Homologues condense and cross over I Homologues a line II I Homologues separate II I Homologues separate into 2 cells II

Self-pollinator Out-pollinator

Factors which influence plant growth Environment Irradiation, Day-length, Temperature, Water availability, Gases Soil, Nutrients Plant Hormones

Growth Hormones

Auxins Cytokinins Gibberellins Ethylene Abscisic

Auxin The term auxin is derived from the Greek word auxein which means to grow. Compounds are generally considered auxins if they can be characterized by their ability to induce cell elongation in stems and otherwise resemble indoleacetic acid (the first auxin isolated) in physiological activity. Auxins usually affect other processes in addition to cell elongation of stem cells but this characteristic is considered critical of all auxins and thus "helps" define the hormone

Auxins Stimulates cell elongation. Mediates the tropistic response of bending in response to gravity and light. Indol-3-acetic acid - IAA

Auxin Charles Darwin was one of the first to examine phototropism in plants. Studies showed that growth towards light is caused by elongation of the cells at the side that is shielded from the light. The phototropic reaction does not happen if the coleoptile s tip is removed, though it can be induced again by the replacement of the tip. This indicates the existence of a substance that is spread from tip to bottom and that causes the elongation.

Phototropism

Phototropism

Auxins Remove coleoptile tip Growth stops Place tip on agar block for 1-4 hours Place agar onto coleoptile Growth resumes

Gravatropism Gravity causes localized expression of auxin in plant roots. Plants grown on an angled show increased accumulation of growthpromoting auxin on the lower side of the root (green staining). Plant roots will grow in the direction of increased auxin expression

Gravatropism

Auxins Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture. Stimulates differentiation of phloem and xylem. Stimulates growth of flower parts. Stimulates the production of ethylene at high concentrations

Auxins The auxin supply from the apical bud suppresses growth of lateral buds. Delays leaf senescence. Can inhibit or promote (via ethylene stimulation) leaf and fruit abscission. Promotes (via ethylene production) femaleness in dioecious flowers.

Auxins Can induce fruit setting and growth in some plants. Delays fruit ripening. Stimulates root initiation on stem cuttings and lateral root development in tissue culture.

Auxins Normal Fruit (seed) Development Seed Removed Seed Removed auxin applied

Auxin No auxin

2,4-Dichlorophenoxyacetic acid (2,4-D)

2,4-Dichlorophenoxyacetic acid (2,4-D) 2,4-D became the first successful selective herbicide and allowed for greatly enhanced weed control in cereal crops. 2,4-D was developed in Britain during World War II to increase crop yields for a nation at war. 2,4-D is a synthetic auxin. It is absorbed through the leaves and is translocated to the meristems of the plant. Uncontrolled, unsustainable growth ensues causing stem curl-over, leaf withering, and eventual plant death.

Cytokinins Cytokinins are compounds with a structure resembling adenine which promote cell division and have other similar functions to kinetin. Kinetin was the first cytokinin discovered and so named because of the compounds ability to promote cytokinesis (cell division). The most common form of naturally occurring cytokinin in plants today is called zeatin which was isolated from corn (Zea mays).

Cytokinins Cytokinin is generally found in higher concentrations in meristematic regions and growing tissues. They are believed to be synthesized in the roots and translocated via the xylem to shoots. Zeatin

Cytokinin Stimulates cell division. Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture. Stimulates the growth of lateral buds-release of apical dominance.

Cytokinin Stimulates leaf expansion resulting from cell enlargement. May enhance stomatal opening in some species. Promotes the conversion of etioplasts into chloroplasts via stimulation of chlorophyll synthesis.

A transgenic tobacco plant (right) with a high content of cytokinin shows fewer signs of senescence compared to a wild type plant (left) in which all the lower leaves are dead.

The ratio of auxin to cytokinin in a tissue dictates growth of axillary meristems High auxin / Low cytokinins = meristem remains dormant Low auxin / High cytokinins = meristem starts to under go cell division and starts to grow

Auxin:Cytokinin Association 0.0 0.03 0.18 3.0 IAA Concentration

Gibberellins Unlike the classification of auxins which are classified on the basis of function, gibberellins are classified on the basis of structure as well as function. The gibberellins are named GA 1...GA n in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterised, is GA 3. There are currently 136 GAs identified from plants, fungi and bacteria.

Giberellic acid Stimulate stem elongation by stimulating cell division and elongation. Stimulates bolting/flowering in response to long days. Breaks seed dormancy in some plants which require stratification or light to induce germination. GA 1

GA 3 Control

Giberellic acid Stimulates enzyme production (a-amylase) in germinating cereal grains for mobilization of seed reserves. Induces maleness in dioecious flowers (sex expression). Can cause parthenocarpic (seedless) fruit development. Can delay senescence in leaves and citrus fruits.

Gibberellins Inhibitors

The eui mutant of rice is deficient in a gibberellin deactivation enzyme and hence grows taller than the wide type (WT)

Harvest-aids are used to manipulate the concentrations of the plant hormones auxin and ethylene so all the leaves are dropped at approximately the same time

Abscisic Acid Stimulates the closure of stomata (water stress brings about an increase ). Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. Induces seeds to synthesize storage proteins. ABA

Abscisic Acid Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase. Has some effect on induction and maintanance of dormancy. Induces gene transcription especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.

Ethylene H 2 C=CH 2 Ethylene, unlike the rest of the plant hormone compounds is a gaseous hormone. Like abscisic acid, it is the only member of its class. Of all the known plant growth substance, ethylene has the simplest structure. It is produced in all higher plants and is usually associated with fruit ripening.

Ethylene Ethylene has been used in practice since the ancient Egyptians, who would gas figs in order to stimulate ripening. The ancient Chinese would burn incense in closed rooms to enhance the ripening of pears. In London in 1864, gas leaks from street lights showed stunting of growth, twisting of plants, and abnormal thickening of stems

Ethylene Stimulates the release of dormancy. Stimulates shoot and root growth and differentiation. May have a role in adventitious root formation. Stimulates leaf and fruit abscission.

Ethylene Induction of femaleness in dioecious flowers. Stimulates flower opening. Stimulates flower and leaf senescence. Stimulates fruit ripening.

Ethylene

Ethylene

Brassinosteroids Regulate cell expansion and are one of the most important hormones that regulate stature. Without them, plants are tiny dwarves, with reduced vasculature and roots, and are infertile. They also regulate senescence or aging.

The dx mutant of tomato is severely dwarfed due to brassinosteroid deficiency. However the fruits develop normally

Jasmonic Acid Major functions is in regulating plant growth include growth inhibition, senescence, and leaf abscission. It has an important role in response to wounding of plants plant resistance. When plants are attacked by insects, they respond by releasing Jasmonic Acid, which inhibits the insects' ability to digest protein. It is also responsible for tuber formation in potatoes, yams, and onions.

Jasmonic Acid Jasmonate ZIM-domain Protein Degradation

Factors which influence plant growth Environment Irradiation, Day-length, Temperature, Water availability, Gases Soil, Nutrients Plant Hormones