Physiology of Seed Plants

Regulating Growth and Development: The Plant Hormones Auxins Cytokinins Ethylene Abscisic Acid Gibberellins

Hormones Chemical signals that help both plants and animals regulate and coordinate metabolism, growth, and differentiation. Phytohormones- plant hormones

Three basic elements of Hormones Synthesis of the hormone in one part of the organism Transport of the hormone to another part (target tissue) Induction of chemical response

Phytohormones Produced in tissues or glands Very active in small quantities Pineapple Ananas comosus for example are only 6 micrograms of indoleacetic acid (IAA) a common plant hormone per kg of plant material. (analogous to a needle in 20 metric tons) Can stimulate or inhibit depends on chemical structure and how it is read by the target tissue

Five classes of plant hormones The “Classic Five” Auxins- today Cytokinins- today Ethylene- thursday Abscisic acid- thursday Gibberellins- thursday

Auxins Charles Darwin and Francis Darwin- The Power of Movement in Plants 1881 In response to light an “influence” that causes bending is transmitted from the tip to area below the tip

The Principle naturally occuring Auxin- Indoleacetic Acid In plants variety of pathways to produce tryptophan usually precursor All tissue produces IAA but typically found in shoot apical meristems, young leaves and developing fruit and seeds. Mutants lacking either auxin or cytokinin Have yet to be found- mutations eliminating Them are lethal

Auxin synthesis- the site of auxin synthesis along the margin of a young leaf Site corresponds to the location of cells that will differentiate into a hydothode (gland like structure) GIS reporter gene detects auxin synthesis

Auxin transport- experimental demo of polar auxin transport in stems represented here by a segment of hypocotyl from a seedling

In the root, nonpolar transport of IAA takes place in the phloem of the vascular cylinder whereas the polar transport occurs in the epidermis and cortical parenchyma cells

Schematic model for polar auxin transport

IAA induced xylem regeneration around a wound Arrowhead- vascular regeneration Basipetal polar movement of auxin from Above the arrow and then around the wound IAA induced xylem regeneration around a wound

- When the apical bud is cut off a plant, the development of axillary buds in lateral branches is observed. - If the apical bud is replaced by cotton impregnated with auxin, no axillary bud development is observed. - So the auxin replaces the apical bud. - It can be deduced that this hormone is produced in the apical part of the plant.

The inferior part of the plant including roots ( or root cap) is cut off and the plant is put in a medium containing auxin or free of it. Without auxin, adventitious roots can developed. This is the principle of cuttings. However, with auxin, root development is much better.  

Auxin promotes fruit development Auxin is involved with the formation of fruit Parthenocarpic fuit- by treating a female flower parts (carpels) of certain species with auxin it is possible to produce a fruit (without fertilization- a virgin fruit) i.e. seedless tomatoes, cucumbers and eggplants. Developing seed is a source of auxin

Auxin and fruit development- Normal strawberry

Strawberry with all seeds removed

Strawberry with horizontal band of seeds removed

Other characteristics of Auxin Auxin provides chemical signals that communicate information over long distances Promotes the formation of lateral and adventitious roots Synthetic auxin (2,4 dichlorophenoxyacetic acid) are used to kill weeds (broad leaf) Mechanism unk.

Cytokinins In 1941 Johannes van Overbeek found that coconut milk (liquid endosperm) contained potent growth factors Factors greatly accelerated the development of plant embryos and promoted the growth of isolated tissue and cells in vitro (test tube)

Discovery had two affects It gave impetus to studies of isolated plant tissues Launched the search for another major group of growth regulators

Basic medium used for tissue culture of plant cells Contained sugar Vitamins Various salts Grown in this culture, growth slowed or stopped Thus some growth stimulus declined and the addition of IAA had no affects Adding coconut milk encouraged the cells to divide and growth to resume

Growth factor from DNA Isolation of growth factor from DNA identifying its chemical nature called kinetin and the group of regulators called cytokinins because its involvement with cytokinesis

Kinetin Resembles purine- adenine Probably does not occur naturally in plants Has relatively simple structure Chemist able to synthesize a number of related compounds Zeatin- most active naturally occurring cytokinin (maize)

Cytokinins Found in active dividing structures, seeds, fruits, leaves and root tips Found also in SVP horsetail, fern Central to tissue culture methods and extremely important in biotech. Tx of lateral buds causes growth even in the presence of auxin thus modifying apical growth.

Undifferentiated plant cell has two courses open to it The cytokinin/auxin ratio regulates the production of roots and shoots in tissue cultures Undifferentiated plant cell has two courses open to it It can enlarge, divide, enlarge and divide again (undifferentiate) or without undergoing cell division, it can elongate and differentiate

In tobacco stem tissue Applicatin of IAA causes rapid cell expansion- giant cells are formed Kinetin alone has little or no effect IAA + Kinetin results in rapid cell division, so that large numbers of relatively small, undifferentiated cells are formed. High IAA, callus tissue- a growth of undiff. Cells in tissue culture frequently gives rise to roots.

Callus development- effects of increasing [IAA] at various kinetin

- At the top, the apical dominance have been annulled by cutting the apical bud. - At the centre, the terminal bud and the root cap have been cut. No futher development of axillary buds on the explant can be observed. So the roots are necessary for the development of buds. - on the other hand (at the bottom), if one identical explant is introduiced in a medium containing cytokinins, the development of axillary buds occurs. So the cytokinins replace the roots and it can be deduced that these hormones are produced in the roots. It is observed also that there is less root development in the presence of cytokinins.

Summary Auxins: - They are produced in the apical part of the plant. - They prevent the development of the axillary buds. - They favour the rhizogenesis ...(development of roots). Cytokinins: - They are produced in the roots. - They prevent the development of roots. - They favour the development of the axillary buds.

With large concentrations of cytokinins and low concentrations of auxins, the development of axillary or adventitious buds can be obtain and in this way the plants are multiplied. With large concentrations of auxins and low concentrations of cytokinins or no cytokinins, the rooting of the shoots can be arrived at. - With equal concentrations of the two hormones, a callus is obtain. The callus is the result of the anarchic proliferation of cells which are more or less differentiated but which cannot organized them and form tissues and distinct organs.

Cytokinins delay leaf senescence Yellowing (loss of chlorophyll) of leaf can be delayed with the addition of cytokinins Xanthium strumarium Leaves turned yellow in about ten days in plain water Add 10mg of kitenin help retain green

Additional chemicals used by plants Brassinosteroids- naturally occuring polyhydroxyl steroids (tissue growth) Salicylic acid- phenolic compound similar structure to aspirin implicated in defense responses Jasmonates- class of compounds known as oxylipins plant growth regulation and defense Polyamines- strongly basic molecules found in all organisms (bacteria, fungi, animal, plants) are essental for growth and development and affects cell division Systemin- a polypeptide functions as a long distance signal to activate chemical defenses against herbivores. Nitric oxide (NO) serves as a signal in hormonal defense responses.

Ethylene Plays a role in fruit ripening Promotes abscission (shedding of leaves, flowers and fruit) Triggers enzymes that promote fruit loosening from trees Auxin prevents abscission (prevents preharvest) High concentration does the opposite

Abscisic Acid (ABA) First discovered as dormin and abscisin Dormin ash and potatoes In other plants abscisin Identical compounds now called Abscisic acid

Abscisic Acis Levels increase during early seed development Stimulates the productio of seed storage protein Prevents premature seed germination Decline in ABA leads to germination

Gibberellin Found in immature seeds- highest concentration Stimulates cell division and cell elongation Application to dwarf mutants cause them to grow tall Plays a role in breaking seed dormancy and germination