Introduction

Hormones (plant growth regulators)

A hormone is a naturally occurring organic compound that acts at low concentrations to produce profound effects on physiology, growth and development (and also behaviour in animals). Since plants do not 'behave' and since plant hormones tend to differ in many respects from animal hormones e.g. may have many sites of synthesis, multiple functions and modes of transport, we refer to them as plant growth regulators.

The five main kinds of plant growth regulators and their effects are:

Auxins e.g. Indole Acetic Acid (IAA) – enlargement and differentiation of cells (growth)
Gibberellins – growth, development (germination, dormancy and flowering)
Cytokinins – cell division in roots and shoots
Abscisic Acid (ABA)– many processes, bud dormancy
Ethylene – many processes, ripening of fruit

Plant growth regulators are responsible for growth promotion and/or growth inhibition and often act in combination or antagonistically.

Plant growth regulators bind to proteins associated with membranes of the cells they will affect i.e. an extracellular signalling molecule activates a membrane receptor, that in turn alters intracellular molecules (second messengers) and creates a response - the signal transduction pathway. The signal transduction pathway involves 3 stages. 1. Receptor binds to menbrane of cell 2. message is transduced 3. a response is illicited

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Plant responses

Plants grow in response to environmental stimuli, inlcuding light and gravity as well as in response to hormones (plant growth regulators). In addition to photosynthesis, plants repsond to light in a number of ways:

Phototropism (directional growth in response to light)
Germination (emergence from seed or spore)
Shade detection (competition for light, day length and seasonal effects)
Photomorphogenesis (light mediated development)
Photoperiodism (physical reaction to length of day or night)

Often environmental stimuli, such as light, cause plant growth regulators to be synthesised and transported to the affected areas of the plant, where they illicit a growth response.

Auxin (Indole Acetic Acid)

Auxins are synthesised in embryos, meristems (growing tips), young leaves and buds. They are transported from apex to base (basipetal) in parenchyma surrounding vascular bundles (xylem and phloem). Their main effects are cell enlargement and tissue differentiation, including response to tropisms. Auxins are also important in apical dominance, ensuring that the plant continues to grow upright (unless apical meristem removed by a grazing animal, in which case lateral growth occurs). Auxins are also imortant when a plant is damaged, it is transported to the site of damage and signals for cells in pith to differentiate into new vascular tissue and connect with vascular tissue above and below wound. Auxins interact with cytokinins during root and shoot growth.

The curvature resposne: auxin and phototropism

The diagram below shows the effects of auxin and why plants show positive phototropism (grow towards the lgiht).

1. This is a normal plant that has the sun positioned almost directly overhead. During this time, the auxin (pink dots) within the plant is evenly distributed. 2. The sun is now positioned at an angle to the plant. The repositioning of the sun causes the auxin to move to the shaded side of the plant, and becomes more concentrated. This overload of auxin next to these cells causes them to start to grow or elongate, the cell walls loosen and water enters via osmosis, causing the cells to enlarge. 3. This results in the the plant growing toward the sun. 4. If the sun moves to the other side of the plant, the auxin would redistribute to the other side of the plant and become concentrated on the side of the plant that is farthest away from the sun. 5. The same growth or elongation of the cells on this side of plant would continue to grow towards the sun.

Gibberellins

Gibberellins are synthesised in meristems, young leaves and embryos. They are transported both ways (bidirectional) in the phloem. Their main effects are (growth promotion) cell division and enlargement, seed germination, environmental effects (vernalisation, photoperiodism), maturation, flowering initiation, fruit formation. Gibberellins stimulate internode elongation (the figure of the flowers below demonstrates the effects of gibberellins 1. Shows a plant lacking gibberellins and has a internode length of "0" as it is a dwarf plant. 2. Shows a typical plant with a moderate concentration of gibberellins and an average internode length. 3.Shows a plant with a high concentration of gibberellins and so has a much longer internode length because gibberellins promotes cell division in the stem.). They also promote the activity of the enzyme alpha-amylase in seeds, breaking down the starchy endosperm which releases sugars to the developing embryo.

Barley seed germination.gif

Cytokinins

Cytokinins are synthesised in roots, seeds, fruits and leaves. They are involved in cell division, germination, retards senescence, root growth and differentiation, overcomes bud dormancy and are transported in the xylem. Cytokinins interact with other growth regulators such as auxin to control root and shoot growth. Additionally, cytokinins promote cell expansion in dicot cotyledons (neither IAA nor GA do this in cotyledons), they promote chloroplast development, they delay leaf senescence and they promote movements of nutrients.

The figure above shows the realtionship between abiotic stress signals and cytokinin. Summary of interactive points between cytokinin metabolism and signalling pathways (as currently modelled in Arabidopsis [5]) and abiotic stress response pathways (Pavlu et al., 2018).

Abscisic acd (ABA)

Abscisic acid (ABA) is synthesised in leaves, stems, roots and fruits. It is a growth inhibitor and controls seed and bud dormancy, embryo development, senescence, stress responses and stomatal closure. It is transported root to shoot in xylem, shoot to root in phloem.

ABA levels increase during early seed development. This promotes production of seed storage proteins. It also prevents premature germination. Breaking dormancy corresponds with decrease in ABA in seeds. Bud dormancy also under ABA control. ABA effects overcome by Gibberellic acid. Stressed plants contain much higher amounts of ABA than non-stressed plants. Roots respond to drought stress by increased synthesis of ABA. ABA transported from root to shoot in xylem and reaches leaves where it induces stomatal closure.

The role of ABA is stomatal closure is illustrated below.

1-Epidermal cell 2-Guard cell 3-Stoma 4-K+ ions 5-Water 6-Vacuole

a. Open stoma: stomata are the small pores in the epidermis of leaves. They are bordered by guard cells. The stomata open when the turgor pressure increases in the guard cells, causing the cells to buckle outward. This happens when water flows into the guard cells. Water flows into the guard cells after K⁺ ions enter the cells, because the water is flowing down its concentration gradient to keep the solute level the same inside the cell as outside. The K⁺ ions are actively transported into the cells. A proton pump moves H⁺ ions out of the cell, which is powered by the hydrolysis of ATP. This creates an electrochemical gradient that allows the K⁺ to flow into the cells through a channel protein. A signal that begins the process is a blue-light component of sunlight.

b. Closed stoma: stomata close when the turgor pressure decreases because water exits the cell. The water flows out because the K⁺ ions exit the cell. They flow out when the proton pump is deactivated. There are a number of signals that can cause stomata to close, these include: a rise in CO₂ concentration and the hormone abscisic acid. Even if plants are kept in the dark, the stomata still open and close about every 24 hours, meaning they are regulated by the Circadian rhythms of the plants. The opening and closing of stomata are also influenced by temperature, humidity, and stress.

Ethylene

Ethylene is synthesised in ripening fruits and aging leaves and flowers. Its main effects are fruit ripening, leaf and flower senescence, abscission, root hair development, stress responses (waterlogging) and inhibition of cell expansion. It differs from other plants regulators in that it is a gas and therefore moves by diffusion.

The following figure shows how ethylene induces leaf shedding (abscission). Note the interaction between promotion effect of auxin (leaf maintenance) and inhibition effect of ethylene (leaf abcission).

Ripening bananas release a lot of ethylene. This causes other fruits to ripen prematurely. So if the only thing you learn from this module is to keep your bananas separately from your other fruit, at least your pears and apples will last a little longer!

The following videos summarise the activity and effects of plant growth regulators.