Introductory Crop Physiology (1+1)

Classes of plant hormones

Classes of Plant Harmones

In general, it is accepted that there are five major classes of plant hormones, some of which are made up of many different chemicals that can vary in structure from one plant to the next. The chemicals are each grouped together into one of these classes based on their structural similarities and on their effects on plant physiology. Other plant hormones and growth regulators are not easily grouped into these classes; they exist naturally or are synthesized by humans or other organisms, including chemicals that inhibit plant growth or interrupt the physiological processes within plants. Each class has positive as well as inhibitory functions, and most often work in tandem with each other, with varying ratios of one or more interplaying to affect growth regulation.

The five major classes are:

1. Auxins
2. Cytokinins
3. Gibberellins
4. Ethylene
5. Abscisic acid
6. Brassinosteroids


AUXIN

• The concept of chemical messengers in plants was proposed by Charles Darwin and his son, Francis in 1881, who spent time looking at the phenomenon of phototropism in wheat seedlings.
• Plant shoots are positively phototropic.
• When a seedling is illuminated from one side the shoot will bend towards the light.
• The directional growth makes sense, since plants need light for photosynthesis.
• However, Darwin found that if the coleoptiles of the wheat seedling were removed the plant no longer curved towards light.
• They did a number of experiments and determined that a chemical located in the colepotile travelled to the region of elongation and effected a differential elongation of cells furthest from the light sources.
• The chemical was subsequently studied and named it as auxin by Frits Went in 1926.
• Chemically auxin is indolacetic acid (IAA) which is synthesised from tryptophan.
• Auxin promotes growth in molar concentrations of 10-3 to 10-8. A primary site of auxin production is the apical shoot meristem.
• Auxin moves down the stem parenchyma cells by polar transport (auxin become negatively charged) using proton pumps, an energy requiring process




Functions of Auxin:

• Stimulates cell elongation
• Stimulates cell division in the cambium and, in combination with cytokinins in tissue culture
• Stimulates differentiation of phloem and xylem
• Stimulates root initiation on stem cuttings and lateral root development in tissue culture
• Mediates the tropistic response of bending in response to gravity and light
• 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
• Can induce fruit setting and growth in some plants
• Involved in assimilate movement toward auxin possibly by an effect on phloem transport
• Delays fruit ripening
• Promotes flowering in Bromeliads
• Stimulates growth of flower parts
• Promotes (via ethylene production) femaleness in dioecious flowers
• Stimulates the production of ethylene at high concentrations

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GIBBERLINS

• The discovery of gibberellins (GA) is created to Ewiti Kurosawa who found that a fungus was responsible for abnormal rice seedling growth, called the “foolish seedling” disease.
• The fungus secreted a chemical that caused the rice plant to grow abnormally long and then collapse frm weakness.
• The fungus was Gibberella fujikuroi, hence the hormone named as Gibberellin. Many seeds contain a variety of different gibberellins.
• Over 100 different GA’s (organic acides synthesized from mevalonic acid) are known. GA’s are produced in roots and younger leaves.
• Most effects of GA are shown only in concert with Auxins
The Nature of 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.
• All gibberellins are derived from the ent-gibberellane skeleton.
• The structure of this skeleton derivative along with the structure of a few of the active gibberellins are shown above.
• The gibberellins are named GA1....GAn in order of discovery. Gibberellic acid, which was the first gibberellin to be structurally characterized, is GA3.
• There are currently 136 GAs identified from plants, fungi and bacteria.
• GA's are widespread and so far ubiquitous in both flowering (angiosperms) and non-flowering (gymnosperms) plants as well as ferns.


Functions of Gibberellins
Active gibberellins show many physiological effects, each depending on the type of gibberellin present as well as the species of plant. Some of the physiological processes stimulated by gibberellins are outlined 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.
• 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.


CYTOKININS
Nature of Cytokinins
Cytokinins or CKs are a group of chemicals that influence cell division and shoot formation. 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). They were called kinins in the past when the first cytokinins were isolated from yeast cells. They also help delay senescence or the aging of tissues, are responsible for mediating auxin transport throughout the plant, and affect internodal length and leaf growth. They have a highly-synergistic effect in concert with auxins and the ratios of these two groups of plant hormones affect most major growth periods during a plant's lifetime. Cytokinins counter the apical dominance induced by auxins; they in conjunction with ethylene promote abscission of leaves, flower parts and fruits. The most common form of naturally occurring cytokinin in plants today is called zeatin which was isolated from corn (Zea mays).



Functions of Cytokinin

• Stimulates cell division.
• Stimulates morphogenesis (shoot initiation/bud formation) in tissue culture.
• Stimulates the growth of lateral buds-release of apical dominance.
• 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
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Role of cytokinins
1) Cutokinins (together with auxin) probably regulate the cell cycle
2) Cytokinins (together with auxin) may regulate tissue morphogenesis, since the ratio of auxin: cytokinin in tissue culture medium determines root or shoot production

• High auxin : cytokinin = root production
• Intermediate auxin : cytokinin = callus growth
• Low auxin : high cytokinin = shoot production
3) Cytokinins delay senescence and promote nutrient uptake

• Application of cutokinin to a leaf on an aging plant can allow that leaf to stay green while others turn yellow and die
• Cytokinin application to later buds can promote their growth
• Some pathogens produce cutokinins that attract nutrients to that tissue or cause extensive growth of lateral buds (leads to witch’s broom)
4) Cytokinins promote chloroplast maturation and cell enlargement in leaf cotyledons

• Cytokinin application promotes development of chloroplast from etioplasts
• Sections of leafy cotyledons enlarge when treated with cytoikinin (but not acid growth)
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